New User Special Price Expires in

Let's log you in.

Sign in with Facebook


Don't have a StudySoup account? Create one here!


Create a StudySoup account

Be part of our community, it's free to join!

Sign up with Facebook


Create your account
By creating an account you agree to StudySoup's terms and conditions and privacy policy

Already have a StudySoup account? Login here

Vertebrate Histology

by: Peyton Schimmel

Vertebrate Histology ZOO 4753

Peyton Schimmel
University of Central Florida
GPA 3.65


Almost Ready


These notes were just uploaded, and will be ready to view shortly.

Purchase these notes here, or revisit this page.

Either way, we'll remind you when they're ready :)

Preview These Notes for FREE

Get a free preview of these Notes, just enter your email below.

Unlock Preview
Unlock Preview

Preview these materials now for free

Why put in your email? Get access to more of this material and other relevant free materials for your school

View Preview

About this Document

Class Notes
25 ?




Popular in Course

Popular in Animal Science

This 149 page Class Notes was uploaded by Peyton Schimmel on Thursday October 22, 2015. The Class Notes belongs to ZOO 4753 at University of Central Florida taught by Staff in Fall. Since its upload, it has received 38 views. For similar materials see /class/227481/zoo-4753-university-of-central-florida in Animal Science at University of Central Florida.

Similar to ZOO 4753 at University of Central Florida

Popular in Animal Science


Reviews for Vertebrate Histology


Report this Material


What is Karma?


Karma is the currency of StudySoup.

You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!

Date Created: 10/22/15
VERTEBRATE HISTOLOGY ZOO 4753 TABLE OF CONTENTS Unit One An Introduction to Histology Unit Two Epithelial Tissues Unit Three Connective Tissues Unit Four The Special Connective Tissues Unit Five Muscle Tissue Unit Six Nervous Tissue Unit Seven The Lymphatic System Unit Eight The Circulatory System Unit Nine The Respiratory System Unit Ten The Digestive System Unit Eleven The lntegument Unit Twelve The Urinary System Unit Thirteen The Reproductive System Unit Fourteen The Endocrine System Unit Fifteen The Special Senses The Eye And The Ear page 1 page 16 page 26 page 35 page 52 page 61 page 74 page 86 page 97 page 108 page 132 Page 144 page 158 page 178 page 189 Unit 1 AN INTRODUCTION TO HISTOLOGY A An Overview ofthe Relationship Between Cells Tissues and Organs 1The Cell a Cytology is the study of the cell the basic unit of life b In unicellular organisms the cell must be able to perform all ofthe functions essential for life c In multicellular organisms cells can cooperate allowing for a division of labor 1 Different cells will have functional differences since they will perform different tasks and as a result will often display morphological differences ex agella 2 When functionally different and structurally similar cells form groups within an organism those groups of cells are known as Tissues a Through specialization cells have lost the multifunctional capacity found in unicellular organisms in favor of fewer emphasized properties 1 Their organization into tissues and organs allows the multicellular organism to perform biological functions with a greater economy 2 The Tissues a Three primordial germ layers will develop in the embryos of vertebrates 1 They are the a Ectoderm the outer layer b Mesoderm the middle layer a Endoderm the inner layer 2 The three germ layers will give rise to the four classes oftissues a Epithelium arise form all three of the germ layers 1 The endoderm gives rise to a The epithelium lining most of the GI tract and it s glands b The epithelium lining the respiratory tract and it s glands c The epithelium lining the bladder and certain portions ofthe urogenital systems a The epithelium lining the lumenal surfaces ofthe blood vessels and heart called endothelium 2 The mesoderm gives rise to a The epithelium lining the serous membranes of the body called mesothelium c The epithelium lining many portions ofthe urogenital systems 3 The ectoderm gives rise to a The epithelium covering the body s surface eg the skin b The epithelium lining select portions ofthe GI tract such as the the anal canal the extrinsic glands ofthe oral cavity the taste buds and the enamel of teeth 2 c The epithelium lining portions ofthe eye ear and nose d The neuroepithelial cells b Connective Tissue arise form primarily the mesoderm 1 The exception are certain ofthe neuroglia 0 Muscle Tissue arise from primarily the mesoderm 1 The exceptions are the smooth muscles ofthe sweat glands and ofthe pupil d Nervous Tissue arises only from the ectoderm b Each ofthe four classes oftissues can be further subdivided based on functional and morphological variations due to their different specializations 1 For example there are three classes of muscle tissue 2 This follows an old adage that form follows function 0 The study of the four classes of tissues and their specializations is called Histology 3 The Organs 1 The four tissues classes will be further organized into organs which in turn will be organized into systems 2 An organ is de ned as a group of functionally similar tissues working together to perform a certain task B Technigues of Microscopy 1The Light Microscope a The light microscope allows the researcher to observe a specimen that is too small to study by the unaided eye 1 The light microscope projects a beam of light through the specimen a To facilitate this the specimen must be thin enough for light to pass through it b The beam oflight will pass through the specimen and then through a series of magnifying lenses and a prism which directs the light towards the researcher s eyes 1 The basic light microscope has three lenses arranged in the following sequence a Condenser Lens focuses the light from the source into a cone which will illuminate the specimen b Ob39ective Lens collects the light which has passe through the specimen 1 It magni es the specimen s image and directs it towards the ocular lens a Often a light microscope will have several objective lenses with varying degrees of magnifying ability 0 Ocular Lens receives the image from the objective lens and transmits it towards the eye or camera 1 The ocular lens will often also magnify the image 3 2 The arrangement of lenses will also allow for resolution ofthe image a Resolution the smallest distance that can be seem between two visibly separate objects b Histoloqical Preparation for Light Microscopv 1 The specimen can be prepared in a number of ways but generally histological preparation will follow these steps a Fixation 1 Fixation must occur either before or soon after the removal of the tissue specimen to prevent degradation 2 One draw back to xation is that xative agents kill cells and so will prevent the observation of cellular processes a In cases where the full enzymatic activity ofthe tissue must be preserved the specimen can be frozen in liquid nitrogen or liquid helium 3 One ofthe most commonly used fixatives is Formalin a concentrated formaldehyde solution a It does not denature proteins or alter tissue structure as severely as does other xatives b Other xatives include picric acid paraformaldehyde and gluteraldehyde b Dehydration 1 Dehydration will often be a prerequisite for subsequent steps in histo prep a Many embedding media require waterto be removed from the specimen a Many staining procedures also require water to be removed from the specimen 2 Dehydration is most often accomplished by immersing the tissue in a series of increasingly more concentrated solvents such as ethyl alcohol a By gradually increasing the strength of the solvent you reduce the chance andor degree of damage to the tissue 1 Even so dehydration will often cause Artifacts to occur such as tissue shrinkage or the false separation of cellstissue layers 0 Clearing 1 The specimen may be treated by a variety of chemicals to prepare it for staining This process is called Clearing d Embedding and Sectioning 1 Sectioning is the cutting ofthe tissue into sections thin enough to transmit light 1 to 20 um 2 Sectioning requires the tissue to be supported and rigid enough to slice uniformly This is the purpose to embedding a The specimen is embedded prior to sectioning b A variety of embedding media are used The two most commonly used are 4 1 Paraf n a wax 2 Epoxy a harder substance used to make thinner sections ie 1 5 um c By dehydrating the tissue prior to embedding the embedding media can enter the tissue to give it the required support 3 Sectioning of the embedded specimen is accomplished by means of a Microtome a Typically the microtome will have a very ne edged metal blade but for thinner sections sometimes glass and even diamond blades are used 4 The sectioned specimen is then placed on a slide often on a drop of water to await staining e Staining 1 To improve the visibility and clarity of cytological structures the specimen will often be stained 2 There are a wide variety of histological stains a Many stains attach to structures by electrostatic forces of attraction ie opposites attract 1 So cationic stains will be attracted to anionic structures and visa versa b Many times a variety of dyes are used that will impart different colors 1 Often a basophilic cationic dye is used in combination with an acidophilic anionic dye a The basophilic dye will attach to negatively charged structures ex the nucleus b The acidophilic dye will attach to positively charged structures ex the mitochondria 2 The most frequently used combination is the H amp E hematoxylin and eosin stain a Hematoxylin a cationic dye will stain structures blue to purple b Eosin an anionic dye will stain structures red to orange c In some case a Metachromatic Stain is used 1 A metachromatic dye will stain differently charged structures differently Ex toluidine blue c Other Types of Light Microscopes 1 Fluorescence Microscopy a Unlike the typical light microscope the uorescent microscope uses ultraviolet light 1 The uv light will be of a wavelength so as to excite a known uorescent marker within the tissue a The fluorescent marker responds by emitting a wavelength of visible light ie fluorescence b Fluorescent microscopy is used quite frequently today 5 1 EX fluorescent tagged antibodies can be made which are specific for any protein 2 Phase Contrast Microscopy a Phase contrast microscopy uses a special pair of condenser and objective lenses to differentiate between structures based on their different reflective properties 1 This makes staining unnecessary 2 It is used for example where staining procedures may disturb or obscure cellular processes 2 Electron Microscopy a Transmission Electron Microscopy TEM 1 TEM uses a beam of electrons instead of light to illuminate the specimen a This improves the resolution and thusly the degree of magni cation which can be achieved b Also instead of using a series of magnifying lenses in TEM the electron beam is shaped by electromagnets to provide magnification 2 Staining is accomplished by the use of heavy metals such as uranyl and osmium which will obstruct the passage ofelectrons through the tissue a Different portions ofthe cells will have different affinities for these metal Ions b Due to the different resulting contrasts we see a two dimensional image of varying electron density which will be photographed 3 Since electrons on a conventional TEM do not have high penetration the tissue sections must be thinner than those used in light microscopy a Scanning Electron Microscopy SEM 1 SEM differs from TEM in that it makes visible only the surface ofthe specimen a An electron beam is bounced off ofthe specimen producing a three dimensional image which will be photographed 1 To accomplish this the specimen must be coated in precious metal such as gold silver or copper 3 The Freeze Fracture Technigue a Certain cellular structures can be best studied by freezing the cell in liquid nitrogen breaking it with a knife and then looking at the components under a microscope EX the cell membrane C The Cell 1ntroduction a The cell is the basic unit of life 1 In all vertebrates the cell is eucaryotic having a distinct nucleus 2 A cell could be considered to be a mass of protoplasm surrounded by a cell membrane a The protoplasm can be divided into 6 1 Cytoplasm which lays between the cell membrane and the nucleus 2 Nucleoplasm which lays within the nuclear membrane 2 The Cell Membrane a The cell membrane is also known as the plasmallema or plasma membrane b The cell membrane separates the cell from the external environment allowing it to maintain it39s own internal environment 1 This is essential for the cell to conduct metabolism and to maintain homeostasis cross 2 It is semipermeable in nature allowing some things but not all things to c The Structure ofthe Cell Membrane 1 The structure of the cell membrane is explained by The Fluid Mosaic Model which describes it as a phospholipid bilayer interdigitated by proteins and other lipids 2 The Phospholipid Bilayer forms the quotbackbonequot ofthe cell membrane a The phospholipid bilayer is the result of the amphipathic nature ofthe phospholipid molecule 1 The phospholipid molecule has a hydrophilic polar quotheadquot and a hydrophobic nonpolar quottailquot a The 39headsquot orientate toward the aqueous external and internal environments b As a result the quottailsquot are positioned in an nonaqueous region between the two layers ofquotheadsquot 2 The amphipathic nature ofthe phospholipid molecule will cause the cell membrane to spontaneously form and to spontaneously repair itself b The hydrophobic tail is attached to the hydrophilic head ofthe phospholipid molecule by a single covalent bond 1 This single covalent bond allows the tails to rotate and accounts for the quot uidityquot ofthe Fluid Mosaic Model 3 Other Lipids a Lipids otherthan phospholipids also interdigitate the cell membrane 1 Triglycerides serve a variety of purposes 2 Cholesterol stabilize the degree of uidity to the cell membrane 4 Membrane Proteins a Membrane proteins are the proteins ofthe cell membrane and comprise approximately 50 ofthe plasmallema b Membrane proteins serve a wide array of functions such as recognition identification and the transportation of substances across the cell membrane c The membrane proteins are divided into two classes based upon the extent to which they penetrate the phospholipid bilayer 1 Peripheral Proteins are the class of membrane proteins which DO NOT extend into the hydrophobictail portion of the phospholipid bilayer They are not embedded in the cell membrane 7 2 Integral Proteins are the class of membrane proteins which DO extend into the hydrophobictail portion ofthe phospholipid bilayer They are embedded in the cell membrane a Some integral proteins will extend through the entire bilayer This subclass of integral proteins is called the Transmembrane Proteins 1 Ex the sodiumpotassium pump 5 Carbohydrates a Carbohydrates are also associated with the cell membrane 1 Typically they are linked to proteins or lipids forming glycoproteins or glycolipids respectively b Most are located on the external surface the E face of the plasmallema c Often the membrane sugars serve as speci c receptor sites 1 Ex for hormones d Structural Specializations ofthe Cell Membrane lntercellular Junctions 1 Gap Junctions a Gap junctions are also known as nexus communicating junctions or macula communicans b These junctions also consist of a quotpassagewayquot for intercellular communication and allow for the ow of chemical substances 1 This is especially true for ions c Structure 1 The two adjacent cell membranes are within 2nm of each other 2 This 2 nm gap is bridged by proteinaceous structures called connexons 2 Desmosomes a Desmosomes are also known as zona adherens b Desmosmes are junctions holding two adjacent cells together c Structure 1 Each membrane has a protein unit a desmosome on it39s inner or P face 2 Radiating between these two desmosomes are proteinaceous laments made up of actin that serve to hold the two cells together a The desmosomes are the anchor for these connecting laments 3 There is a small gap of 5nm between the two cells at the point of the desmosomal junction 3 Tight Junctions a Tight Junctions are also known as zona occludens b Like desmosomes tight junctions serve to hold two cells together c They differ dramatically however in structure 1 The tight junction is a point where the two adjacent cell membranes actually fuse together becoming in effect one membrane at that point a This is due to the amphipathic nature of the phospholipid molecule b As a result there is no space between the two opposing cells at the 8 tight junction e The Function of the Cell Membrane 1 Although the cell membrane serves as a barrier between the cell39s external and internal environments it is not totally impermeable Instead the cell membrane is semipermeable a Semipermeability means that the cell membrane allows certain substances but not all substances to cross 2 Passive Transport is the movement of molecules across the cell membrane without the use of cellular energy Instead the energy of movement comes from the kinetic energy ofthe molecules themselves a Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration 1 There are two types a Osmosis is the diffusion of solvent molecules usually water b Dialysis is the diffusion of solute molecules b Facilitated Diffusion is diffusion requiring the cell membrane to be made permeable to the diffusing molecules due to the action of a carrier protein 1 These special membrane proteins will temporarily bind to the diffusing molecule to accelerate it39s movement intoout ofthe cell a Often the carrier protein changes the charge ofthe molecule so it can then travel along the osmotic gradient b These carrier proteins will be speci c for specific molecules 3 Active Transport a Active transport requires the expenditure of cellular energy 1 This is often due to the fact that the movement ofthe molecules is against the concentration gradient 2 In some cases ATP may also be required since the molecule would normally be unable to cross the cell membrane 3 Ex the sodiumpotassium pump 4 The Transport of Large Molecules Across the Cell Membrane a Exocytosis is the movement of large molecules out ofthe cell 1 Often these large molecules are secretory products or waste products 2 Process The substance is enclosed in an vesicle within the cytoplasm and moved to the cell membrane The vesicle will fuse to the cell membrane due to the amphipathic nature of the phospholipid molecule which releases the substance into the external environment b Endocytosis is the movement oflarge molecules into the cell There are three strategies of endocytosis used by cells 1 Phagocytosis quotcell eatingquot a Process 1 Out foldings ofthe cell membrane surround and engulfthe quotfood substancequot 9 2 The food is now surrounded by a vesicle which pinches of from the cell membrane 3 This vesicle will move deeper into the cytosol where it will become surrounded by enzyme lled vesicles lysosomes which will digest the food b Ex39 A neutrophil consuming a bacterium 2 Pinocytosis quotcell drinkingquot a The mechanism of pinocytosis is similar to that of phagocytosis except that the object to be brought into the cell is already in solution 1 So lysosomes are not required Instead the molecules can simply diffuse out of the vesicle into the cytosol 3 Receptor Mediated Endocytosis a Receptor mediated endocytosis is very similar to phagocytosis except that it requires special receptor proteins on the cell membrane to recognize the substance to be brought into the cell 1 The vesicles which form will be covered with these receptor proteins and so are called quotwhiskeredquot or quotcoatedquot vesicles a These receptor proteins are lost from the vesicle and return to the cell membrane prior to the lysosome mediated degradation ofthe object 3 The Organelles a The Nucleus 1 Introduction a The nucleus is typically the most prominent organelle in a vertebrate cell 1 It averages 5 um in diameter b It is typically spherical in shape 0 Typically vertebrate cells are uninucleated but there are some exceptions 1 Ex Erythrocytes and thrombocytes are anucleated 2 Ex Skeletal muscle cells are multinucleated 2 The nucleus is surrounded by a double membrane structure called the Nuclear Envelope aka Nuclear Membrane Nuclemma a This nuclear envelope is even more selectively permeable than is the cell membrane 1 It does allow some substances to pass through by means of openings called Nuclear Pores 3 The nucleus contains the bulk of a cell39s genetic information a This genetic material consists of DNA RNA and histone proteins b The term that is used to describe this genetic material is based on it39s degree of condensation 1 Chromosomes are the most condensed form ofthis genetic material and is observed priorto and during cell division 2 Chromatin is a relatively less condensed form of the genetic material and is it39s normally occurring state a It comes in two forms 10 1 Euchromatin extended or not very condensed chromatin It does not stain well 2 Heterochromatin very condensed chromatin It does stain well 0 Transcription will produce a copy of a particular genetic code on a chromatin strand 1 This copy is the mRNA molecule and it will exit the nucleus into the cytoplasm 4 The Nucleolus a nonmembranous intranuclear organelle ofthe nucleus responsible for the synthesis of rRNA and thusly of ribosomes b The Ribosome 1 Ribosomes are small organelles being made up of two subunits a These two subunits are composed of rRNA and protein b The two subunits come together only in the presence ofan mRNA molecule 1 They are responsible for translation the conversion of the genetic code held on the mRNA into a proteinpeptide 2 Ribosomes can exist either freely in the cytoplasm or attached to another class of organelle the endoplasmic reticulum c The Endoplasmic Reticulum ER 1 The ER is a maze of parallel interrnal membranes that serves to connect the nucleus to other structures in the cell and to the cell membrane a The ER allows for compartmentalization within the cell 1 It forms chambers called Cisternae for particular chemical reactions b The ER and it39s cisternae contain a variety ofenzymes to facilitate a variety of chemical reactions 2 The ER is divided into two types based upon the presence or absence of attached ribosomes a Rough Endoplasmic Reticulum RER 1 This ER is studded with numerous ribosomes giving it a quotroughenedquot appearance a These ribosomes covey certain peptides produced by translation into the RER for the continuation of protein synthesis 2 RER is also involved in transport b Smooth Endoplasmic Reticulum SER 1 This form of ER lacks ribosomes on it39s surface 2 SER functions include a the primary site of phospholipid sterol and triglyceride metabolism b transport 0 the detoxification of certain harmful substances 1 Ex SER is quite abundant in hepatocytes d The Golgi Organelle 1 The Golgi organelle is also known as Golgi apparatus Golgi complex or Golgi a It was first described in 1898 by Camillo Golgi 2 The Golgi organelle is a stack of membranes that packages the products of 11 the cell a Actually the Golgi is a stack of smooth membraned cisternae which receive proteins from RER for required posttranlational modi cations 1 These modi cations include a the addition of carbohydrate groups to certain proteins to form glycoproteins b the addition of fatty acid chains to certain proteins to form glyceroproteins c the packaging of the protein products into vesicles aka sacs 1 Vesicles are spheroids of membrane containing some substance 2 The modi cation and destination ofthese protein products is determined by their amino acid sequence which is determined by the genetic code 2 The protein is carried in transport vesicles and approaches the Golgi from the RER at the E side of the organelle a The protein is processed packaged and released from the Golgi at it39s m side e The Lysosome 1 Lysosomes are specialized vesicles containing digestive enzymes a There are a wide variety of enzymes found in lysosomes designed to work on different substrates such as fats carbohydrates proteins and nucleic acids 1 Presently over 40 different enzymes have been identified in the lysosomes of human cells a Most ofthese enzymes work best at a pH of 5 b These enzymes are responsible for lysosome functions within the cell 1 the digestion of food substances 2 the digestion of foreign invaders 3 the digestion ofthe cell39s own organelles to release energy during periods of quotstarvationquot 4 the digestion ofthe cell itself when it dies a which gives lysosomes the nickname quotsuicide sacsquot f The Microbody 1 Microbodies are another special group of vesicles containing a variety of enzymes for a variety of metabolic reactions 2 The most common type of microbody in mammalian cells is the perioxysome a Perioxysomes are used in the break down of fats and may also serve to detoxify substances ion the liver and kidneys 1 Perioxysomes get their name from an intermediate product ofthis break down peroxide a The peroxide is further broken down by the microbody g The Mitochondrion 1 The mitochondrion is a double membraned capsuleshaped organelle where the chemical energy present in food is converted into ATP 12 a It is the site ofthe Kreb39s cycle and oxidative phophorylation stages of aerobic respiration b Since they are energy producing organelles mitochondria are most numerous in the most active cells ex spermatozoa 2 Mitochondria contain their own DNA called mitochondrial DNA which is maternally inherited 3 The mitochondrion has an outer and an inner mitochondrial membrane a Between these two membranes is the intermembrane space b The inner mitochondrial membrane encloses a second space called the matrix c The inner mitochondrial membrane is actually larger than is the outer mitochondrial membrane and so is arranged into folds projecting into the matrix called cristae 1 The greater size ofthe inner mitochondrial membrane is to afford more surface area for oxidative phosphorylation h The Cytoskeleton 1 The shape of the cell it39s mobility and the mobility and location of it39s organelles is due to a complex matrix of protein filaments forming a framework called the quotcytoskeletonquot 2 There are three classes of cytoskeletal components based on structure and diameter size a Microfilaments 1 Microfilaments are the smallest of the three classes being 7 nm in diameter 2 They are solid protein bers composed of globular protein subunits 3 One example is actin 4 Functions a When associated with the microtubule myosin contractile structures are formed b When cross linked to other proteins it forms a meshwork that gives support to many other cellular structures 1 Ex stress bers in ct cells b Intermediate Filaments aka Tono laments 1 Tono laments are intermediate in size ranging between 8 to 10 nm in diameter 2 Intermediate laments are formed from brous proteins ie keratin and so are the more stable of the three classes a As a result they serve to help strengthen the cytoskeleton b They are also solid rods of protein c Microtubules 1 Microtubules are the largest of the three classes being 25 nm in diameter 2 Microtubules are hollow tubelike structures composed of globular proteins called globulins a The globulin proteins come in two forms alpha and and 13 arranged into pairs or dimers 1 Each dimer has one alpha and one beta globulin b Microtubules grow by the addition of dimers 3 To function microtubules need an anchoring point called the Microtubule Organizing Center or Centrosome in nondividing cells a A microtubule organizing center consists ofa pair of centrioles organized at right angles to one another 1 A centriole consists of9 groups of 3 microtubules arranged to form a hollow tube a The centriole may also function in microtubule assembly 3 Cilia and Flagella a Cilia and agella may initially look different but are structurally quite similar They are microtubular structures used in locomotion by the cell 1 Cilia are numerous and short 2 Flagella are few often singular and long b Cilia and agella both consist of a slender stalk surrounded by an extension ofthe cell membrane 1 The quotstalkquot consists of a group of microtubules arranged in a 9 2 conformation called the Axoneme a This is 9 pairs of microtubules arranged in a ring around a a central pair of microtubules 1 The microtubules have extensions called dynein arms which allow them to slide past one another by a ratcheting motion of the arms a This produces the movement of a cilium or agellum 2 At the base of the cilia or flagella is a Basal Body a The basal body is organized like a centriole b It serves as an anchor for the cilium or agellum 4 Cytoplasmic Inclusions a Unlike organelles cytoplasmic inclusions are variably present in cells 1 They occur only in specific cell types or in association with a speci c stage of functional activity b Types of Cytoplasmic Inclusions 1 Pigments a Melanin 1 Melanin is contained in membranelimited granules called Melanosomes a Melanosomes are only found in certain cells such as the melanocytes ofthe epidermis 2 Melanin determines skin hair and eye color and shields the body from ultraviolet radiation b Lipofuscin 1 Lipofuscin is a goldenbrown pigment which is an end product of 14 lysosomal activity a Lipofuscin accumulates as a cell ages b Since it is derived from lysosomes lipofuscin is contained in membranous vesicles c Hemosiderin 1 Hemosiderin is another goldenbrown pigment a It gets it39s color due to it39s high iron content 1 The high iron content is due to the fact that hemosiderin is a product of the breakdown of erythrocyte 2 Lipids a Oil droplets within the cytosol can serve a variety of functions such as 1 energy stores 2 sources of lipid for membrane turnover 3 otation devices in unicellular organisms 3 Glycogen a Glycogen is a polymer of glucose and serves as the storage form of glucose in cells 5 Cells reproduce by means of Mitosis a Mitosis however makes up only a short portion ofthe cell cycle Students are responSIbe for the cell cycle and mitosis D Tissues 1Tissues are groups of similar cells working together to perform a particular task a Tissues are divided into four classes epithelial connective muscle and nervous l A 01 A 00 0 0 Unit 2 EPITHELIAL TISSUES A General Features Epithelia are single or multiple sheets of cells where the cells are closely opposed to one another with little intercellular material All surfaces of the body excepting the joint cavities are covered or lined by epithelium a Epithelium then serves as a barrier to seal and to separate the organism from various external and internal environments b Epithelium covers all of the body39s exterior surfaces ex epidermis cornea c Epithelium lines all passageways which connect either directly or indirectly to the exterior ex digestive respiratory and urogenital tracts d Epithelium lines the blood vessels and the heart39s interior ie Endothelium e Epithelium lines all of the closed coelomic cavities eg pleural pericardial and peritoneal 1 This epithelium is called Mesothelium f Epithelium lines all invaginations ofthe body39s surface epithelia eg glands Epithelia rest on a Basement Membrane and an underlying layer of loose c t a The basement membrane is acellular Epithelial are generally avascular a Blood vessels from the underlying ct do not penetrate the epithelium 1 Instead materials diffuse between the epithelial cells and the blood The majority of the body39s glands develop as invaginations of epithelia into the underlying ct a There are two classes ofglands Exocrine and Endocrine All bodily surfaces are more or less quotactivequot with a continuous ux of materials across the epithelium in either one or two directions a Virtually everything that enters or exits the body passes through is synthesized by or is modified by the epithelium Epithelial tissues have a strong regenerative capability Epithelial tissues are quotplasticquot being able to undergo Metaplasia when local environmental conditions become chronically altered a Metaplasia is the morphological andor functional transformation from one type of tissue into another type ofthe same class Epithelia are diverse in origin a They are derived from all three ofthe primordial germ layers b This property helps to contribute to the diverse structures of epithelium 10 Epithelial tissues have a diversity of functions including a protection b lubrication c digestion d absorption e transport f excretion g sensory reception and transduction h reproduction i secretion ex sweat oil milk HCI mucus enzymes hormones bile salts urine and gametes B The Classes of Epithelia 1 Epithelial tissues are broadly divided into three superclasses which in turn are subdivided The three superclasses are a surface epithelia b glandular epithelia c special epithelia 2Surface Epithelia a The surface epithelia are distinguished and classified primarily on the basis of two criteria the number of cell layers and the heightshape of the cells 1 Also in certain epithelia the free surface ofthe outermost cells may be structurally specialized for particular functions a In such cases these surface specializations are also used to classify the epithelium b The Types of Surface Epithelia 1 Simple Epithelia simple epithelia consists of a single layer ofepithelial cells over a basement membrane a Simple Sguamous Epithelium a single sheet of attened scalelike cells 1 quotSquamousquot means scalelike and refers to the fact that these cells are much wider than they are tall 2 Simple squamous epithelium is found in areas such as a small glandular ducts b the mesothelium lining the closed coelomic cavities eg pleural pericardial and peritoneal c the endothelium lining the blood vessels heart and lymph vessels d respiratory bronchioles and alveoli e Bowman39s capsules and loops of Henle in the kidneys 3 Functionally simple squamous epithelium is well suited for sites of uid metabolite and gas exchange b Simple Cuboidal Epithelium a single sheet of hexagonshaped cells 1 These cells are called quotcuboidalquot because they appear to be cube shaped under the light microscope being of equal height and width a Cuboidal epithelial cells will typically have a centrally positioned nucleus 2 Simple cuboidal epithelium is found in areas such as a glandular termini and ducts b the parenchyma cells of the liver ie hepatocytes c rete testis d covering the free surface ofthe ovary ie germinal epithelium e certain portion ofthe renal tubules ie thick segments 3 Functionally simple cuboidal epithelium may simply line conducting passageways ex glandular ducts or may be structurally adapted to play important roles in secretion or absorption ex the proximal and distal convoluted tubules of the kidneys 0 Simple Columnar Epithelium a single sheet of polygon shaped cells 1 These cells are called quotcolumnarquot because they appear to be column shaped under the light microscope being ofa much greater height than width a Typically columnar epithelial cells will typically have a basally positioned ovoid nucleus 2 Simple columnar epithelium is found in places such as a the ducts of many glands b lining the stomach intestines and gall bladder 0 some of the small respiratory passageways d portions of the oviducts and uterus 3 Functionally simple columnar epithelium is well designed for absorption ex intestinal epithelium and secretion ex uterine secretory cells a In some cases simple columanr epithelial cells will have cilia on their apical surfaces d Pseudostrati ed Columnar Epithelium a single sheet of irregularly shaped cells giving the appearance of more than one cell layer 1 Pseudostrati ed epithelium is not truly strati ed a It only appears to be since 1 not all ofthe cells reach the free surface of the tissue 2 the nuclei of the cells are on two orthree levels 3 the cells appear to be crowded due to their varying shapes b However if observed closely all of the cells are in contact with the basement membrane and so are only one layer 2 Pseudostrati ed columnar epithelium can be found in places such as a most ofthe respiratory passageways b eustachian tube and portions of the middle ear 0 portions of the male urethra d portions of the male accessory sex organs 3 Functionally pseudostratified columnar epithelium is designed for lining secretory and absorptive roles a The pseudostrati ed columnar epithelium lining the respiratory tract are ciliated and the tissue will contain mucus producing goblet cells 18 1 So this epithelium is called pseudostratified ciliated columnar epithelium with goblet cells 2 The cilia will move substances across the surface of the tissue adding to it39s functionality b The pseudostrati ed columnar epithelium ofthe ductus epididymides and ofthe ductus defrens possess numerous elongated microvillae called stereocilia and so the tissue is termed pseudostratified columnar epithelium with stereocilia 2 Strati ed Epithelia consists of two or more sheets of epithelial cells with only the basal layer being in contact with the basement membrane This is largely a protective group guarding the body from wear and stress a Strati ed Sguamous Epithelium this multilayered tissue39s outermost cells have a attened squamous appearance 1 The deeper cells may be less squamous in appearance being cuboidal or even columnar in shape in certain cases but classi cation is based on the outermost layer in strati ed epithelia 2 There are two groups of strati ed squamous epithelia a Strati ed Sguamous Nonkeratinized or Mucus Type 1 The surface cells contain visible nuclei and lack keratin 2 It is found on the surfaces of moist cavities or passageways which open onto the body surface such as the mouth pharynx esophagus vagina and anal canal b Strati ed Sguamous Keratinized or Cutaneous Type 1 The surface cells are dead being anucleated and lled with keratin 2 It covers the entire exposed surface ofthe body except for the cornea a It is well designed to deal with abrasion and dessication b Strati ed Cuboidal Epithelium multiple layers of epithelial cells where at least the surface cells are cuboidal 1 Strati ed cuboidal epithelium is oflimited distribution throughout the body a Where it is found it is typically only two layers thick b It is found in areas such as larger glandular ducts c Strati ed Columnar Epithelium multiple layers of epithelial cells where at least the surface cells are columnar the cells ofthe deeper layers are typically cuboidal in shape 1 Strati ed columnar epithelium is of limited distribution throughout the body a Where it is found it is typically only two layers thick b It lines small portions ofthe pharynx larynx the largest glandular ducts and portions of the male urethra 1 It also occurs in certain regions of transition between two two different types of epithelia d Transitional Epithelium multiple layers of variable appearing cells The 19 appearance of the cells is based upon the tissue39s location and action 1 It is a exible layer which can expand and contract which is responsible for the variable appearance ofthe cells a The surface cells are typically domeshaped while the underlying cells may be columnar cuboidal or even squamous in appearance 1 Occasionally two nuclei per surface cell is observed b Transitional epithelium is speci cally adapted for exibility and for stretching 2 The distribution of transitional epithelium is limited primarily to the urinary system It lines the renal pelvis ureters bladder and portions of the urethra based on gender 3 Glandular Epithelia a Single cells or groups of cells designed for secretion are defined as Glands 1 The products of glands may either be secreted into ducts in the case ofthe Exocrine Glands or into the blood in the case of Endocrine Glands a Not all endocrine glands are ofepithelial origin and so will be studied in their respective organ systems b Exocrine Glands 1 Exocrine glands can be classified based on four factors a cell number ie multicellular vs unicellular b duct system ie simple vs compound and secretory portion ie tubular acinar ortubuloacinar 0 nature of secretion ie mucus serous or seromucus d mode of secretion ie merocrine apocrine or holocrine 2 Types of Exocrine Glands a Types of Exocrine Glands Based on Cell Number 1 Unicellular Glands are composed ofa single secretory cell interposed in an epithelium of cells having other functions ex a goblet cell 2 Multicellular Glands a multicellular gland is composed of many cells and will vary in complexity occurring as a Epithelial Sheet Glands ex surface mucus glands ofthe stomach b lntraepithelial Glands ex urethral mucus glands 0 Complex Glands with Ducts by far the most numerous these glands are distinguished by their duct systems and by their secretory portion b Types of Exocrine Glands Based on Duct System and Secretorv Portion 1 General Comments a Based on duct system glands can be considered to be 1 Simple if the duct is unbranched 2 Compound if the duct branches b Based on secretory portion glands can be considered to be 1 Tubular if the secretory portion is quottest tubequot shaped 2 AlveolarAcinar if the secretory portion is quot askquot shaped 20 3 TubuloacinarTubuloalveolar is the secretory portions ofthe gland are of both types 2 Classifications a Simple Glands these are glands whose ducts are unbranching 1 Simple Tubular Glands ex crypts of Leiberkuhn ofthe small intestine 2 Simple Coiled Tubular ex eccrine sweat glands 3 Simple Branched Tubular ex Brunner39s glands fundic glands 4 Simple AcinarSimple Alveolar a These glands are not found in humans or possibly in mammals at all b Ex poison glands in certain amphibians 5 Simple Branched AcinarAlveolar Glands ex sebaceous glands b Compound Glands these are glands whose ducts display definite branching 1 Compound Tubular Glands ex cardiac glands of the stomach 2 Compound AcinarAlveolar ex salivary glands 3 Compound TubuloacinarTubuloalveolar ex pancreas 0 Types of Exocrine Glands Based on The Nature of the Secretion 1 Mucus Glands secrete the glycoprotein mucin which forms mucus when combined with water a Due to the high quantity ofmucinogen present in these cells mucus glands stain poorly 2 Serous Glands secrete serous uid a watery clear uid containing a proteinaceous component 3 Seromucus or Mixed Glands have both a serous and a mucus component to their secretion a Both types of secretory units occur as either 1 Mixed Alveoli 2 Demilunes mucus units with serous quotcapsquot having a half moon appearance ex submandibular gland d Types of Exocrine Glands Based on the Mode of Secretion 1 Merocrine Glands release their secretory product by typical exocytotic techniques a The entire cell remains intact only the product is released b Merocrine secretion is typical for glands involved in protein or mucus release 0 Ex exocrine pancreas salivary glands 2 Apocrine Glands release their products with the loss of a portion of the cell a The product is released from the apical surface of the cell along with either a thin rim of cytoplasm or a larger cap of lumenal 21 cytoplasm b EX certain sweat glands ceruminous glands and the lipid secretions ofthe mammaries 3 Holocrine Glands the release oftheir products involves the destruction ofthe entire cell a The secretory product and the cell are released into the glandular lamina 1 The cell is in effect a portion of the secretory product b Accumulation of the product results in organelle disintegration and the ultimate death ofthe cell c EX sebaceous glands d Some refer to the testes and the ovaries as highly specialized holocrine glands due to the nature oftheir exocrine product the gametes 3 Myoepithelial Cells a Myoepithelial cells are also known as basal cells or basket cells b Myoepithelial cells are specialized epithelial cells which are contractile in nature 1 They are located between the basal portions of secretory cells and the basement membrane of certain glands a EX sweat salivary mammary lacrimal and ceruminous glands 2 They help to squeeze the glandular product out of the secretory unit a Because they are contractile myoepithelial cells are described as being quotmuscle likequot Myoid 1 Like muscle cells myoepithelial cells contain myo brils 2 Unlike muscle cells however they are of ectodermal origin 4 Special Epithelia a In addition to the aforementioned epithelia a few epithelia possess unique structures functions and special properties These are the quotspecial epitheliaquot 1 The most notable of these are concerned with sensory reception neuroepithelia and with reproduction germinal epithelium lining the seminiferous tubules b The surface epithelia will be considered under the appropriate topics C Epithelial Specializations 1 Introduction a As a class epithelial tissues is the most diverse in function 1 This is partially due to it39s origin from all three germ layers 2 Related to this property is the multiformity of the various epithelial membranes as well as their internal cellular features b Cytological specializations of the epithelial cell may occur at the various interfacial areas of 1 apical cell surface between the cell and the surface environment 2 lateral cell surfaces between the cell and adjacent cells 3 basal cell surface between the cell and the underlying basement membrane c In addition there may be specializations of the cytoskeleton 2 Specializations ofthe Apical Surface a Microvillae 1 Microvillae are delicate fingerlike extensions ofthe cell39s apical surface formed by multiple evaginations of the plasmallema and cytoplasm a Microvillae contain a core of fine 6 nm micro laments running along their longitudinal axis 1 This core of micro laments is called the Actin Bundle a The actin bundle is attached to the apical tip ofthe microvilla in a dense structure called the Dense p bThe basal portion ofthe actin bundle is embedded in and interconnected with the cytoskeleton of the cell 1 The meeting point is called the Terminal Web 2 Microvillae increase a cell39s surface area and are found in epithelial cells concerned with secretion and absorption a Ex The quotstriated borderquot of intestinal absorptive cells the quotbrush borderquot of the kidney39s proximal tubules and the stereocilia of the ductus defrens 1 Note stereocilia are much longer and more specialized than are the typical microvillae b 1 Like microvillae cilia are plasmallema covered evaginations ofthe apical cell surface 2 Cilia are motile structures a Beating ofthe cilia consists ofa rapid fonNard motion called the Effective Stroke and a slower backward motion called the Recovem Stroke 1 This metachronal rhythm provides the means by which mucus and particulate matter is moved along the epithelial surface a This action is termed Mucociliam Clearance b Ex respiratory epithelium c Glycocalyx 1 The glycocalyx is a surface coat consisting of complex carbohydrates in 23 association with either structural proteins embedded in the cell membrane or with proteins located on the surface ofthe cell membrane a Although this glycoprotein encrustation occurs on all surfaces ofepithelial cells it is most prominent on the apical surface 2 Functions a cellular recognition b adhesion and binding of various molecules c concentrate certain ions for absorption by the cell d chemical reactions 1 EX the enteric surface coat of intestinal absorptive cells contain hydrolytic enzymes to aid in digestion 3 Specializations ofthe Lateral Surface a As mentioned previously epithelial cells have many and well developed intercellularjunctions 1 tight junctions or zona occludens 2 desmosomes or macula adherens 3 zonula adherens an intermediate junction also serving to hold epithelial cells together a Like the desmosome there is some space between the two cells however it is structurally more similar to the tight junction 4 gap junctions or macula communicans 4 Specializations ofthe Basal Surface a The basal surface specializations include the basement membrane and the hemidesmosome both of which serve to bind the epithelium to the underlying connective tissue b Basement Membrane 1 The basement membrane is also sometimes referred to as the basal lamina 2 The basement membrane is an acellular structure composed of two layers a Basal Lamina Proper the layer closest to the epithelium 1 It is composed ofglycoprotiens 2 It is produced by the epithelium itself b Reticular Lamina the layer closest to the underlying loose ct 1 It is composed of reticular fibers embedded in a matrix of glycoprotiens and polysaccharides 2 It is produced by the loose ct 3 The basement membrane serves as an underlying support and cushion for the epithelium and to attach the epithelium to the underlying ct a In special cases it may also act as a ltration barrier ex in the kidney c Hemidesmosome 1 As the name indicates this is in essence halfa desmosome 2 The hemidesmosome serves to attach the epithelial cells to the basement membrane 5 anniali atinn ofthe Cy 39 39 L I a Epithelial cells especially squamous type possess many intermediate filaments b Terminal Web a dense accumulation of laments running parallel to and immediately below the apical surface of the cell 1 The terminal web is particularly prevalent in cells having apical appendages a It serves to provide support and anchorage for apical appendages Unit 3 CONNECTIVE TISSUES A Introduction 1 Connective tissues are a diverse class oftissues including bone blood cartilage adipose and areolar ct a The types of connective tissues are classi ed based on the relative proportions of three components ground substance bers and lls 1 The ground substance and fibers are the extracellular components of the ct and make up the Matrix B The Components of the Connective Tissue Proper 1The Ground Substance of the Matrix a The ground substance functions as a molecular sieve 1 It facilitates the diffusion of metabolites between the blood and the tissues 2 It acts as a physical barrier to prevent the spread of large particles such as bacteria and other pathogens b Ground substance is an amorphous viscous gel 1 It is produced primarily by a connective tissue cell called the Fibroblast a Fibroblasts are derived from embryonic mesenchyme as are most ct cells c Components of Ground Substance 1 water 2 salts a such as calcium phosphate 3 glycosaminoglycans GAGs a The particular type of GAGs present in a particular ct vary in occurrence and proportion Some types are 1 Hyaluronic Acid is the most common and largest type 2 Sulfated Proteoglycans is a class of GAG consisting ofa sulfated gycosaminoglycan chain covalently bound to a core protein a There are four major classes 1 chondroitin sulfate 2 dermatin sulfate 3 keratin sulfate 4 heparin sulfate b GAGs have several functions 1 They contribute to the viscosity of ground substance a This acts as a barrier to the spread of pathogens following tissue 26 injuries 2 They provide structural support to cts 3 They act as a medium for the diffusion of nutrient and gases 4 They exert a direct in uence on the surrounding cells a This is especially true during the development of cells 1 A hyauronic rich matrix is especially suitable for cell migration and proliferation It may also prevent precocious differentiation b Speci c interactions between the surface receptors of cells and GAGs may play a role in the regulation of cellular functions 4 structural proteins a Structural proteins serve to bind the ct cells to the surrounding collagen bers of the matrix b Some structural proteins are 1 Fibronectin can be considered to be a cell matrix ligand since it promotes the attachment of ct cells especially fibroblasts to collagen bers a It also plays a role in other processes such as cell migration cell differentiation phagocytosis and chemotaxis b It also directly in uences the cytoskeletal organization of ct cells which can change their shapes andor their functions 2 Laminin is associated with the basement membrane where it is a specific attachment protein for epithelial cells to the type IV collagen of the basement membrane 3 Chondronectin promotes the attachment of cartilage cells to collagen 2 The Fibrous Component of the Matrix a General Comments 1 Connective tissue bers provide general support for other tissues a Ex ct fibers form a dense supporting framework in the integument 2 ln hollow organs and blood vessels which expand and contract ct bers allow for exibility 3 Three types of fibers are found in cts collagen reticular and elastic bers a The density proportion arrangement and occurrence of each type vary depending on the functional requirement ofthe tissue b The Three Classes of Connective Tissue Fibers 1 Collagen Fibers a Collagen fibers are the most abundant and strongest of the three classes 1 Collagen is found in all types of ct in varying degrees 2 Collagen has a tensile strength greater than does a similar sized strand of steel a 80 it adds strength to the tissue and will decrease the effects of mechanical forces on the tissue b Collagen is a protein polymer composed of monomeric units of the protein Tropocollagen 1 Tropocollagen is produced by the broblasts typically a Although it is made by osteoblasts in bone and by chondroblasts in 27 cartilage 2 Collagen gets it39s strength from it39s structural arrangement a Collagen is arranged into micro brils b Microfibrils are arranged into fibrils c Fibrils are grouped into a ber d Fibers are grouped into a collagen bundle c There are ve identi ed molecular types of collagen 1 Type Collagen is the most abundant and widespread type 2 Type II Collagen is found in cartilage and in certain tissues of the eye 3 Type III Collagen is found in the ct components ofthe skin GI tract cardiovascular system and uterus a Type III collagen is often found in association with reticular fibers 4 Type IV Collagen is found in the basement membrane 5 Type V Collagen is found primarily in fetal tissues although vestigial amounts remain in the adult 2 Reticular Fibers a Reticular bers are actually thin collagen fibers arranged into delicate networks not into bundles 1 They form a delicate supporting network around individual cells of many tissues and organs a EX they constitute the inner stroma of lymphoid and hemopoietic organs 2 Reticular bers also are a significant portion of the reticular lamina of the basement membrane 3 Elastic Fibers a Elastic fibers are composed of the protein Elastin 1 Elastin gives these bers the ability to stretch a However elastic fibers are not as strong as are collagen bers b Elastic fibers are fond in most brous ct but are most abundant in those tissues requiring exibility 1 EX trachea elastic arteries skin and uterus 3 The Cells of the Connective Tissue Proper a General Comments 1 The cell types and their relative densities are highly variable and depend largely on fiber density location and functional state 2 The cells ofthe ct proper can be divided into two groups permanent and transient cells a Permanent cells can be thought of as being involved with the long term maintenance of the ct 1 They include mesenchymal cell broblasts and adipocytes b Transient cells can be thought as being involved with short term events such as a reaction to an injury orto an invasion by pathogens 1 They include macrophages mast cells plasma cells and cells that invade from the blood stream such as neutrophils eosinophils monocytes and lymphocytes These last four W7 be studied under the 28 topic of the circulatey system b Permanent Cells 1 Fibroblasts and Fibrocytes a Fibroblasts are immature ct cells which will mature into brocytes 1 As is typical for immature connective tissue cells broblasts are responsible for producing at least some of the organic component of the matrix a They produce all three ber types as well as the ground substance 2 Fibroblasts are widely distributed cells a Their appearance may vary but overall they are long tapered cells with thin at nuclei b They are often found in association with collagen bundles b Fibrocytes are mature broblasts 1 They are less active than are fibroblasts a They may help to maintain the ct by aiding in it39s repair 2 They are similar in appearance to broblasts a One difference is that they have fewer cytoplasmic granules 2 Mesenchymal Cells aka pericytes perivascular cells a Mesenchymal cells are less widely distributed than are broblasts 1 They are often found in association with blood vessels hence the names pericytes and perivascular cells b Mesenchymal cells are not very differentiated 1 They retain the multipolarity of embryonic mesenchyme cells which can develop into other ct cell types during gestation 2 As a result mesenchymal cells can develop into other cell types under the appropriate conditions a They can develop into a variety of ct cell types including adipocytes and mast cells and can even develop into smooth muscle cells c Mesenchymal cells are similar in appearance to fibroblast but they can easily be distinguished by their stellate shape 3 Adipocytes a Adipocytes are specialized for the synthesis and storage of lipid 1 They may occur singly but more often in groups throughout loose connective tissues a Loose connective tissues having a high density of adipocytes are called Adipose Tissue b Adipocytes will alternately store fat and become depleted 1 Lipid deposition and mobilization is controlled by neuroendocrine secretions and by the organism39s nutritional state 2 The pattern of lipid is storage differs between the two classes of adipose tissue a In white fat tissue the lipid is stored in a single large droplet described as Unilocular 1 This gives the adipocyte it39s classic quotsignet ringquot appearance 29 when full b In brown fat tissue the lipid is stored in several smaller vesicles 1 This pattern is termed Multilocular 2 Brown fat is a specialized type of adipose tissue a It is thermogenic in that it can convert stored energy in the lipid into heat b Brown fat is found in some mammal species especially in hibernating species 1 It is found in humans only in the fetus and the neonate c The size of adipose tissue reflects the number of adipocytes present and their size 1 Adipose tissue demonstrates two patterns ofgrowth a Hyperplastic Growth adipocyte precursors proliferate for a limited period postnatally b Hypertrophic Growth change in the size of adipocytes due to lipid accumulation 1 Hypertrophic growth and it39s reversal can occur throughout an individual39s lifetime 2 The result is that both the size and the number of adipocytes can be in uenced during the postnatal period but in uences occurring later in life affect only cell size c Some ofthe Transient Cells 1 Mast Cells a Mast cells are widely distributed throughout the connective tissues 1 They are usually found in close association with blood vessels 2 They are particularly numerous in sites close to the outside ofthe body ie the dermis b Mast cells are very similar in appearance to basophils and share certain similar functions and products with these white blood cells 1 This is the reason why it was once incorrectly believed that mast cells were derived from basophils which had migrated into the ct from the blood stream c Mast cells are involved in the immune response mechanism ofthe body 1 ln particularthey play a role in the inflammation reaction and produce a variety of chemicals which promote it a histamine to increase vascular permeability b eosinophil chemotaXic factor to attract eosinophils which will phagocytosize certain pathogens and trigger the reverse of the in ammation response c prostaglandins to increase edema d heparin which serves as an anticoagulant e and a number of enzymes which degrade various ct components 2 Lymphocytes a Lymphocytes are found in small numbers throughout the connective tissues where they perform much oftheir immunological functions 30 1 They are generally more abundant in the specialized loose connective tissues ofthe alimentary canal and respiratory tract called the Lamina Propria a In these passageways there is contact with the external environment and so a greater need for protection ofthe underlying tissues from pathogenic invasion b Lymphocytes will commonly penetrate the epithelium to perform their functions We Will see more of this under the lymphoid system c Lymphocytes of the connective tissues are similar in appearance to what they look like in the blood stream a cell having a prominent nucleus and little basophilic cytoplasm around it 1 It is very dif cult to differentiate between B and T lymphocytes under the light microscope 3 Plasma Cells a Plasma cells are actually antibody secreting B lymphocytes b Plasma cells are widely dispersed throughout the connective tissues 1 They are particularly abundant in the lamina propria ofthe alimentary canal and respiratory passageways a They are also found in the lymphoid organs b They are rarely found in the blood 0 Plasma cells are unlike other lymphocytes in appearance 1 They look more like basophilically staining broblasts being at tapered cells a The plasma cell nucleus is pinwheel shaped 4 Macrophages a Macrophages are longlived highly mobile phagocytic cells found throughout the cts of the body 1 Macrophages are derived from the monocyte class of leucocyte which have migrated into the ct from circulation b Macrophages move about by means of a special type of ameboid movement called Diapedesis 1 This allows them to travel to the site ofthe infection 0 Macrophages are similar in appearance to mast cells in the both possess prominent cytoplasmic granules 1 They can be differentiated by staining and when present by pseudopodia which are found only in macrophages 2 The cytoplasmic granules of the macrophage contain a variety of substances a Some contain digestive enzymes to facilitate phagocytosis 1 These include lysosomes b Some produce immunoregulatory substances as well as a vast array of unrelated substances such as interferon lysozyme prostaglandins peroxide and a variety ofgrowth factors C The Classification ofthe Connective Tissues 1The Embrvonic Connective Tissues a Mesenchymal Connective Tissue 1 Mesenchyme fills the spaces between developing organs and is largely composed of mesenchymal cells and a fluid ground substance a Mesenchymal cells are stellate shaped and multipotent being able to give rise to all ofthe other connective tissue cell types 2 Mesenchyme will give rise to all of the adult connective tissue types a As development proceeds the formation of various bers the formation of various cell types and the formation of a relationship between fibers and cell types eventually yields an adult connective tissue b Mucus Connective Tissue 1 Mucus connective tissue is similar to mesenchymal connective tissue except that it contains collagen fibers and a more viscous ground substance a It develops from mesenchyme 2 Mucus connective tissue is typified by Wharton39s Jelly ofthe umbilical cord 2 The Mature or Adult Tvpe Connective Tissues a Mature connective tissues are classified based on 1 relative density of bers ie loose vs dense cts 2 predominant cell type ex adipose tissue 3 predominant fiber type if that is other than collagen ex elastic ct b The Loose Connective Tissues have a high cell content and a relatively low ber content 1 Areolar Connective Tissue a Areolar connective tissue is widely distributed throughout the body 1 It is the most common type ofloose ct by far 2 It is the class of ct supporting the epithelia 3 It composes the stroma ofvarious organs and glands a In certain organs and glands the areolar ct will also form septa which will divide the structure into lobes andor lobules 4 It makes up some of the super cial fascia and dermis b Functions 1 acts as a barrier to infection up to a point 2 cushions the body up to a point 3 allows for the passage of nerves blood vessels and lymph vessels 2 Adipose Tissue a Adipose tissue is a loose ct with adipocytes as the predominant cell type b Adipose is found in a variety of places such as 1 the hypodermis 2 surrounding and protecting certain organs 3 the medullary cavity of long bones 0 Functions 1 stores energy 2 insulates the body form heat loss 32 3 cushions the body and protects delicate organs ex the kidney from mechanical trauma 3 Reticular Connective Tissue a Reticular is a loose ct where reticular fibers are the predominant ber type 1 The reticular fibers form a delicate supportive framework 2 Associated with these reticular fibers are Reticular CellsReticulocytes which maintain the bers and can also perform phagocytic functions b Reticular tissue is found in areas such as 1 the inner stroma of many solid organs 2 the inner stroma ofthe lymphoid organs 3 the inner stroma of hemopoietic tissues such as red marrow c The Dense Connective Tissues have a high ber content and a relatively low cell content 1 lrreqular Dense Dense lrreqular Connective Tissue a This class of dense connective tissue has a random weave of bers and very few cells 1 Collagen makes up the vast bulk or often all ofthe fibers 2 Fibroblasts and brocytes are the most common cell types b Dense irregular connective tissue is found in areas such as the sheaths of tendons and nerves the capsules of organs and the dermis 1 It is found in areas subject to stress from a number of directions or that need a protective barrier 2 Regular Dense Dense Regular Connective Tissue a This class of dense connective tissue has a regular repeating pattern of bers and very few cells 1 Collagen also makes up the vast bulk or often all of the bers 2 Fibroblasts and brocytes also are the most common cell types b Dense regular connective tissue is a flexible tissue with a great resistance to mechanical forces 1 The parallel arrangement of it39s bers gives a great deal of strength in one direction a 80 it is found in structures that must be able to resist a great deal of stress but primarily from one direction such as tendons and ligaments 3 Elastic Connective Tissue a Elastic tissue is a dense connective tissue where the predominant ber type is the elastic ber b It is found in areas that must be able to deal with a high degree of mechanical stress but also be highly resilient 1 EX elastic cartilage and elastic arteries d The Special Connective Tissues these are the connective tissues which are the most modi ed from the mesenchymal format We W7 discuss these under the next unit topic 1 Cartilage 2 Bone 3 Blood Unit 4 THE SPECIAL CONNECTIVE TISSUES A Cartilage 1 Introduction a Cartilage is a highly resilient ct that provides strength and support in areas of the body requiring a certain degree of flexibility b Cartilage has two types of cells 1 Chondrocytes are the mature cells of the cartilage a Chondrocytes mature from chondroblasts 2 Chondroblasts are the immature cartilage cells and are responsible for the production of bers and ground substance a During cartilage development this extracellular material combines with water to form a hydrated amorphous gel termed the Matrix 1 It is the matrix which gives cartilage it39s properties of resiliency weight bearing capability and high tensile strength 0 All types of cartilage develop either directly or indirectly from mesenchyme in a process called Chondrogenesis 1 During early chondrogenesis newly proliferating chondroblasts develop and will form cartilage a After the chondroblasts have secreted enough matrix components they will be completely surrounded by the matrix and become chondrocytes 1 The space in the matrix occupied by the chondrocyte is termed the Lacuna quotlakequot and also contains interstitial uid a NOTE During histo prep the chondrocytes will often shrink due to dehydration making the lacunae more pronounced b Since the chondroblasts produce fibers and ground substance it is logical that they show extensive RER well developed Golgi and numerous vesicles containing matrix materials 2 Developmentally cartilage arises from mesenchyme in two patterns Appositional Growth and Interstitial Growth a Appositional Growth 1 ln appositional growth cells in the perichondrium will differentiate into chondroblasts which will secrete new matrix material a This new material is apposed to the preexisting cartilage and will add to it39s mass b The chondroblasts will eventually develop into chondrocytes c The perichondrium is a membranous structure forming the outer covering of cartilage organs 1 It has two layers a The outer layer is a brous layer of dense irregular ct which gives strength and support to the cartilage organ 35 b The inner layer is a cellular layer made up ofa single layer of cells having chondrogenic potential b Interstitial Growth 1 ln interstitial growth chondrocytes within the cartilage undergo mitosis producing new chondroblasts which will secrete new matrix material a This process increase cartilage from within the cartilage organ b These chondroblasts will also develop into chondrocytes 1 Daughter cells from mitotic divisions tend to remain near one another forming small clusters termed lsogenic Groups a The cells within an isogenic group are all the daughters of the same original chondrocyte d The Nature of Cartilage Matrix 1 The components of cartilage matrix include a high component of bers and proteoglycans a Proteoglycans are a class of glycosaminoglycans 1 The proteoglycan chondroitin sulfate is particularly prevalent 2 Based on the intensity of staining during histo prep there are two regions of matrix recognized a Territorial Matrix an extremely basophilic region which immediately surrounds the chondrocytes 1 The greater degree of basophilic staining demonstrated by the territorial matrix may be due to a higher GAG content b lnterterritorial Matrix a weakly basophilic region located between cells and groups of cells 3 Due to the density of the matrix cartilage lacks nerves and blood vessels a The chondrocytes are nourished by the diffusion of oxygen and nutrients through the matrix from the blood vessels in the perichondrium 2 Types of Cartilage cartilage classi cation is based on the amount of fibers and ground substance present as well as the types of bers present a Hyaline Cartilage 1 Hyaline cartilage is the most abundant and widespread class of cartilage a It is the quotcharacteristicquot cartilage b Hyaline cartilage forms the bulk ofthe embryonic skeleton c Hyaline cartilage is found in the adult in nasal septum larynx trachea bronchi articular surfaces ofjoints and the costal cartilages 2 Functionally hyaline cartilage provides exible support and weight bearing surfaces in joints a In addition it forms the framework ofthe developing embryo b It also provides a mechanism for bone growth in terms oflengthening 3 The Nature of Hyaline Cartilage a The chondroblasts develop directly from mesenchyme b The matrix is clear quothyalinequot in appearance 1 The ground substance is abundant and contains delicate type II collagen brils a The collagen does NOT form bundles and so are not as visible 36 2 The matrix also contains the proteoglycans chondroitin sulfate and keratin sulfate 3 As one ages calcium begins to in ltrate the matrix increasing it39s rigidity and thus progressively reducing the cartilage39s ability for interstitial growth a Further growth can only be appositional b Elastic Cartilage 1 Elastic cartilage is considered to be a modification of hyaline cartilage a However it39s chondrocytes are derived from broblasts rather than directly from the mesenchyme 2 Elastic cartilage is found in the external ear auditory tubes eustachian tubes epiglottis corniculate cartilages and cuneiform cartilages 3 Functionally elastic cartilage provides an extremely exible support 4 The Nature of Elastic Cartilage a The ground substance of elastic cartilage is sparse and extensively in ltrated with elastic fibers which are randomly arranged c Fibrocartilage 1 Fibrocartilage is a combination of hyaline cartilage and dense regular connective tissue in both appearance and function a The Nature of Fibrocartilage 1 The chondrocytes of brocartilage arise form broblasts 2 The ground substance is extremely sparse and heavily in ltrated with dense collagen bers arranged into parallel bundles 3 The chondrocytes are arranged into rows between the collagen bundles b Functionally brocartilage combines the stress bearing properties of cartilage and tendonsligaments for a rm but not a rigid support 1 It also allows forthe repair of all types of cartilage a The repair of cartilage occurs by the formation of fibrocartilage b The ability to repair our cartilages reduces as we age 2 Fibrocartilage is found in the intervertebral discs pubic symphysis and certain tendinous insertions ie the border between hyaline cartilage and dense regular connective tissue B Bone 1 Bone can be considered to consist of quotbone tissuequot and quotbone organsquot a Bone tissue is the mineralized supportive cconnective tissue forming the framework of bone organs b Bone organs provide the supporting framework ofthe body 2 Bone Organs or Bones a As organs bones consist of bone tissue external and internal connective tissue investments tendinous insertions ligamentous attachments blood vessels nerves and bone marrow which consists ofeither bone forming elements or 37 adipose deposits 1 In certain bone organs there are additional specializations for articulation b Bone tissue consists of cells fibers and ground substance as does any ct 1 However in bone tissue the matrix becomes mineralized with inorganic salts giving bone it39s rigidity 2 Morphologically bone tissue is organized in two ways a The two ways are 1 Compact Bone when the tissue forms a compact solid mass with relatively few intervening spaces 2 Spongy Bone aka trabecular bone cancellous bone when the tissue forms a three dimensional network of intercommunicating osseous projections termed quottrabeculaequot b General distribution in a bone organ forthese two morphologies of bone tissue 1 Compact bone forms an outer protective shell for bone organs a This is called Cortical Bone 2 Spongy bone forms a branching internal framework a This is called Medullam Bone c Organization ofthe Bone Organ ex the femur 1 Bone Tissues a Compact bone forms the cortex ofthe epiphyses and diaphysis b Spongy bone forms an internal supportive framework 1 In the case ofthe femur a long bone it is located within the epiphyses a The hollow core ofthe diaphysis called the Medullam Space is in direct communication with the cancellous spaces in each opposing epiphysis 2 Bone Marrows a Bone marrow occupies the intertrabecular spaces and the medullary cavity b In adults there are two distinct types of bone marrow 1 Red Marrow hemopoietic tissue located in the intertrabecular spaces that will give rise to blood cells throughout one39s lifetime 2 Yellow Marrow an inactive fat infiltrated tissue found in the medullary cavities of long bones 3 Connective Tissue Membranes of Bone a Periosteum a layer of specialized dense connective tissue which covers the external aspect of the bone organ It covers the cortical bone 1 Like the perichondrium the periosteum has an outer layer ofa brous nature and an inner layer of a cellular nature a This inner layer is made up of cells having an osteogenic potential 1 It will allow for the remodeling and repair of bone 2 Tendons and ligaments attach to the bone by way of the periosteum 3 The periosteum attaches to the cortical bone by means of Sharpey39s Fibers aka Perforating Fibers a Sharpey39s bers are dense collagen bundles running from the 38 brous layer of the periosteum into the mineralized bone matrix b Endosteum a layer of attened cells having osteogenic potential lining the inner aspects of the bone organ ie medullary cavity intertrabecular spaces bone canals 4 Blood Vessels Servicing the Bone a In long bones the vascular supply is from vessels that penetrate the diaphysis and marrow through the Nutrient ForamenNutrient Canal 1 These blood vessels arise from vessels of the periosteum 2 Nerves will often accompany the blood vessels b V thin the bone tissue we see the Haversian systems 5 Enclosed in fluidfilled joint cavities the articular surfaces of bone organs are covered by a cap of hyaline cartilage called Articular Cartilage 3 Bone Tissue a General Comments 1 Bone tissue is a very specialized hard connective tissue of both complex structure and function 2 In some ways bone is similar to cartilage in that it consists of cells and an intercellular matrix of fibers and ground substance a However certain cells induce the mineralization of bone tissue giving it a hardness and strength beyond that of cartilage b Also compared to cartilage bone tissue has a high degree of structural organization in it39s matrix 1 This is especially true in regards to the packaging of it39s collagen bers c These two features allow bone to withstand and recover from tension and compression and to perform it39s mechanical role with a minimum of weight and material 3 Bone is a metabolically active tissue being continuously remodeled throughout life and being capable of repair a This is accomplished by the resorption of old bone and the deposition of new bone b Bone responds to mechanical stresses by reorganizing bone tissue to bear weight more efficiently 4 Bone also serves as a reservoir for minerals especially calcium and phosphate b Constituents ofthe Bone Tissue Matrix 1 Bone matrix is a mineralized organic matrix a The mineral component of bone matrix is termed Hydroxyapatite 1 Hydroxyapatites constitute 65 of bone matrix volume and accounts for the rigidity and hardness of bone 2 The mineral component also serves as a reservoir for minerals especially calcium 3 Although the matrix of bone primarily contains the minerals calcium phosphate carbonate and citrate other minerals such as uorine sodium and magnesium also occur 39 a The principle bone mineral is calcium phosphate b In some cases small amounts of harmful radioactive minerals which occur in the environment can become incorporated into the bone b The hydrated organic component bone matrix is termed the Osteoid 1 The osteoid constitutes 35 of bone matrix volume a 95 of the osteoid is collagen b 5 ofthe osteoid is the amorphous ground substance consisting of proteoglycans such as chondroitin4 sulfate and keratin sulfate c The Cellular Component of Bone Tissue 1 Osteogenic Cells aka osteoprogenitor cells are pleuripotent stem cells derived from mesenchymal cells which can develop into either chondroblasts or osteoblasts a They maintain their pleuripotent capability throughout life so as to allow for bone repair and remodeling b In the adult osteogenic cells are located in the cellular layer of the periosteum and in the endosteum c Appearance Under the light microscope osteogenic cells appear as at inconspicuous cells with a pale staining elongate nucleus and a sparse eosinophilic cytoplasm 2 Osteoblasts are immature bone cells derived from osteogenic cells which will develop into osteocytes a Osteoblasts secrete the osteoid and participate indirectly in the subsequent calci cation of the matrix 1 Once they have become isolated in the matrix which they have deposited osteoblasts will undergo morphological changes and develop into osteocytes b In the adult osteoblasts are located in the cellular layer ofthe periosteum and in the endosteum as well as in the bone tissue itselfwhere needed c Appearance since they are secretory cells osteoblasts possess well developed rough endoplasmic reticuli and Golgi as well as many secretory vesicles 1 Under the light microscope osteoblasts appear as plump polygonal cells usually having one side quotattachedquot to the matrix which they are secreting 2 Their cytoplasm is extremely basophilic 3 One special group of secretory vesicles present in osteoblasts are called Matrix Vesicles a Matrix vesicles contain alkaline phosphatase 1 Alkaline phosphatase liberates phosphate ions from organic phosphate by enzymatic hydrolysis a This allows for the calci cation of bone matrix since it provides adequate amounts of organic phosphates to facilitate the precipitation of calcium ions 1 The phospholipids ofthe osteoblast39s cell membrane 40 bind to calcium to accelerate this process 3 Osteocytes are the principle cell type of bone tissue a They are derived from osteoblasts b Osteocytes exist in Lacunae surrounded by matrix in the bone 1 Bone matrix however does not allow forthe diffusion of materials between the blood vessels and the osteocytes a Instead the lacunae have tiny radiating canals called Canaliculi 1 These canaliculi interconnect osteocytes since they allow cytoplasmic processes from osteocytes to reach one another a There are gap junctions between the osteocytic processes b The processes are surrounded by a nonmineralized intralacunar matrix 2 The canaliculi open up near blood vessels traveling through canals in the bone a 80 they serve to transport materials between the blood and the osteocytes 4 Osteoclasts are specialized cells ofthe bone which will degrade the matrix a The available evidence today indicates that osteoclats are derive from circulating monocytes not from osteogenic cells as are the other bone cell types b Osteoclasts degrade bone at sites where the bone is being remodeled 1 The remodeling of bone is a twofold activity a Osteoclasts breakdown bone matrix in a process termed Osteoclastic Resorption b Osteoblasts secrete new bone matrix NB39 Osteoporosis occurs when osteoclastc resorption occurs ata greater rate than does osteoblastc deposition 2 Osteoclasts are also involved in the release of minerals from bone 0 Osteoclasts are generally located against the surfaces of bone where resorption is occurring 1 They are found in shallow depressions in the bone matrix which they have excavated termed Resorption Bags or Howship39s Lacunae d Appearanc osteoclasts are multinucleated giant cells 1 The cell often has 30 or more nuclei a This is believed to be due to the fusion of monocytes during their formation 2 The cytoplasm of the osteoclast is acidophilic a The cytoplasm along the bone matrix is organized into an elaborate Ruf ed Border 1 Electron microscopy indicates that this quotruf ingquot may be an array of villus like structures 2 V thin the ruffled border is an area of cytoplasm rich in vesicles containing catabolic substances 3 Between the ruffled border and the matrix apposed to the osteoclast is a villus free zone called the Clear Zone 41 a The clear zone anchors the osteoclast to the matrix e Activities of Osteoclasts 1 Osteoclasia or Osteoclastic Resorption a Osteoclastic Resorption occurs in two stages 1 Demineralization of Bone the osteoclasts secrete one or more varieties of acids which increase the solubility of bone mineral salts along the ruffled border 2 Degradation of Collagen the collagen which has become exposed due to the demineralization ofthe matrix located between the villi ofthe ruf ed border is degrade by the action of acid hydrolases released from osteoclast vesicles at the intervillar spaces b Bone reabsorption can occur during bone remodeling but also by a process called Osteocytic Osteolysis 1 ln osteocytic osteolysis involves old osteocytes located deep in old heavily mineralized bone a They are termed resorptive Osteocytes and will cause calcium to be released into the blood b This release of calcium occurring in osteoclasia is from superficial less mineralized bone 2 The Release of Calcium from Bone Matrix a The release of calcium from bone matrix is under hormonal control 1 Calcitonin from the parafollicular cells ofthe thyroid gland stimulate a decrease in osteoclastic resorption a Calcitonin reduces the size of the ruffled border and the size ofthe clear zone and decreases osteoclastic activity 2 Parathyroid Hormone from the parathyroid gland stimulates an increase in osteoclastic resorption a Parathyroid hormone increases the size of the ruf ed border and the size ofthe clear zone and increases osteoclastic activity d The Arrangement of the Constituents of Bone 1 The Arrangement of the Constituents of Bone in Sponqv Bone a Spongy bone demonstrates a relatively simple structure consisting ofan interconnected network of bony bars Trabeculae with many intervening spaces lntertrabecular Spaces b The bone tissue ofthe trabeculae is arranged into microscopic layers called Lamellae 1 This laminated structure is due to the orientation of the tissue39s collagen bundles a the collagen of one lamella are all parallel b The collagen within a lamella may all be parallel but they will also be at a 90 angle to those of the neighboring lamellae 1 This arrangement increases the strength of bone 2 Located along the lamellae are the lacunae and their resident 42 osteocytes a Radiating from the lacunae to the surface of the trabeculae are the canaliculi 2 The Arrangement of the Constituents of Bone in Compact Bone a The structure of compact bone is much more complex than is that of spongy bone mostly due to the problem involved in nutrients and oxygen reaching the osteocytes in this denser bone b There are three patterns of lamellar organization found in compact bone 1 Haversian Lamellae a This is the most prominent ofthe three types of lamellae b Haversian lamellae consists of primary units of compact bone structure called Osteons or Haversian Systems 1 The osteon consists of lamellae arranged concentrically around a central canal a This blood vessel and nerve carrying canal is called a Haversian Canal or a Central Canal 1 The Haversian canal is lined with endosteum c The osteocytes within their lacunae are concentrically arranged around the osteon with their canaliculi radiating towards the Haversian canal and towards one another d Adjacent Haversian lamellae can connect with each other either directly or by canals perpendicular to the Haversian canals called Volksmann39s Canals 1 Volksmann39s canals contain the branches of blood vessels and nerves traveling through the Haversian canals 2 Volksmann39s canals are lined by endosteum 2 Interstitial Lamellae a These are fragments of laminated bone tissue that are packed between osteons b Interstitial lamellae represent remnants of older partially resorbed and remodeled osteons marked by Cement Lines c The boundaries between Haversian and interstitial lamellae are 3 Circumferential Lamellae a Circumferential lamellae are circular lamellae forming the external and internal lamination of cortical bone 1 The outer circumferential lamellae are immediately below the periosteum and the Sharpey39s bers attach to it 2 The inner circumferential lamellae are immediately below the endosteum b The collagen bundles ofthe circumferential lamellae are l0 erpendicular to those of the adjacent Haversian lamellae 4 Osteogenesis aka Ossification Bone organs develop by two mechanisms a lntramembranous Ossification 1 ln intramembranous ossi cation a preformed model of the bone organ 43 composed of mesenchyme is replaced by bone a 80 in intramembranous ossi cation the development of the bone organ is initiated by the formation of bone tissue without a preexisting surface b The osteoblasts develop from osteogenic cells derived from mesenchymal cells and begin the process of osteogenesis 2 Intramembranous ossi cation occurs in the quotMembrane Bonesquot a Ex frontal parietals clavicles facial bones portions of the mandible occipital and temporal bones b Endochondral Ossification aka intracartilagenous ossi cation 1 ln endochondral ossification a preformed model ofthe bone organ composed of hyaline cartilage is replaced by bone a In endochondral ossi cation development of the bone organ is initiated by the formation of bone tissue with a preexisting surface of hyaline cartilage 2 Endochondral ossi cation gives rise to the quotEndochondralCartilage Bonesquot a These are the majority of the axial and appendicular skeletal bones Students are responsible for the detaYs of the two mechanisms of osteogenesis C Blood Blood is a connective tissue composed of free cells in a fluid matrix a Unlike other types of connective tissues blood lacks bers except during the clotting response b Blood can be looked at in terms ofthe extracellular material the Plasma and the cellular component the Formed Elements Plasma a The plasma acts as a medium for the circulation of cells and metabolic substances b The primary components of plasma are water inorganic salts and the plasma proteins 1 Plasma Proteins are special proteins unique to the blood They include a Albumins the most abundant class 1 They serve primarily to maintain blood viscosity and volume b Fibrinogens serve primarily in clot formation c Globulins a class of plasma proteins ofdiverse size and function They include 1 Gamma Globulins such as the antibodies 2 Beta Globulins used in the transport of hormones lipids and metal Ions 2 Plasma also contains certain microscopic particles such as a Chylomicra fatty bodies 1 Chylomicra increase in number after a fatrich meal b Hemoconia small particles of diverse origin possibly the fragments of 44 l cells 3 The Formed Elements a General Comments 1 The formed elements are the blood cells a There are three distinct classes erythrocytes leucocytes and thrombocytes 2 All ofthe formed elements arise from hemopoietic tissue a In the embryos fetus and even the neonate there are a number of hemopoietic organs spleen liver bone marrow and yolk sac b In the adult hemopoiesis is restricted only to the red marrow 1 All ofthe various formed elements begin there a However one class of leucocytes complete their maturation outside of the red marrow c All ofthe various formed elements arise from a multipotent stem cell called the Hemocytoblast 1 The hemocytoblast is derived from mesenchymal cells 2 The hemocytoblast will give rise to ve lineages of stem cells producing all of the various formed elements b Euthrocytes 1 The erythrocytes are the red blood cells and are the most numerous of the formed elements 2 Appearance erythrocytes are anucleated red colored cells shaped like biconcave discs a They are 8 um by 2 um in dimension b Their shape increase surface area for gas exchange 0 The mature erythrocyte lacks most ofthe typical organelles to allow it to hold more Hemoglobin 1 Hemoglobin is a complex protein composed of four globular polypeptide chains each bearing a Heme Group a The heme group contains iron 1 Iron binds to oxygen allowing for it39s transport through the blood stream a It will also bind to carbon dioxide 2 The iron ofthe heme group gives the red colorto erythrocytes a Since erythrocytes are by farthe most abundant of the formed elements they give the red color to blood d Erythrocytes are exible cells which allows them to travel through the smaller capillaries 1 This exibility is due to a subplasmalemmal framework of microfilments made up ofthe protein Spectrin 3 The Life Cycle ofthe Euthrocyte a Erythrocytes are short lived cells typically having a life span of approximately 120 days b The steps of hemopoiesis which gives rise to erythrocytes is termed Emthropoiesis and occurs in the red marrow 45 1 Erythropoiesis is stimulated by the hormone Euthropoietin a Erythropoietin is produced by the kidneys in response to a decrease in erythrocyte numbers 2 The Steps of Erythropoiesis a The multipotent stem cell the hemocytoblast differentiates into a Proemthroblast in the red marrow b The proerythroblasts will differentiate Basophilic Euthroblasts 1 Basophilic erythroblasts have smaller nuclei and an increased number of ribosomes a This increase in ribosome number allows forthe synthesis of hemoglobin and is responsible for the basophilic nature of these cells c The basophilic erythroblasts develop into Polychromatophilic Euthroblasts 1 At this point in erythropoiesis the cells begin to show two patterns of staining basophilic staining due to the presence of ribosomes and eosinophilic staining due to the presence of hemoglobin 2 The nucleus of these cells is even smaller having been further reduced and it39s functions have been terminated d The polychromatophilic erythroblasts develop into Normoblasts 1 During the normoblast stage the nucleus is very reduced and will be eventually extruded form the cell e The normoblasts will differentiate into Reticulocytes 1 Due to the dual nature oftheir staining reticulocytes are also known as quotpolychromatophilic erythrocytesquot 2 Typically the reticulocyte will mature into a erythrocyte and then leave the red marrow to enter into circulation a However about 1 to 2 ofthe erythrocytes in circulation are really reticulocytes which are still completing maturation unless there is a disorder 0 When erythrocytes become Senescent they have used up all oftheir enzymes necessary to maintain ATP and can not replace them 1 The cells become fragile 2 These senescent cells are trapped engulfed and degraded by phagocytic cells ofthe liver spleen and bone marrow a Hemoglobin is degraded into Bilirubin 1 Iron is released from the degrading hemoglobin and is complexed with protein to be stored as Ferritin or as Hemosiderin which will be available for erythropoiesis 1 General Comments a Leucocytes are the white blood cells b They are the only complete cells ofthe formed elements 0 Leucocytes c Leucocytes can be described as connective tissue cells which utilize the 46 blood stream for transport from the hemopoietic red marrow to areas where they are required 1 Leucocytes are a variety of motile nucleated cells which serve in the defense of the body from disease causing organisms ie the immune system d Leucocytes care divided into two large groups based on their appearance under light microscopy 1 Granular Leucocytes or Granulocytes eucocytes having prominent cytoplasmic granules and a lobulated nucleus a They are basophils eosinophils and neutrophils 2 Agranular Leucocvtes or Agranulocvtes eucocytes lacking prominent cytoplasmic granules and having a spherical nucleus a They are lymphocytes and monocytes e The steps of hemopoiesis which give rise to the leucocytes are broadly called Myelopoiesis and occur at least initially in the red marrow 1 ln myelopoiesis the hemocytoblast can follow three distinct pathways giving rise to different leucocytes a The hemocytoblast can differentiate into a Myeloblast which can ultimately develop into any of the three granulocytes b The hemocytoblast can differentiate into a Monoblast which will eventually develop into the monocyte c The hemocytoblast can differentiate into a Lymphoblast which will eventually develop into the lymphocytes 2 Granulocytes leucocytes having prominent cytoplasmic granules and a lobulated nucleus They derive from myeloblasts and develop and mature in red marrow a Neutrophils 1 Neutrophils are the most abundant of the granulocytes and the most numerous type of leucocyte in the blood stream a Neutrophils are chemotatically attracted to areas in which bacteria and other foreign substances are concentrated such as sites of in ammationinfection 1 They are attracted there by substances known as Chemotactic Factors generated at the in ammation site and diffusing into the surrounding tissues a Chemotactic factors may be chemicals released by phagocytized bacteria or by a wide range of host factors eg complement activation 2 In the mature neutrophil the nucleus has a highly lobulated appearance a On average the nucleus will have three to five lobes b This is why neutrophils are also called quotpolymorphonuclear leucocytesquot c The lobulated nucleus allows the neutrophil to be used in gender determination since the Barr Body is visible in the neutrophils of 47 women 3 Neutrophils demonstrate two populations of cytoplasmic granules a 80 of the granules appear as tiny pink granules containing antibacterial substances such as lactoferrin lysozyme and cobalaminbinding protein b 20 of the granules are larger and blue in color 1 These granules are primarily lysosmes containing peroxidase acid hydrolase acid phosphatase and other enzymes involved in antibacterial digestive actions a These granules will also contain lysozyme b Eosinophils 1 Eosinophils usually have bilobed nucleus and an abundance of orangered granules a The granules will obscure portions ofthe nucleus 1 In some respects these granules are like the typical lysosome in that they contain hydrolytic compounds 2 In other ways though they are different form the typical lysosome since they contain a number of substances that moderate the in ammation response and aid in the rejection of parasites by the body a They contain histaminase amlsulphatase and the hormone hydrocortisone to depress the allergic and immune reactions b They also contain m Major Basic Protein which is released onto the surfaces ofa parasite to promote antibody mediated immune response 2 Eosinophils are rare cells a They are found in low numbers in chronic inflammatory reactions 1 They phagocytize antigenantibody complexes such as antibody coated bacteria b Eosinophils occur in the connective tissues ofthe respiratory tract and the digestive tract c They are chemotactically attracted to areas where allergic reactions are occurring c Basophils 1 Basophils are rare leucocytes having a distinctive appearance a They have a bilobed or quotUshapedquot nucleus and prominent basophilically staining granules 1 They also have metachromatically staining granules 2 The granules of basophils contain a number of pharmacologically active substances which promote in ammation and some GAGs 2 Basophils in many ways resemble mast cells but they are unrelated 3 Basophil functions include a increasing vascular permeability in the in ammation response 48 b binding to the immunoglobulin lgE which is produced by the plasma cells 3 Agranulocytes leucocytes lacking prominent cytoplasmic granules and having a spherical nucleus They initially develop in red marrow but migrate elsewhere to mature a Monocyte 1 Monocytes could be considered to be immature macrophages that are in transit to the connective tissues a Even at this time however they are still highly phagocytic b After having been in circulation for 1 to 2 days they enter into the connective tissues and differentiate into macrophages 1 they may undergo further division and enzyme synthesis 2 As macrophages they are highly mobile moving about by diapedesis 2 Appearance monocytes have a pale basophilic cytoplasm lacking granules easily discernible under light microscopy and a large kidneyshaped nucleus a They do possess numerous small granules but again these can not be seen under the typical light microscope b Lymphocytes 1 General Comments a Appearance lymphocytes look quite a bit like monocytes 1 They also have a pale basophilic cytoplasm lacking granules easily discernible under light microscopy and a large indented nucleus a The nucleus is not as strongly indented kidney shaped in lymphocytes b They also possess numerous small granules but again these can not be seen under the typical light microscope 2 Lymphocytes can be identified according to size a They range from 6 to 18 um in diameter b Based on size they are termed small medium and larg 1 Only small and medium sized lymphocytes are found in blood circulation b Most ofthe lymphocytes in the blood or lymph are actually recirculating immunocompetent cells 1 These cells have developed the capacity to recognize and respond to foreign invaders and antigens 2 There are two types of lymphocytes a The two types are 1 T Lymphocytes long living cells which mature in the thymus and are involved in cellmediated immunity 2 B Lymphocytes are cells having a varying longevity which mature in some still undetermined area quotbursaequivalentquot and are involved in antibodymediated immunity 49 b Both types are involved in the memory ofthe immune system and will develop to respond to speci c antigens 1 Some B cells and some T cells will develop into Effector Cells a A B cell that has become exposed to an antigen will undergo mitosis producing more B cells Some of these will become Plasma Cells which will secrete antibodies specific to the antigen b T cells can also undergo mitosis producing a number ofT T cell varieties involved in cellmediated immunity 1 Cytotoxic T Cells aka Killer T Cells recognize other cells with foreign antigens on their cell membranes and destroy them by lysis 2 Helper T Cells assist B cells and T cells in the immune response a They secrete factors in response to antigens which will stimulate B cells and other leucocytes to destroy the invader 3 Suppressor T Cells suppress the immune response 2 Some cells become Memom Cells a Memory cells are long lived cells that allow the immune system to respond more swiftly and more ef ciently when exposed to the same pathogen again d Thrombocytes 1 Thrombocytes also called quotplateletsquot function to arrest bleeding and to cause thrombosis clot formation 2 Thrombocytes are the smallest ofthe formed elements a Appearance they are 2 to 4 um long and shaped like flattened discs b They are actually fragments of the cytoplasm of large Megakamocytes which explains their lack of organelles 1 Each of these large multilobed cells will give rise to between 1000 and 5000 thrombocytes c Underthe light microscope they show two regions 1 Granulomere the central portion ofthe thrombocyte a It stains purple 2 Hyalomere the peripheral portion of the thrombocyte it is peripheral to the granulomere a It stains pale blue 3 About a third ofthe body39s thrombocytes are located in the spleen Unit 5 MUSCLE TISSUE A Skeletal Muscle 1 Introduction a The functional cellular component of muscle tissue is the muscle cell or myo ber 1 In all three classes of muscle tissue the myofiber synthesizes and maintains a group of proteins responsible for the contractile nature of this tissue b Skeletal muscle is described as quotstriatedquot due to the intracellular arrangement of the contractile protein laments which will form alternating bands when viewed under light microscopy in a longitudinal section 1 The term quotskeletalquot comes from the muscle typically being attached to the skeleton a There are some exceptions such as the upper esophagus b As a result skeletal muscle is primarily involved in the initiation of body movement and locomotion c Skeletal myo bers are long cylindrical cells arranged parallel to one another along the long axis of the muscle organ 1 The longest ofthese cells will stretch the length of the muscle organ from origin to insertion but shorter cells are more common d Three levels of connective tissue organization confer structural integrity to the contracting muscle 1 Epimysium a fibrous connective tissue sheath surrounding the external surface ofthe entire muscle organ a Epimysium consists of collagen and broblasts 1 It is a dense regular connective tissue b Epimysium serves to bind the muscle organ together 0 Epimysium is continuous with the tendons attaching the muscle to the origininsertion 1 An extension of the tendon39s collagenous fibers will enter into the body of the muscle organ to increase the anchoring surface area d Epimysium is sometimes referred to as quotdeep fasciaquot 2 Perimysium a brous connective tissue sheath surrounding bundles of myo bers termed Fascicles a Perimysium also consists of collagen and fibroblasts 1 It is a dense regular connective tissue contiguous with the epimysium 3 Endomysium a loose connective tissue sheath having reticular bers surrounding each individual myofiber 4 Deep to the endomysium is the External Lamina 1The external lamina surround each myofiber just deep to the endomysium 2 The external lamina is similar in composition and appearance to the 51 basal lamina of the epithelial basement membrane e The Origin of Skeletal Myo bers 1 Skeletal myo bers arise from Myoblasts a Myoblasts are round noncontractile cells with a single central nucleus 1 Myoblasts are derived from mesenchymal cells a Mesenchyme is of mesodermal origin and so muscle is of mesodermal origin 1 The mesenchyme that gives rise to the myoblasts arises from various parts ofthe embryo a Trunk musculature arises from myotomal mesoderm b Limb musculature arises from somatic mesoderm c Facial muscles muscles of mastication and muscles ofthe larynx and pharynx arise from the pharyngeal arch aka branchial mesoderm b During development myoblasts will fuse together forming increasingly larger mulitnucleated Myotubes which will eventually become mature myo bers 1 These myotubes synthesize the contractile proteins 2 Subsequent to the establishment of innervation the myotubes begin to display contractile behavior 3 This fusion of myoblasts during the development of skeletal myo bers explains their multinucleated nature a They average between 50 to 100 nuclei per myo ber b The nuclei are located immediately deep to the muscle cell membrane the Sarcolemma f The major contractile proteins ofthe myo ber are m a micro lament and Myosin a microtubule also termed m and Thick Filaments respectively 1 These contractile proteins are arranged into parallel bundles called Myo brils running along the long axis of the muscle cell a Each myo bril is associated with a quotsleevequot of sarcoplasmic reticulum which will supply it with calcium for contraction b Mitochondria are located between myofibrils with their long axis being parallel to the myo brils 2 The actin and myosin laments form alternating regions in the myo bril a At rest they slightly overlap but they strongly overlap during contraction b The basic functional unit is the Sarcomere 1 Many sarcomeres can make up one myofibril 2 The sarcomere is composed of a central array of 15 um long myosin laments interdigitated at both ends by 10 um long actin laments a These actin filaments are anchored into a transverse structure called the Z lineZ disc 1 So the Z lines form the borders ofa sarcomere 3 At rest a sarcomere is 25 um long When contracted it becomes 20 um long 3 The banding or striated pattern of skeletal muscle is due to the arrangement 52 of actin and myosin within the sarcomere a There are alternating dark bands A bands and light bands I bands 1 These bands are named for their refractile properties a A bands are anisotropic b l bands are isotropic 2 The A band runs the length ofthe myosin a It includes those portions of actin which overlap with myosin b The l band is an actin puremyosin free zone 1 One band actually overlaps between two adjacent sarcomeres with a Z line being central to the l band 3 The actin and myosin laments remain parallel within a sarcomere and between sarcomeres due to intermediate laments a These intermediate filaments are Desmin and Vimentin 4 During contraction the actin laments of the sarcomere slide past the myosin laments towards the center of the A band shortening the myo bril a The shortening ofthe myo brils will cause the myofiber to shorten which causes the muscle to contract b V thin the A band is the centrally located H zone 1 The H zone is a central portion ofthe A band into which actin does not extend when the muscle is at rest a 80 it is a myosin pure region in the center ofthe A band 2 V thin the H zone is a myosin cross bridge free central area called the M line c The Al Junction is that portion of the sarcomere where the actin and myosin laments overlap 1 During contraction extensions of the myosin laments called myosin cross bridges engage the actin filaments and pull them into the A band towards the M line a These cross bridges are composed of heavy meromyosin HMM 2 The Intracellular Control of Muscle Contraction a During contraction ATPase on the myosin laments is activated due to elevated calcium ion levels resulting in the production ofATP to power contraction 1 This is stimulated by the actin filaments a The actin laments in turn are controlled by the TroponinTropomyosin SystemComplex which confers calcium dependency on actin and myosin 1 The actin lament is a double helix of globular actin monomers arranged around a long filament oftropomyosin a The regulatory protein Troponin is found at periodic intervals along the tropomyosin lament 2 Troponin will regulate contraction by exerting a conformational constraint on the actin filament a Troponin will sterically mask the binding sites where the myosin cross bridges will attach on the actin lament 53 b During contraction calcium will bind to troponin causing a conformational shift which will expose the actin39s binding sites to the myosin cross bridges 1 This allows for the sliding ofthe filaments past one another b Regulation of Calcium Concentration 1 Calcium ion concentration is controlled by the sarcoplasmic reticulum SR a The sarcoplasmic reticulum shuttles calcium between the cytoplasmicsarcoplasmic compartment and the sarcoplasmic reticulum compartment 1 Calcium interacts with the myo brils in the sarcoplasmic compartment 2 Calcium is stored in the sarcoplasmic reticulum compartment a Calcium is released from the sarcoplasmic reticulum during contraction and retrieved to the sarcoplasmic reticulum to allow for muscle relaxation 2 The Skeletal Muscle Triad a The triad is made up ofthree components 1 a TTubule aka Transverse Tubule which is an invagination of the sarcolemma which penetrates deep into the myo ber 2 and a pair of Terminal Cisternae flanking the ttubule a The terminal cisternae are dilated sac like expansions ofthe sarcoplasmic reticulum that store calcium during muscle relaxation b The triad is located at each Al junction 0 During contraction a wave of depolarization travels along the sarcolemma down the ttubules to the sarcomere 1 This wave of depolarization is caused by the release of neurotransmitters from the motor neuron at the neurosynaptic junction 2 This wave of depolarization will cause the terminal cisternae to release calcium into the sarcoplasmic reticuli 3 Calcium will travel through the sarcoplasmic reticuli to the sarcoplasmic compartment where calcium will interact with the myo brils to facilitate contraction 3 Motor Innervation of Skeletal Muscle a The junction between a nerve process and a myo ber is termed the Neuromuscular Junction NMJ 1 The NMJ is the site of neuromusculartransmission 2 The NMJ consists of a Branches of the motor neuron39s terminal axon 1 One branch will synapse with an individual myo ber b a myo ber 0 Motor End plate a region of the sarcolemma specialized for nerve transmission 1 The motor end plate has two levels oforganization a Primary Svnaptic CleftPrimarv Svnaptic Trouqh a 39 r 39 I in 54 the sarcolemma occupied by the innervating axonal branch b Secondam Synaptic Cleft highly infolded areas ofthe primary synaptic cleft 2 Associated with the motor end plate are membrane molecules involved in neurotransmittertransmission a Acetylcholine Receptors membrane proteins that recognize and bind to the neurotransmitter acetylcholine b Acetylcholinesterase an enzyme deployed along the inner aspect of the secondary synaptic cleft which destroys the neurotransmitter after contraction has been stimulated 3 The structural integrity ofthe NMJ is maintained by the external lamina b The development of skeletal muscle is in uenced by neural control 1 This in uence occurs after the establishment of the myotube 2 This in uence results in the establishment of three physiological categories of skeletal myo bers a Slow Twitch Muscle Fibers 1 These bers have a slower rate of contraction higher stamina and display less power 2 It has oxidative properties which increase stamina ie it is aerobic b Fast Twitch Muscle Fibers 1 These bers demonstrate a faster rate of contraction lower stamina and more power 2 It ha a glycolytic enzymatic nature ie anaerobic 0 Intermediate Fast Twitch Fibers 1 As the name indicates these fibers are intermediate in rate of contraction stamina and power 2 It has an anaerobic glycolytic nature as does the fast twitch class 4 Sensory innervation of Skeletal Muscle a Introduction 1 The sensory nerves of skeletal muscles may be myelinated or unmyelinated a Unmyelinated neurons are much less common and found in the perimysium 2 Sensory nerves can serve a number of functions a Some located in tendons and joints provide proprioceptive information b Some are involved in neural re ex arcs that adjust tension and the length of skeletal muscles 1 Le neuromuscular spindles and Golgi tendon organs b Neuromuscular Spindles 1 Neuromuscular spindles are stretch receptors located within skeletal muscles especially slow twitch extensor muscles used to maintain posture a They monitor static and dynamic aspects of muscle length b They also respond to passive increases in muscle length through a nervous reflex arc called the Stretch reflex 1 The stretch re ex allows the muscle to resist excessive and injurious over stretching 55 2 Composition a Neuromuscular spindles are composed of between 3 to 12 lntrafusal Fibers 1 lntrafusal fibers are specialized striated muscle bers encapsulated by perimysium 2 They are located within the belly of skeletal muscles and are orientated parallel to the surrounding regular myo bers a These myo bers surrounding the intrafusal bers are termed Extrafusal Fibers 3 There are two morphotypes of intrafusal bers within each spindle a Nuclear Bag Fibers larger in diameter than the other type they have their nuclei located in an noncontractile equatorial region of the ber b Nuclear Chain Fibers smaller in diameter than the other type they have their nuclei arranged into a single row among the myo brils b Two types of sensory nerves innervate the neuromuscular spindle 1 Primam Fibers heavily myelinated and so fast conducting nerves 2 Secondam Fibers unmyelinated and so slow conducting nerves c The intrafusal fibers receive motor innervation from the Gamma Neurons aka fusiomotor nerves 1 Gamma neurons alter the sensitivity ofthe neuromuscular spindle to stretch by regulating the contraction ofthe intrafusal bers c Golgi Tendon Organs 1 Golgi tendon organs are encapsulated sensory nerve endings which monitor increase in muscle tension a They are located within the tendons near the musculotendinous junction 2 The Golgi tendon organ consists of unmyelinated nerve endings encapsulated by a connective tissue termed the Endoneurium and enmeshed among the collagen bundles of the tendon 3 Mechanism Muscle tension is transferred to the tendon This causes the tendon39s collagen bers to compress the nerve ending B Cardiac Muscle 1 Cardiac muscle bers possess many characteristics which are similar to those of skeletal muscle bers but with modi cations a Cardiac muscle cells are typically uninucleated but can be binucleated b They are smaller cells than are skeletal myo bers and they branch c Myocardial cells are attached to one another at lntercalated Discs 1 lntercalated discs serve both to bind cells and to allow for communication 2 The intercalated disc consists ofthree structurally distinct regions a desmosomesmacula adherens b fascia adherens 1 The fascia adherens is the site of insertion of actin filaments from the 56 myo brils into the sarcolemma 0 gap junctionsmacula communicans 1 Gap junctions allow for cardiac myo bers to be electrically coupled for uniform contraction of the heart chambers d Myo bril organization is similar in cardiac myo bers to that of skeletal myo bers 1 They show the same striation patterns and their sarcomeres are identical to those of skeletal muscle cells 2 There are some differences a The sarcoplasmic reticulum is less well developed since cardiac muscle contraction depends to a greater degree on the in ux of extracellular calcium b The ttubules are further apart being located at the Z lines 0 Instead of a triad cardiac muscle cells possess a made up of one ttubule and one terminal cisterna e Unlike the skeletal muscle39s neuromuscular junction the nervecardiac muscle junction lacks the motor end plate f The cardiac muscle cells have many more mitochondria than do skeletal muscle cells 2 Specialized Cardiac Muscle Fibers a The cardiac conduction system consists of highly specialized muscle fibers designed to distribute the impulse for contraction throughout the heart 1 Nodal Fibers a Nodal bers compose the sinoatrial and atrioventricular nodes b Nodal bers are specialized cardiac myo bers which exhibit an inherent contractile rhythmicity due to their ability to spontaneously depolarize 1 They depolarize quicker than do the typical cardiac muscle cells a As a result they are termed quotpacemaker cellsquot 2 Purkin39e Fibers a These specialized cardiac myofibers make up the rest of the cardiac conduction system AV bundle bundle branches and Purkinje bers b They are elongated thick cardiac myo bers specialized for conduction not for contraction 1 So Purkinje cells have few myo brils c Purkinje bers are not connected to each other by the typical intercalated discs but by desmosomes and gap junctions scattered along the opposing cell membranes d Purkinje bers are designed to ensure the ef cient contraction of the ventricles C Smooth Muscle 1 Smooth muscle forms the bulk of the visceral musculature a The only notable exception is the myocardium b Smooth muscle bers can occur individually or most often in sheets 57 1 These sheets will be found in a the muscular walls ofthe digestive organs and the ducts ofthe associated glands b lining portions of the respiratory tract c lining the urinary and reproductive tracts d the muscular walls of blood vessels and ofthe larger lymph vessels e the arector pili muscles c As is the case with all muscle most smooth muscle is of mesodermal origin 1 However some is of ectodermal origin dermis nipple prepuce scrotum glans penis and the intrinsic muscles of the eye 2 Appearance smooth muscle cells are small fusiform cells with a single elongate centrally located nucleus a They average between 20 and 50um in length 1 But they can range from 20 um in the vascular walls to 500 um in a gestational uterus b The single nucleus will often have several nucleoli 3 Smooth muscle sheets are associated with small amounts of collagen and elastin bers as well as a few scattered broblasts a Individual smooth myofibers are supported by reticular bers 1 Also closely opposed to the cell surface is an external lamina 4 The Cytoskeleton of Smooth Muscle Cells a Actin and myosin are present in smooth myo bers 1 However they are not organized into myo brils at least when at rest and so smooth muscle is unstriated in appearance a The smooth myofibers lack sarcomeres when at rest and so are unstriated in appearance b Smooth muscle cells are believed to contract by the sliding filament mechanism as do the other two classes of muscle tissue but it is not well understood c It may be that the myosin exist in a disaggregated or morphologically dispersed state when the cell is at rest and will aggregate into distinct thick laments due to nervous or hormonal stimulation b There is also a network of intermediate laments present 1 These intermediate filaments are anchored in dense bodies distributed through both the cell and the sarcolemma a This intermediate lamentdense body arrangement i believed to represent an intracellular cytoskeleton b The interdigitating thick and thin myo laments instead are arranged so as to lay nearly parallel to the long axis ofthe myofiber 1 Also running along the length of the myofiber are intermediate laments a These intermediate filaments have short globular segments called dense bodies along their length 1 The thin laments anchor into these dense bodies so they serve as Z discs 58 2 Also along the length ofthe intermediate laments are structures called dense plagues a Dense plaques also anchor the actin laments so they also act as Z discs but in addition anchor on to the collagen of the surrounding endomysium 3 It is believed that this arrangement serves to anchor a functional sarcomere and to allow for contraction by a sliding filamentlike mechanism 5 Smooth muscle cells lack ttubules a Ttubules may not be necessary due to the small size of the cells and the reliance of smooth muscle on intercellular communication 6 Smooth muscle is innervated by unmyelinated postganglionic nerve of both the sympathetic and parasympathetic divisions of the ANS a This nervemuscle junction lacks a motor end plate b Also one axon will service many myo bers due to their high degree ofelectronic coupling which allows the muscle sheet to contract as a unit 1 This is single unit innervation 0 Smooth muscle may also contract due to nonnervous in uences such as hormones stretch and spontaneous contractile rhythmicity Unit 6 NERVOUS TISSUE A Introduction 1 The neuron is the functional and the structural unit of the nervous system a It displays two highly developed physiological traits 1 lrritability the capacity to generate a nervous impulse in response to various stimuli 2 Conductivity the ability to transmit these impulses along it39s cellular processes b Neurons allow for communication between the CNS and the rest of the body via the PNS 2 Nerve tissue also includes a class of nonneuronal cells called the Supporting Cells a Supporting cells assist the neurons in their functioning B The Neuron 1 Introduction a The neuron is composed of three integral parts 1 the Perikamon or nerve cell body 2 one or more Dendrites a Dendrites are typically receptive structures 1 Normally they will respond to a stimulus and conduct an impulse towards the perikaryon b Dendrites are branching structures having numerous secondary processes 3 and usually one Axon a Axons typically send information out from the axon towards another cell b Axons rarely branch along their length 1 In some cases one or even two branches may arise from the axon a These are called Axon Collaterals b Axon collaterals always branch at a right angle to the axon 2 At the end ofthe axon near the site of synapse there will be small branches radiating off from the end ofthe axon a This is termed the Telodendria 1 Each branch ofthe telodenria is short and will end in a swollen knob called an Axon Terminal Axonal Buoton or End Bulb a The end bulb will be in contact with the cell that the axon is communicating with b Histologically neurons are classi ed based on the number of processes radiating from the perikaryon 1 Unipolar Neurons aka pseudounipolar neurons these neurons have a single radiating process a Unipolar neurons are at their greatest numbers in the embryo before they develop into bipolar or multipolar neurons b In the adult unipolar neurons are much less common and occur primarily in the sensory ganglia 2 Bipolar Neurons these neurons have two processes one dendrite and one axon a Bipolar neurons are a rare class of neurons 1 They are found in places such as the olfactory mucosa retina and the inner ear where they serve a sensory role 3 Multipolar Neurons these neurons have three or more radiating processes typically with one axon and two or more dendrites a These are by far the most numerous class of neuron b Multipolar neurons serve motor sensory and integration roles c The Components of the Neuron 1 Although the shape ofthe perikaryon will vary between the various neurons it will characteristically contain a a large spherical nucleus having a prominent nucleolus b a well developed perinuclear Golgi complex c and many mitochondria scattered throughout the cytosol 1 Mitochondria are also scattered in the dendrites and axons 2 Neurons also have specialized intermediate filaments called Neuro laments a These neuro laments are organized into bundles to make up Neurofibrils which are visible in the cytoplasm 1 The neuro brils are believed to give strength and resiliency to the neuron and it39s processes as well as to play a role in the movement of neurotransmitters through the cell ie axonal transport 3 Endoplasmic reticuli are also present a SER is located throughout the neuron b RER is confined to the perikaryon and the dendrites 1 RER odes not extend into the axon beyond it39s point of origin the Axonal Hillock a As a result the axon is dependent on the perikaryon for protein synthesis 1 These proteins are conveyed throughout the length ofthe axon by axonal transport 2 RER appears under the light microscope as clumps of darkly staining basophilic material called Chromatophilic Bodies or Nissl Bodies 4 The neuron also has cytoplasmic inclusions including a fat droplets b pigments 1 lipofuscin which is abundant in ganglionic neurons 2 melanin which is abundant in certain CNS neurons c glycogen is present only in the embryonic neuron 61 1 In the adult neuron there is a high dependency on oxidative metabolism and limited anaerobic capacity 2 The Synapse a The synapse is the point oftransfer ofthe impulse from one neuron to another cell typically another neuron 1 Usually the synapse is between a presynaptic axon and a postsynaptic neuron an axodendritic synapse but not always 2 The synapse is where the Synaptic Transmission occurs b There are two physiological types of synapses chemical synapses and electrical synapses 1 The impulse is electrical in nature a In a chemical synapse the electrical information ofthe impulse is converted into a chemical message and then converted back into an electrical impulse so as to cross the space between the two cells 1 The space between two synapsing cells is called the Synaptic Cleft b In an electrical synapse the electrical impulse simply jumps the synaptic cleft and continues along in the postsynaptic neuron 2 The Chemical Synapse using the vey common axodendritic synapse as our model a V thin the axon terminal are spherical membranebound structures containing neurotransmitters called Synaptic Vesicles 1 During synaptic transmission the synaptic vesicles will be stimulated by the impulse to fuse with the axon membrane and release their chemicals into the synaptic cleft 2 The presence of synaptic vesicles on only one side ofthe synaptic cleft explains the unidirectional ow of information at a chemical synapse b The neurotransmitters will diffuse across the synaptic cleft and attach to receptor proteins on the dendritic membrane 1 These receptor proteins are attached to ion gateways which will open when the neurotransmitter has bound to the receptor protein a The opening ofthese ion channels will initiate and propagate a wave of depolarization which will conduct the generate and conduct the electrical impulse c Chemical synapses occur extensively in both the PNS and CNS 1 Ex the neuromuscularjunction where the neurotransmitter is acetylcholine 3 The Electrical Synapse a The electrical synapse occurs predominantly in the CNS b The electrical synapse consists ofa gap junction which structurally and electrically couples the presynaptic and postsynaptic membranes c The depolarization is mediated directly by ionic current instead of by chemical messengers d This type of synapse allows for a bidirectional ow of information 3 Myelin a Myelin is a lipoprotein produced by supporting cells of the nervous system which serves to insulate neuronal processes especially axons 1 This insulation allows the neuron to conduct the impulse more efficiently 2 Myelin is a modi cation of the supporting cell membrane b Myelin and PNS Neurons all PNS axons and certain PNS dendrites are surrounded by myelin produced by Schwann Cells 1 These Schwann cells are arranged in sequence along the length ofthe neuronal process and form the Sheath of Schwann or Neurilemma 2 Schwann cells are derived from neural crest cells 3 Functions of Schwann Cells a They insulate neurons from one another b They in uence the conduction velocity of impulses transmitted along the axon 0 They participate in events associated with the regeneration of injured neurons 4 An axon can be ensheathed by Schwann cells in one oftwo configurations a Unmyelinated Axons 1 Unmyelinated axons are slowly conducting 05ms axons 2 ln unmyelinated axons several axons are associated with each Schwann cell ofthe neurilemma a Each axon has invaginated into a separate portion of the Schwann cell39s plasmallema 1 Along it39s length portions of each axon are myelinated by a covering ofthe Schwann cell membrane and portions are uncovered b Myelinated Axons 1 Myelinated axons are faster conducting axons 120ms due to their greater degree of insulation 2 Each Schwann cell is associated exclusively with only one axon a The Schwann cell is wrapped around the axon many times 1 These coils are so tight as to exclude the cytoplasm from all but the outermost layer of the Schwann cell a The outermost layer holds the nucleus and rest ofthe cytoplasmic components b 80 the myelin sheath is constructed of concentric revolutions of the Schwann cell plasmallema 1 These layers have absolutely nothing between them so the cell membranes are fused at their P faces a Under electron microscopy these fused cell membrane surfaces appear as dark lines called Ma39or Dense Lines b Alternating with the major dense lines are are less darkly staining lnterperiod Lines formed by the close association of opposing membrane E faces c The innermost and outermost revolutions of the Schwann cell plasmallema are connected by narrow channels of 63 39 called SchmidtLanterman Clefts c Schwann cell plasmallemae have an unusually high lipid to protein ratio This gives myelin it39s insulating properties 1 As a result myelinated axons can conduct impulses at a higher velocity 3 The Nodes of Ranvier represent small gaps between adjacent Schwann cells along an axon a Here minute portions ofthe axolemma are exposed to the extracellular space b The nerve impulsequotleapsquot over the insulatedportions ofthe axon from node to node 1 This is termed Saltatom Conduction 2 This allows for a faster rate of conduction 4 A small portion of the axon called the Initial Segment remains unmyelinated a The initial segment is where the axon originates at the axonal hillock and is where the impulse is generated c Myelin and CNS Neurons in CNS neurons anothertype of supporting cell the Oligodendrocyte is responsible for myelination 1 The oligodendrocyte extends multiple processes that will ensheath several axons in myelin a Unlike myelinated PNS neurons one oligodendrocyte services more than one axon b Unlike unmyelinated PNS neurons the axons do not invaginate into the insulating cell39s plasmallema 1 Instead the oligodendrocyte sends out extensions to wrap around and insulate the axons 4 Connective Tissue Investments ofthe PNS Nerve 0m a All PNS nerve organs are made up ofgroups of neurons and their supporting cells 1 These groups of neurons have ct investments much like those ofa skeletal muscle b The Three Levels of Connective Tissue Organization 1 Epineurium is the outermost ct sheath and surrounds the entire nerve organ a The epineurium is composed of dense irregular ct rich in collagen bers elastic fibers and fibroblasts b The epineurium extends into the nerve trunk separating it into axon bunches called Fascicles c The epineurium ofa nerve organ can be followed back to the spinal cord where it is continuous with the dura mater 2 Perineurium the ct sheath encasing the individual fascicles a The inner layer ofthe perineurium is distinct from the outer layer 1 The outer layer is a dense irregular ct continuous with that of the epineurium 64 2 The inner layer consists ofa continuous sheet of flat epithelial cells supported by a basal lamina a These epithelial cells are called Perineurial Epithelium 3 Endoneurium is the innermost ct sheath and encases individual axons a The endoneurium is separated from the Schwann cells by a basal lamina b The endoneurium is a loose ct rich in reticular bers c The nervous tissues are well vascularized and the blood vessels travel through connective tissue investments 1 Arterioles and venules will be found in the epineurium and perineurium 2 Capillaries are found in the endoneurium C The General Organization ofthe Spinal Cord and the Outflow of Peripheral Nerves 1 Introduction a The nervous system is divided into two components 1 M the brain and spinal cord 2 w the nerves emanating from the spinal cord and brain that distribute to other areas of the body 2 The Spinal Cord a The spinal cord is the pathway for the transmission of information from the brain to the rest of the body efferentmotor and from the body to the brain afferentsensory 1 Nerves enter and exit the spinal cord at regular intervals a Sensory nerves enterthe spinal cord through the Dorsal Roots b Motor Nerves exit the spinal cord through the Ventral Roots c At a short distance from the spinal cord each pair of dorsal and ventral roots will fuse to form a Spinal Nerve that will carry quotmixedquot both sensory and motor information b The spinal cord is organized into distinct white and gray areas 1 The gray matter is organized into a roughly quotHshapedquot central region a The gray area houses mostly unmyelinated structures such as perikarya and unmyelinated processes b The quotarmsquot or quothornsquot ofthe gray area are designated the Dorsal Horns and Ventral Horns 1 The two sets of horns are connected by the quotbar ofthe Hquot the Gray Commissure a V thin the gray commissure is a hollow canal lled with Cerebral Spinal Fluid and lined by modi ed epithelial cells called Ependymal Cells 1 This canal is called the Central Canal 2 The ventral horns are motorefferent areas a They contain the perikarya of multipolar alpha neurons called Somatic Motor Neurons which will innervate the skeletal muscles 65 3 The dorsal horns are sensoryafferent areas a They contain unmyelinated axons of Somatic Sensom Neurons whose cell bodies are located in an expansion ofthe dorsal root called the Dorsal Root Gang onCranialSpinal Ganglion 1 A ganglion is a term for a collection of perikarya located outside of the CNS 4 In certain regions of the spinal cord between T1 and L2 there are an extra pair of horns situated between the ventral and dorsal horns a These are the Lateral Hornslntermediolateral Gray Horns b The lateral horns are also motor areas and are part ofthe ANS 1 So the lateral horns contain the cell bodies of Visceral Efferent Neurons a Their axons will travel with those ofthe ventral horns39 neurons through the ventral root 2 The white matter surrounds the gray matter and is composed primarily of myelinated structures especially descending and ascending myelinated axons a These axons are arranged into the Dorsal Ventral and Lateral Columns 3 The Autonomic Nervous System a The ANS consist oftwo neurons extending from the CNS to the structure innervated 1 These two neurons will meetsynapse outside ofthe CNS in an autonomic ganglion and so are designated the Preganglionic Neuron and the Postganglionic Neuron a The Preganglionic Neuron 1 The preganglionic arises from the CNS the exact location depends on which division ofthe ANS the nerve belongs to 2 The preganglionic neuron is myelinated 3 The preganglionic neuron will synapse with the postganglionic neuron in an autonomic ganglion through the neurotransmitter Acetylcholine b The Postganglionic Neuron 1 The postganglionic neuron has it39s cell body located in autonomic ganglion and will extend it39s axon into the target organ 2 The postganglionic neuron will be unmyelinated 3 The division of the ANS that the postganglionic neuron belongs to will determine what type of neurotransmitter is used to communicate with the target organ b The ANS has two divisions which often service the same organ but produce radically different responses 1 The Parasympathetic Division a The parasympathetic nerves arise from the brain stem and the sacral portion of the spinal cord and so are called Craniosacral in their origin b This division uses acetylcholine as it39s postganglionictarget organ neurotransmitter 66 1 Acetylcholine promotes the quothouse keepingquot response c The parasympathetic ganglia are located near or within the structures innervated by their postganglionic axons 2 The Sympathetic Division a The sympathetic nerves arise only from the spinal cord from the thoracic and lumbar portions of the spinal cord 1 So it is referred to as being Thoracolumber in it39s origins a The perikarya of it39s preganglionic neurons are located in the lateral horns ofthe spinal cord b This division uses epinepherine and norepinepherine as it39s postganglionictarget organ neurotransmitter 1 Epinepherine and norepinepherine promotes the quotemergency responsequot c Their ganglia are located much closer to the spinal cord than are those of the parasympathetic division and display two patterns 1 Chain GangliaParavertebral Ganglia a These are paired ganglia running parallel on either side of the spinal cord 1 To be more exact they are ventroloateral to the spinal cord b A chain ganglion is attached to the one above and below it by nerve trunks 1 This forms the Sympathetic Chain 2 Prevertebral Ganglia Collateral Ganglia a These are singular ganglia located ventralanterior to the spinal cord 1 They are located much closer to the target than are the chain anglia b They will form nerve plexuses in the abdominopelvic region such as the celiac and hypogastric plexuses 4 The Histology of the Ganglion a All ganglia are contained within a connective tissue capsule that is continuous with the epineurium and perineurium ofthe nerves entering and exiting it 1 V thin the ganglion each perikaryon is surrounded by Satellite Cells the second ofthe two types of PNS supporting cells aSatellite cells separate the perikaryon from capillaries and so may serve to regulate metabolic exchange b This layer of satellite cells is continuous with the sheath of Schwann ofthe axon communicating with the neuron 2 External to the satellite cells is a basal lamina 3 External to the basal lamina is a connective tissue Capsule composed of collagen bers and Capsule Cells a The capsule cells are modi ed broblasts b The capsule is continuous with the endoneurium ofthat neuron39s axon b The Dorsal Root Ganglion 1 The perikarya of the dorsal root ganglion are ovoid in shape with centrally 67 located nuclei a These perikarya are located in the peripheral portion ofthe ganglion while their processes run through the central portion ofthe ganglion 2 The neurons ofthe dorsal root ganglion are unipolar a The axonlike portion oftheir singular process is myelinated and will enter into the spinal cord one of the ascending pathways b The dendritelike portion ofthe central process is also myelinated 1 The dendritic portion will terminate as a sensory receptor in either a the skin Somatic Afferent Terminal b or the a visceral organ Visceral Afferent Terminal 2 The dendritic portion will branch into dendritic endings so as to pick up stimuli along two patterns a as unencapsulatedfree dendritic endings which are receptive to temperature pressure or pain b or as encapsulated dendritic endings which are mechanoreceptors for the sensation of touch and pressure 0 The Autonomic Ganglia 1 Both parasympathetic and sympathetic ganglia contain multipolar neurons having a single unmyelinated postganglionic axon 2 Their perikarya contain acentrically orientated nuclei D The Cerebellum and Cerebrum ofthe Brain 1 The Cerebellum a The cerebellum is primarily concerned with coordination and the refinement of skeletal muscle activity the maintenance of equilibrium and the maintenance of muscle tone 1 It only in uences muscular activity It does not initiate it 2 New studies indicate that the cerebellum may also serve as an extra storage bank for memories particularly about movements b The Cerebellar Cortex 1 The cerebellar cortex is the convoluted outer layer ofgray matter surrounding a central core ofwhite matter populated by small clusters of gray matter a The central core ofwhite matter is called the Cerebellar White Matter and the small clusters of gray mater embedded in it are called Cerebellar NucleiGanglia b This is the arrangement of white to gray matter which we see in the quothigherquot portions ofthe brain 2 Histologically the cerebellar cortex is a trilaminar structure The three layers are the Granular Layer the Purkin39e Layer and the Molecular Layer a The Purkin39e Laye is the middle layer ofthis trilaminar arrangement 1 It consists of a single layer of flaskshaped cells called Purkin39e Cells a These Purkinje cells sit on top of the granular layer b The Purkinje cells have centrally orientated nuclei surrounded by 68 Nissl bodies 1 Their apical dendrite extends into the molecular layer where it will arborize into a many branched dendritic tree 2 Their singular axon extends through the underlying granular layer and into the cerebellar white matter a The axon is myelinated b This is the pathway by which the in uences ofthe cerebellar cortex are accomplished b The Granular Laye is the deepest layer ofthe cortex sitting on top of the white matter 1 It consists of a large population of densely packaged cells called Granule Cells a Granule cells have short dendrites which communicate with axons of the cerebellar white matter b Granule cells have a single long axon that extends super cially through the Purkinje layer and into the molecular layer 1 This axon is unmyelinated 2 In the molecular layer this axon bifurcates and establishes synapses with dendritic branches of Purkinje cells c The Molecular Laye is the super cial most layer of the cerebellar cortex 1 It consists mostly ofthe dendritic processes of Purkinje cells and the axons of granular cells 2 Amid these processes however there are two types of cells Basket Cells and Stellate Cells 2 The Cerebrum a This is by farthe largest and most highly evolved portion ofthe primate brain 1 It is responsible for all of our quothigher brain activitiesquot b The Cerebral Cortex 1 The cerebral cortex like the cerebellar cortex is the convoluted outer layer of gray matter surrounding a central core of white matter populated by small clusters of gray matter a The central core ofwhite matter is called the Cerebral White Matter and the small clusters of gray mater embedded in it are called CerebralBasal NucleiGanglia 2 The cerebral cortex is responsible for analyzing sensory information for initiating muscular activity and for learning memory and the association of information 3 The cerebral cortex contains three major types of nerve cells named for the shape of their perikarya Stellate Cells Fusiform Cells and Pvramidal Cells a Pyramidal cells are by farthe most prominent 1 These abundant cells have a pyramid shaped cell body a They have a large central nucleus surrounded by many Nissl bodies b It has an apical dendrite extending super cially towards the brain39s surface 69 c It has a basal axon that extends deeper into the brain into the cerebral white matter towards the spinal cord 1 This is the principle path for information traveling from the cerebral cortex to the rest ofthe CNS a It is the PyramidalCorticospinal Tract and is motor 2 Their are a variety of pyramidal cells recognized based on size and relative position in the cortex a The largest are the giant Betz Cells found in the motor cortex 4 The cerebral cortex is divided into six major layers of nerve cells a Pyramidal cells are located mostly in cortical layers II III V and VI b Each layer is categorized by the pyramidal cells present and by their degree of axonal and dendritic branching E The Neuroglia 1 Introduction a The neuroglia are the supporting cells of the CNS 1 They include the astrocytes oligodendrocytes microglia and ependymal cells 2 The suppo hg cells off7e PNS the Schwann and satellite cells have already been discussed 2 Astrocytes a Astrocytes are starshaped cells of the CNS b There are two types of astrocytes currently recognized although they may really both be the same type of cell 1 Fibrous Astrocytes are found primarily in white matter a They have several long infrequently branching cytoplasmic processes b They have an ovoid nucleus c They have many laments running throughout the cytoplasm called Glial Filaments 2 Protoplasmic Astrocytes are found primarily in gray matter a They have shorter more frequently branching cytoplasmic processes b They have an ovoid nucleus also c They have many laments running throughout the cytoplasm called Glial Filaments also c Astrocytes have many quot oot processesquot which are the dilated terminal portions of their cytoplasmic extensions 1 Some ofthese foot processes extend into the pia mater some extend to the neurons or their processes and some extend to the capillaries of the CNS d Functions 1 Astrocytes may play a role in the bloodbrain barrier a They may help to regulate the exchange of materials between the blood stream and the neurons 2 Astrocytes are now known to play a major role in neurotransmitter 70 regeneration 3 Oligodendrocytes a Oligodendrocytes have many cytoplasmic extensions which wrap around and insulate CNS neurons in myelin 1 They act as the Schwann cells ofthe CNS 4 Microglia a Microglia are odd cells 1 Their origin is still undetermined but it is known that they are not derived from neuroepithelium as are the other neuroglia b Appearance small cells with deeply staining nuclei and having a few sort cytoplasmic extensions c Functions 1 Microglia act as macrophages of the CNS a They phagocytosize foreign invaders and dead neurons 5 Ependymal Cells a Ependymal cells are derivatives of embryonic neuroepithelium and make up the Ependyma ofthe adult 1 The ependyma is a single layer ofependymal cells that lines the hollow spaces of the CNS ie the brain ventricles the central canals and all of the communicating passageways b Appearance ependymal cells will appear as either cuboidal or columnar ciliated epithelial cells depending on their location 1 The cilia are responsible for the circulation of CSF cerebrospinal uid a CSF is a plasma filtrate that nourishes and protects the CNS 1 CSF is found in all ofthe hollow spaces ofthe CNS and within the surrounding meninges 2 CSF is produced by special capillaries beds called the Choroid Plexus F A Comparison of Afferent and Efferent Nerve Endings 1 EfferentMotor Nerve Endings a Efferent nerve endings are axon terminals on synaptic contact with myo bers and secretory epithelial cells 1 They stimulate contraction and secretion respectivel 2 Their morphology was descrbeo under the neuromuscular junction 2 AfferentSensom Nerve Endings a Afferent nerve endings are dendritic terminals which act as sensory receptors b They are capable of the transduction ofvarious stimuli into nerve impulses that are sent back towards the CNS for processing and evaluation 1 These various stimuli include temperature touch pressure pain odor etc 2 Examples of sensory receptors include neuromuscular spindles Golgi tendon organs and special senses such as taste smell hearing and vision c Sensory receptors come in two classes 71 1 Complete Cell Receptor Cells these are neuroepithelial cells responsible for the detection of some ofthe special senses a They are typically modi ed columnar epithelial cells with elongate stereocilia for the detection of stimuli b They are in close association with a dendritic extension of a sensory neuron which will pick up the stimulus and conduct it towards the CNS c We Will discuss the complete cell receptor cells in more detail under the topic of the speca senses 2 Dendritic Endings these are the terminal portions of dendrites which directly pick up the stimulus They are divided into two morphologically distinct classes a FreeUnencapsulated Dendritic Endings 1 These dendritic endings are unmyelinated lacking Schwann cells and are not covered by a connective tissue capsule 2 They typically monitor the general senses such as temperature pain and pressure b Encapsulated Dendritic Endings 1 These dendritic endings occur at the terminus of myelinated dendrites but are themselves unmyelinated a They are covered however by a many layered connective tissue sheath 2 They also monitor general senses and often act as mechanoreceptors aThey occur frequently in the skin 1 EX Pacinian corpuscles and Meissner39s corpuscles Unit 7 THE LYMPHATIC SYSTEM A Introduction 1 General Comments a The primary functions of lymphoid organs are protective or immunologic in nature 1 They are the source of immunocompetent cells which are capable of neutralizing antigens to which the body has been exposed 2 The actions ofthe lymphatic system also include the actions of lymphocytes plasma cells and a variety of macrophages a These cells and their precursors form the primary cellular populations of lymphoid organs 1 As a result these cells are often referred to as Lymphoid Cells a Many however use the term lymphoid cell only for the lymphocytes and plasma cells not forthe macrophages b The term quotlymphatic systemquot includes not only the cells within distinct lymphoid organs but also the widely distributed cells found in circulation and in the loose connective tissues as well as those in certain epithelial tissues c The term quotimmune systemquot includes the lymphoid cells accessory cells such as macrophages and their secretory products 2 Developmental Overview a The origin ofthe stem cells for the lymphoid organs can be traced to mesenchyme 1 These stem cells are the hemocytoblasts 2 Soon after their development the vasculature will have become con uent and these multipotent cells will migrate to temporary depots such as the liver and spleen a There they will proliferate and differentiate along the various leucocyte lines 3 Later bone marrow becomes the predominant source ofthese stem cells a Red marrow maintains this capability throughout our lifetime b Late in fetal development many stem cells will migrate from the bone marrow into the PrimamCentral Lymphoid Organs 1 V thin the primary lymphoid organs special epithelially derived cells will induce the immigrant stem cells to proliferate and differentiate into immunocompetent stem cells 2 There are two primary lymphoid organs among endothermic vertebrates a Thymus this is the site of T cell maturation b Bursa of FabriciusBursa Eguivalent this is the site of B cell maturation 1 It39s actual location in higher mammals is still undetermined but may be Peyer39s patches liver red marrow lymph nodes or any combination 73 c Shortly after birth cells from the primary lymphoid organs migrate into the SecondamPeripheral Lymphoid Organs 1 The secondary lymphoid organs are the tonsils lymph nodes spleen and GALT gut associated lymphatic tissue 2 In the secondary lymphoid organs the T and B cells set up populations in fairly distinct zones ready for the immune response 3The Two Types Of Immunity a Although morphologically similar T and B lymphocytes are functionally dissimilar 1 T cells are primarily involved in cellmediated immunity 2 B cells and their derivatives the plasma cells are involved in humoral immunity also known as antibody mediated immunity b CellMediated Immunity 1 Activated T lymphocytes called Effector T Cells eliminate antigens either by attacking them directly or indirectly through the release of Lymphokines a Lymphokines are a class of chemical agents that will elicit a number of responses throughout the immune system 1 The effecter T cells that produce lymphokines are called Helper T Cells b Those effecter T cells that attack the antigen directly are called CytotoxicKiller T Cells 2 The primary distinguishing feature of cellmediated immunity is that specifically sensitized lymphocytes seek out the antigen a Contact with the antigen is required to trigger the reaction 1 This causes cellmediated immunity to be a localizedreaction 3 Among the effecter T cells are several varieties that can be distinguished from one another by unique cell surface molecules and by their specialized functions a Killer T Cells attack the antigen bearing agent directly 1 Some will have a speci c af nity for destroying tumor cells 2 Some will have a speci c af nity for fighting infections b Helper T Cells will secrete a variety of chemical agents such as lymphokines to enhance the immune response c Suppressor T Cells will secrete a variety of substances designed to suppress the immune response afterthe infection is under control c Humoral Immunity 1 Activated B cells and plasma cells secrete specific antibodies in response to specific antigens which will bind to those antigens forming inactive complexes 2 A distinguishing feature of humoral immunity is that contact between the lymphocyte and the antigen is not required to incapacitate or kill the antigen a Instead following sensitization speci c antibodies are secreted by plasma cells and distributed throughout the fluids ofthe body 1 The antibodies will seek out the antigens 2 Antibodies belong to a class of plasma proteins called 74 mmunoglobulins a There are ve distinct classes lgG lgA lgM lgE and lgD d Although certain antigens will trigger either a humoral immunity response or a cellmediated immunity response in most cases both immune response must work cooperatively 4 Lymphoid Tissue a The term quotlymphoid tissuequot refers to the parenchyma of lymphoid organs and of the loose connective tissues throughout the body in which lymphoid cells make up a large portion of the population 1 The connective tissue framework ofthese regions will generally consist of a reticular meshwork a The reticular meshwork consists of fine reticular fibers and reticular cells b Lymphoid tissue is usually described in terms of the relative densities of it39s lymphoid cell aggregates 1 Diffuse Lymphatic Tissue refers to a relatively loose aggregate of lymphoid cells 2 Nodular Lymphatic Tissue refers to a denser more organized tissue a A Nodule usually consists of a dense spherical aggregate of lymphocytes with a paler staining center surrounded by a darker staining corona 1 This paler staining center is called the Germinal Center a The germinal center will contain many large lymphocytes and lymphoblasts especially when active and displays a great amount of mitotic activity 2 The corona contains many small tightly packed lymphocytes c A Comparison ofthe Stroma and the Parenchvma Between Primarv and Secondam Lymphoid Organs 1 Introduction a Stroma the supporting framework of an organ 1 Typically the stroma is a connective tissue b Parenchyma the working tissue of an organ 2 Primam Lymphoid Organs a In primary lymphoid organs the stroma lacks reticular fibers and the reticular cells are of epithelial origin 1 These reticular cells are nonphagocytic a They are stellate shaped b They are believed to have an inductive effect on incoming stem cells from the bone marrow b The parenchyma is a diffuse lymphoid tissue 3 Secondam Lymphoid Organs a The stroma is rich in reticular fibers and the reticular cells are of mesenchymal origin 1 Most secondary lymphoid organ reticular cells are quite phagocytic but some are involved in reticular fiber synthesis and antigen trapping b The parenchyma is composed of both diffuse and nodular lymphoid tissue 75 1 Diffuse lymphoid tissue is predominantly occupied by T cells 2 Nodular lymphoid tissue is predominantly occupied by B cells B The Primary Lymphoid Organs 1 The Thymus a The thymus is a bilobed mass located in the midline ofthe mediastinum deep to the sternum 1 It attains it39s greatest relative weight and is at it39s most developed at about the time of birth a The thymus will begin to progressively degenerate with the onset of puberty 2 The thymus is the site of T cell maturation a T lymphocytes begin their development in the red marrow and will migrate into the thymus to mature b The Stroma 1 The stroma of the thymus includes a thin connective tissue Capsule which surrounds the organ a Extensions of the capsule will run into the thymus dividing it up into partial lobules 1 These connective tissue extensions are called Septa 2 To further partition the thymus smaller septa will radiate off of larger septa 2 The stellate shaped epithelialreticular cells will from an delicate inner framework a Though lacking the support of reticular bers these reticular cells are very branched and will connect to one another by desmosomes b The reticular cells ofthe thymus are of epithelial origin 1 In particular they develop from the endoderm 2 They re ect their epithelial origins in their secretory nature a They produce a variety of peptide hormones most of which regulate T cell development and maturation 3 These reticular cells are not phagocytic c The Parenchyma 1 The parenchyma within each lobule ofthe thymus is divided up into a Cortex and a Medulla a Since the lobules are incomplete the medulla is continuous between adjacent lobules 2 The cortex is composed primarily of aggregated lymphocytes called Thymocytes a However the density ofthese aggregates is not as great as that seen in nodular lymphoid tissue so this is still considered to be diffuse lymphoid tissue 3 The medulla will be lighter staining due to having fewer lymphocytes 76 a However the medulla does have more epithelialreticular cells than does the cortex b Scattered throughout the medulla are a third type of cell 1 These are concentrically arranged epithelial cells called Thymic Corpuscles a Their function is unknown 4 The parenchyma will also include macrophages in large numbers and in lower numbers mast cells plasma cells and granulocytes d The Vascular Supplv ofthe Thvmus 1 Vascularization of the thymus consists of small vessels that penetrate the capsule ramify in the interlobular ctsepta and enter into the parenchyma between the cortex and medulla a Capillaries will extend into the cortex from the corticomedullary zone of the lobules and will extensively anastomize with one another 1 These capillaries will drain into postcapillary venules and small veins of the medulla a The postcapillary venules are the route by which lymphoid cells can migrate into or out ofthe thymus 2 The capillaries of the cortex however are impervious to cells and macromolecules which means that they form the BloodThymus Barrier a The bloodthymus barrier prevents antigens from entering the cortex 1 Those antigens which enter into the medulla are phagocytized before they can spread into the cortex e Functional Aspects of the Thvmus 1 The cortex of a newborn39s thymus displays the extensive proliferation of lymphocytes a These proliferating cells are randomly arranged throughout the cortex b This extensive proliferation of lymphocytes is called Blastogenesis 1 Blastogenesis is driven by hormonelike inductive substances produced by the epithelialreticular cells a Over ten factors have been identi ed including thymosin thymopoietin and serum thymus factor 2 The thymus is the site of T lymphocyte maturation 3 At puberty the thymus begins to undergo a progressive degenerative process called lnvolution a Lymphocyte populations become greatly depleted and will largely be replaced by adipose tissue b The thymic corpuscles become greatly enlargened c The gradual age related degeneration ofthe thymus is termed A92 lnvolution 1 In the mature individual immunocompetence has been established and so the thymus is less signi cant a T lymphocytes are generally longlived cells which will migrate 77 extensively between the blood and the secondary lymphoid organs 2 The Bursa Eguivalent a In birds the B lymphocytes migrate from the bone marrow to mature in the bursa of Fabricius 1 In mammals an equivalent to this primary avian lymphoid organ exists but it has not been identified as yet a Candidates range from the spleen or the lymph nodes to the GALT or the diffuse lymphoid microenvironments of certain epithelia C The Secondary Lymphoid Organs 1 The Lymph Nodes a Lymph nodes are small encapsulated organs located along lymphatic vessels through which lymph uid ows 1 Lymph nodes serve to filter lymph and to add plasma cells to the lymph 2 They are located throughout the body but are most prominent in the axillary and inguinal areas b The Stroma 1 The lymph node is covered by a connective tissue capsule 2 Extensions called Trabeculae radiate from the capsule into the parenchyma of the lymph node 3 The lymph node also has a M a The hilus is an area of dense irregular connective tissue on the concave surface ofthe lymph node where lymph vessels exit the organ 1 These lymph vessels are called Efferent Lymphatics and are carrying lymph uid away from the lymph node a Lymph is carried to the lymph node by Efferent Lymphatics which penetrate the capsule on the convex surface ofthe organ 4 The inner stroma is composed of a delicate meshwork of reticular bers and reticular cells a These mesenchymal derived reticular cells are phagocytic and will also trap antigens c The Parenchyma 1 The parenchyma ofthe lymph node is divided into a cortex and a medulla a The cortex is composed primarily of nodular lymphoid tissue 1 It forms prominent nodules called Lymph Nodules having prominent germinal centers 2 Surrounding the lymph nodules is a small amount of scattered diffuse lymphoid tissue called the Paracortical Region 3 The paracortical region is predominantly occupied by T cells while the lymph nodules are mainly composed of B cells b The medulla is composed of diffuse lymphoid tissue arranged into cord or strandlike structures called Medullam StrandsCords 78 1 Typically macrophages are found in great numbers on around the medullary cords d Lymphatic Sinuses 1 Lymph enters the lymph node via the afferent Lymphatics and passes through the organ to the efferent lymphatics by a system of sinuses a Lymph first enter into the SubcapsularMarginal Sinuses located immediately below the capsule b It then percolates into the Cortical Sinuses c From the cortical sinuses it ows into the Medullam Sinuses d The medullary sinuses will drain into the efferent lymphatics at the hilus 2 Lymph will also ow through the diffuse lymphoid tissues ofthe lymph node but at a slower rate 3 Histology ofthe Sinuses a The walls of the sinus are composed ofa reticular ber meshwork b The sinuses are also lined with two types of cells macrophages and nonphagocytic endothelial cells 1 This lining is discontinuous e Functional Aspects of the Lymph Node 1 Lymph borne antigens from the body39s connective tissues are transported by lymph vessels to the lymph nodes where they will induce immune activity a During a peak humoral immune response plasma cells will be found in the medullary cords in large numbers b Typically the antigens will be phagocytized by phagocytic cells ofthe lymph node 2 Many lymphoid cells will enter the lymph node sinuses by way ofthe afferent lymphatics and will exit the lymph node by way of the efferent lymphatics a They will migrate to other lymphoid organs and to connective tissue spaces b Some cells particularly memory cells will recirculate back to the lymph nodes 2 The Spleen a The spleen is the largest lymphoid organ 1 It is located in the left hypochondriac region ofthe abdominal cavity 2 The spleen is interposed in the systemic circulation and so serves to lter blood a It also serves as a source of immunocompetent cells and even has some nonimmunological functions 1 In the fetus it serves as a hemopoietic organ 2 The spleen is a reservoir of blood 3 Along with ltering out antigens the spleen removes senescent blood cells b The Stroma 1 The spleen is encapsulated by a thick capsule of dense irregular connective tissue a Peritoneum will cover the free surfaces of the spleen external to the 79 capsule 1 This is the case for all ofthe organs in the abdominopelvic cavity 2 Trabeculae branch off ofthe capsule and extend into the spleen a Both the trabecular and capsular connective tissues will contain some elastic fibers and smooth muscle 1 This is to allow the spleen to expand to store blood and to contract to release this blood back into circulation 3 The M will allow for passage ofthe splenic artery and splenic vein 4 The inner stroma is composed of reticular bers and reticulocytes c The Parenchyma 1 The parenchyma ofthe spleen is composed of lymphoid tissue called the Splenic Pulp which is divided into the White Pulp and the Red Pulp a White pulp is composed of both diffuse and nodular lymphoid tissue 1 It is organized around the arteries of the parenchyma a Diffuse lymphoid tissue forms a sheath around the artery called the Periarterial Lymphocyte Sheath b Nodular lymphoid tissue surrounds this periarterial sheath 1 These nodules are called Splenic Nodules b Red pulp is composed of diffuse lymphoid tissue surrounding anastomizing venous sinuses 1 The lymphoid tissue ofthe red pulp is arranged into strands between the venous sinuses called Splenic CordsStrands 2 Between the red and white pulp are poorly defined regions called Marginal Zones a Marginal zones receive much of the blood entering the spleen d The Organization of the pleen in Reference to the Vasculature 1 The splenic artery enters the spleen at the hilus and then branches a These branches are called Trabecular Arteries and they will travel with the trabeculae into the spleen b Central Arteries will branch off of the trabecular arteries branching out of the trabeculae and into the parenchyma 1 The central arteries will have their outermost tunic the tunica adventitia be largely replaced by reticular bers which are highly in ltrated with lymphocytes a This cuff of lymphocytes surrounding each central artery is the periarterial lymphocyte sheath 1 Surrounding the sheath will be a large nodule of nodular lymphatic tissue the splenic nodule with pronounced germinal centers c The central arteries branch into many smaller vessels which service the capillaries of the marginal zones 1 The central arteries finally terminate in the red pulp as highly branched small vessels called Penicilli 2 The cells lining the venous sinuses are discontinuous and so present little barrier to the movement of cells between the red pulp and the venous 80 sinuses a These venous sinuses anastomize freely b The venous sinuses are drained by the Red Pulp Veins c The red pulp veins will drain into the Trabecular Veins which will travel with the trabeculae out ofthe parenchyma d The red pulp veins will drain into the splenic vein at the hilus e Functional Aspects ofthe Spleen 1 The dense reticular meshwork in the marginal zones and in the red pulp cords functions as an effective trap for antigens and for senescent blood cells a These regions contain large numbers of macrophages and phagocytic reticular cells for the removal oftrapped cells and antigens 1 The cells lining the venous sinuses are nonphagocytic 2 As an effector organ in immunity the spleen has much in common with the lymph nodes a The diffuse lymphoid tissue of the spleen is predominantly populated by T lymphocytes b The nodular lymphoid tissue of the spleen is predominantly populated by B lymphocytes 3 Gut Associated Lymphatic Tissue GALT a Introduction 1 The lamina propria of the digestive tube and ofthe respiratory tract is often considered to be a lymphoid tissue since it is rich in lymphoid cells and has a reticular stroma a Most of the plasma cells of this area secrete the antibody class lgA which is very effective in preventing bacterial and viral invasion b At different portions ofthe digestive tube the lamina propria is greatly enlargened by the presence of more highly organized lymphoid tissue 1 These tissue expansions form the tonsils Peyer39s patches appendix and solitary lymph nodules scattered throughout the entire tract but most numerous in the colon 2 Like a typical secondary lymphoid organ the organs of GALT also are composed of both diffuse and nodular lymphoid tissue a Their cellular proliferation is antigen driven 1 Particularly by ingested antigens b lmmunocompetent cells will recirculate through postcapillary venules in diffuse lymphoid tissue 3 These organs maintain a close association with an epithelial tissue which is also highly in ltrated by lymphocytes b The epithelium associated with nodules of GALT called GALT NodulesGALT Follicles is histologically distinct from the surrounding epithelium and is called Follicular Epithelium 1 Follicular epithelium is specialized so as to allow for the enhanced take up of lumenal particles and microorganisms a This is accomplished by a localized decrease in the epithelial 81 barrier b This will stimulate the response ofthe underlying lymphocytes and macrophages b The Tonsils 1 The tonsils from a ring of lymphatic tissue around the entrance into the throat a The tonsils consist oftwo paired structures the Palatine and Lingual Tonsils and a singular Pharyngeal Tonsil 1 The tonsils are overall quite similar with only slight differences between them 2 The lymphoid tissue oftonsils will contain nodules having unusually large germinal centers surrounded by diffuse lymphoid tissue a In many ways it looks much like the cortex ofa lymph node b The tonsils will have a scanty capsule located deep in the lamina propria 1 The capsule will have short trabeculae radiating off from it c The inner stroma ofthe tonsil will be composed of reticular bers and reticulocytes 3 The tonsils are closely associated with the overlying epithelium a The type of epithelium present depends on the tonsil 1 The pharyngeal tonsil is covered by pseudostrati ed ciliated columnar epithelium with goblet cells 2 The lingual and palatine tonsils are covered by stratified squamous nonkeratinized epithelium b The epithelium is highly infolded and forms deep Cmpts 1 The trabeculae ofthe tonsil are orientated towards these crypts 2 A number of small glands are associated with this region a Their ducts will open into the bases ofthe crypts b Skeletal muscle is located immediately below these glands c The Peyer39s Patches 1 Peyer39s patches are also known as aggregated nodules and are found tin the ileum of the small intestine 2 The Peyer39s patch will consist of between about 30 to 40 nodules a They are histologically similar to the tonsils b They are typically con ned to the lamina propria 1 However some of the larger Peyer39s patches may extend into the surrounding tissue layers ofthe ileum a The largest ones will even bulge into the lumen attening the villi in this area c The nodules are interspersed with diffuse lymphoid tissue 3 The follicular epithelium of Peyer39s patches is highly specialized a These are areas where the villi are often absent b While most ofthe constituent cells of follicular epithelium are the simple columnar epithelial cells typical forthe small intestine there are some modi ed cells 1 These unique epithelial cells have cytoplasmic extensions forming bridges between them 82 a This allows these cells to trap antigens from the intestinal lumen and to present them to the cells of the underlying lymph nodules b Macrophages and lymphocytes are often found in close association with these modified epithelial cells 1 Together they form structures called lntraepithelial Lymphoid Nests d The Appendix 1 The appendix is a blind evagination of the cecum located near the ileocecal junction 2 The lamina propria of the appendix is composed entirely of lymphoid tissue similar histologically to the tonsils and Peyer39s patches 3 In certain cases there are crypts associated with the appendix but they are often absent or poorly developed D The Lymph Vessels 1 Lymph vessels serve as a drainage system for interstitial uid returning it to the blood stream a Interstitial uid is a ltrate of plasma and leaks out ofthe capillaries into the interstitial spaces 1 It is not picked up by the postcapillary venules since it may now carry pathogenic agents which have entered the connective tissue spaces a During lymph circulation lymph is not only returned to the blood stream but also is cleansed of foreign invaders and augmented by antibodies and immunocompetent cells to deal with any invaders 2 Lymph Vessels a Lymph circulation begins with the Lymph Capillaries 1 Lymph capillaries are blind ended microscopic tubes which will drain the interstitial uid from the interstitial spaces 2 The lymph capillary is composed of endothelium a simple squamous epithelium and Lymphatic Anchoring Filaments a The lymphatic anchoring laments are extracellular laments which terminate in the basal lamina ofthe endothelium 1 They give structural support to these thin vessels b The endothelial cells will slightly overlap forming Minivalves to prevent the back ow of lymph 3 Unlike blood capillaries lymph capillaries are more variable in size and shape and usually have a larger lumenal diameter a Also they branch and anastomize more extensively b Lymph capillaries will drain into larger vessels which in turn will drain into larger lymph vessels called Lymphatics 1 Lymphatics are similar to veins except a They have larger lumens b They have simpler walls 1 The walls are composed ofendothelium a few smooth muscle cells and a few bundles of collagen and elastic fibers c They have many more valves 2 Periodically along the lymphatics will be lymph nodes forthe cleansing of the lymph and the addition of immunocompetent cells c The lymphatics will drain into larger vessels called Lymph Ducts 1 The largest of the lymph ducts are the Right and Left Lymphatic Ducts a The left lymphatic duct is also known as the Thoracic Duct 1 It is much larger than is the right lymphatic duct since it drains a greater portion of the body alt drains lymph from all regions of the body inferiorto the diaphragm and the left portion ofthe body superior to the diaphragm b The right lymphatic duct drains lymph from only the right portion of the body superior to the diaphragm c The right and left lymphatics will drain into the right and left subclavian veins respectively d Structure 1 The lumen is lined by endothelium 2 The next layer is a thicker component of smooth muscle than was found in lymphatics and also includes some elastic fibers 3 The outer layer ofthe lymphatic duct is very similar to the outer layer of a vein the tunica adventitia a It has a heavy connective tissue component and a longitudinal array of smooth muscle bers 3 Lymph fluid is not pumped through the lymph vessels a Lymph fluid is moved through lymph circulation by contractions of surrounding skeletal muscles Unit 8 THE CIRCULATORY SYSTEM A Introduction 1 The circulatory system consists of a muscular pulsing heart and a system of blood vessels a The blood vessels include 1 Arteries which will carry the blood and it39s dissolved constituents from the heart to the capillaries 2 Capillaries which will serve as the site of exchange between the tissues of the body and the blood stream a In the capillary beds oxygen and nutrients are delivered to the tissues and wastes are removed from the tissues 3 Veins which will drain the capillary beds and return the blood to the heart B The Heart 1 Introduction a The heart is a muscular organ consisting of four pumping chambers 1 The Right Atrium receives blood from the superior and inferior vena cava and sends it to the right ventricle a This blood is deoxygenated and from systemic circulation 2 The Right Ventricle receives blood from the right atrium and will pump it into the pulmonary trunk so that it can enter pulmonary circulation a This blood will travel to the lungs where it will dump carbon dioxide and pick up oxygen where it will become oxygenated 3 The Left Atrium receives this oxygenated blood from the pulmonary veins and sends it to the left ventricle 4 The Left Ventricle receives blood from the left atrium and pumps it into the aorta so that it can enter systemic circulation 2 The Three Lavers ofthe Heart Wall a Endocardium 1 The endocardium is the innermost layer of the heart wall a It forms the internal lining of the atria and ventricles 2 Composition a The endocardium has a layer of Endothelium a simple squamous epithelium 1 This endothelium is continuous with the endothelium lining all of the blood vessels of the circulatory system 2 Along with lining the chambers this endothelium also lines the heart valves the chordae tendineae and the papillary muscles 85 a These heart valves are the Right AtrioventricularTricuspid Valve the Left AtrioventricularBicuspidMitral Valve the Aortic Semilunar Valve and the Pulmonam Semilunar Valve 1 They are connective tissue structures that serve to prevent the back ow of blood b Deep to the endothelium is a dense irregular connective tissue layer consisting ofelastic and collagen bers 1 This dense irregular connective tissue layer is often called the Subendothelial Connective Tissue 2 This dense irregular connective tissue forms the Cardiac Skeleton a The cardiac skeleton forms 1 rings around the AV valves called Annuli Fibrosi 2 reinforcement forthe origins ofthe aorta and the pulmonary trunk 3 the inner core ofthe interventricular septum 4 and the central core of the heart valves a The core ofthe AV valves are dense collagen bundles 1 Extensions ofthese collagen bundles form the chordae tendineae a The chordae tendineae attach the AV valves to the papillary muscles and prevent them from prolapsing during ventricular systole 1 The papillary muscles are conical projections of smooth muscle in the ventricles b The semilunar valves have a strong component of elastic bers in their cores particularly on the ventricular aspect 1 These elastic bers help them to do their job 0 Supporting this layer of dense irregular connective tissue is the Subendocardium 1 The subendocardium is an irregular layer composed of adipose and irregularly arranged bundles of collagen 2 Branches of the coronary blood vessels will travel through the subendocardium 3 Deeper portions ofthe subendocardium may contain bundles of smooth muscle or elements of the cardiac conduction system a This portion ofthe subendocardium is thicker in the atria than in the ventricles 1 It contains the musculi pectinati b Myocardium 1 The myocardium is the middle and by farthe thickest layer of the heart wall a It is responsible forthe pumping action of the heart b It is ofvarying thickness based on functional needs and is responsible for the disparate sizes ofthe heart chambers 2 It is a muscular layer composed primarily of cardiac myo bers whose endomysia are anchored into the subendocardium and to each other 86 a The endomysium of cardiac myo bers may or may not contain some elastic fibers 1 They do in the atrial myocardia 2 They don39t in the ventricular myocardia b The more superficial cardiac myo bers will have their endomysia inserting into the super cialmost layer of the heart wall the epicardium 0 Most of the cardiac myofibers are anchored to each other by their intercalated discs 1 Some will insert into the cardiac skeleton ofthe heart 3 Medium sized coronary blood vessels will penetrate the myocardium c Epicardium 1 The epicardium covers the outer aspects ofthe heart 2 Composition a The super cialmost portion ofthe epicardium is the Visceral Pericardium 1 The visceral pericardium is a serous membrane a A serous membrane consists of a layer of mesothelium a simple squamous epithelium sitting on a thin layer of loose connective tissue b The mesothelium is secretory producing a serous uid called Pericardial Fluid 2 The visceral pericardium is contiguous with at least part of the Parietal Pericardium a Togetherthe visceral and parietal pericardia form the Pericardium c The parietal pericardium has two layers 1 It has a delicate inner layer called the Serous Layer which is contiguous with the visceral pericardium and also secretes pericardial uid 2 It has a tough fibrous outer layer composed of dense irregular connective tissue called the Fibrous layer 3 Between the visceral and parietal pericardia is a uid lled space called the Percardial SpaceCavity b Deep to the visceral pericardium is a thin layer of dense irregular connective tissue 0 Immediately deep to this brous layer is an irregular layer composed of loose connective tissue containing adipose and the larger coronary blood vessels 1 The connective tissue fibers ofthe this layer are continuous with the endomysia ofthe superficialmost myofibers ofthe myocardium 2 The adipose tissue of this layer is often found in close association with the coronary blood vessels 3 The Cardiac Conduction System a The cardiac conduction system consists of modi ed cardiac muscle cells called Nodal TissueNodal Fibers 1 Cardiac myo bers have the capability of contracting independently called lnherent Rhythmicity 87 a They coordinate the heartbeat through the intercalated discs 2 Nodal tissue is modi ed cardiac myo bers a They take inherent rhythmicity a step further being more nervouslike in it39s ability to initiate a wave of depolarization that will stimulate the contraction ofthe entire myocardium ofthe heart chamber b Overall nodal tissue resembles cardiac myo bers under the light microscope b The Components of The Cardiac Conduction System 1 The Sinoatrial Node a mass of nodal tissue located in the right atrium near the opening ofthe superior vena cava that will stimulate atrial systole in both atria simultaneously 2 The Atrioventricular Node a mass of nodal tissue located in the right atrium near the right AV valve that will stimulate ventricular systole in both ventricles simultaneously a It is stimulated by the SA node b Due to the greater thickness ofthe ventricular myocardia the distribution of the depolarization initiated by the AV node needs some assistance 3 The Bundle of HisAtrioventricular Bundle a brous mass of nodal tissue located in the upper interventricular septum that receives the depolarization from the AV node and carries it into the interventricular septum 4 Bundle Branches branches of the atrioventricular bundle that branch off of it at about the middle of the septum and carry the depolarization to the apices of the ventricles 5 Purkin39e Fibers branches ofthe bundle branches which carry the impulse from the apices to the lateral walls ofthe ventricles a This allows the entire muscle mass of both ventricles to contract as a singular coherent unit B The Blood Vessels 1 The wall ofthe quottypicalquot blood vessel called the Vascular Wall is divided into three tunics however their nature will vary between the three classes of blood vessel a Tunica lntimaTunica lnterna the innermost tunic it lines the lumen and is in direct contact with the blood 1 The tunica intima consists of a Endothelium a layer of simple squamous epithelium continuous throughout the circulatory system b Subendothelial Connective Tissue a layer of loose connective tissue b Tunica Media the middle tunic 1 It typically consists of smooth muscle and variable amounts offdiffering connective tissue components c Tunica AdventitiaTunica Externa the outermost tunic 1 It typically consists of connective tissue which will serve to anchor the blood 88 vessel to surrounding structures 2 The tunica adventitia of larger blood vessels will be perforated by numerous small blood vessels which service the tissues of the vessel a These vessels will penetrate the tunica media but will not enter into the tunica intima b These small vessels servicing the tunics of larger vessels are called the Vasa Vasorum 1 The vasa vasorum includes small arteries small veins and capillaries 2 The Arteries a Introduction 1 Arteries are the vessels which will carry blood from the heart towards the tissues of the body a As a result they will carry blood at it39s highest pressure and this is reflected in the structure of their vascular wall 1 They are the thickest walled blood vessels 2 There are three classes of arteries elastic arteries muscular arteries and arterioles b ElasticConducting Arteries 1 Elastic arteries are the largest arteries of the body and the rst to receive blood from the heart a EX aorta brachiocephalic a common carotid a common iliac a pulmonary trunk and pulmonary a 2 Structure a Tunica lntima endothelium and a thin layer of subendothelial connective tissue which will progressively thicken with age b Tunica Media it is the thickest layer and is rich in elastic bers 1 It consists of bands of elastic membranes called Elastic Laminae alternating with bands of smooth muscle cells and a limited amount of collagen 2 The elastic component allows theses arteries to expand when receiving blood and to recoil to help propel the blood c Tunica Adventitia contains mostly longitudinally arranged bands of collagen 1 ln larger elastic arteries this connective tissue layer will also contain some elastic fibers c MuscularDistributing Arteries 1 Muscular arteries are branches of elastic arteries and could be described as quotmedium sized arteriesquot a EX splenic and brachial b Due to their high muscle component these arteries are proportionately the thickest walled of all the arteries and therefore ofall blood vessels 2 Features of Muscular Arteries a One diagnostic feature of muscular arteries is that their tunica media has predominantly circularlyspirally arranged smooth myo bers forming a 89 discrete muscular compartment b Another diagnostic feature found in all muscular arteries is a well developed lnternal Elastic MembraneLamina 1 The internal elastic membrane is located between the tunica intima and the tunica media 2 The internal elastic membrane is often considered to be a portion of the tunica intima a However since the elastic tissue is most likely synthesized by the smooth muscle myofibers ofthe tunica media it is actually a portion ofthe tunica media 1 In fact the tunica media of the larger muscular arteries will contain a few scattered elastic lamina throughout 2 The smooth muscle cells will also produce a few reticular bers and the proteoglycans of the tunica media c The tunica adventitia will also contain a series ofthinner elastic lamina called the External Elastic LaminaMembrane d The tunica adventitia is dominated by abundant longitudinally arranged bundles of collagen and occasional bundles of smooth muscle within the inner adventitia 1 The collagen will reinforce the walls to withstand high blood pressure 2 The smooth muscle is restricted to the inner adventitia 3 Vasomotor nerve bers will enter the adventitia a Their ganglia are also located in the adventitia b From the adventitia the nerves will penetrate the tunica media where they will innervate the smooth muscle so as to regulate vasoconstriction and vasodilation 1 These nerves will innervate only the outermost layers of smooth muscle a Smooth myofibers will be coupled by gap junctions so all cells will receive the impulse 4 The tunica adventitia can be quite thick in some muscular arteries but not as thick as the tunica media d Arterioles 1 Arterioles are the smallest of the arteries a They are generally between 20 um to 100 um in diameter b The arterioles will deliver blood to the capillaries 1 They also serve to regulate the ow of blood into the capillary beds 2 Structure of the Typical Arteriole a Arterioles have a well developed internal elastic membrane separating the tunica intima and the tunica media 1 It gives a distinctive crenelated appearance to the tunica intima b The tunica media of arterioles is composed of circularly arranged smooth muscle which will regulate the ow of blood to the capillaries 1 A few small unmyelinated axons ofvasomotor nerves innervate the outer circumference of the tunica media 90 c The tunica adventitia is usually poorly developed with little or no external elastic membrane component 3 Terminal Arterioles a Terminal arterioles are the arterioles which give rise to the capillaries 1 Theirtunica media is reduced to a cuff of smooth muscle located near the origination ofthe capillaries called the Precapillam Sphincter a The precapillary sphincter controls the ow of blood into the capillary bed 2 The tunica adventitia is extremely reduced b Along with giving rise to capillary branches terminal arterioles will give rise to another type of branch 1 This branch is called the Metarteriole a The metarteriole will branch off of the terminal arteriole close to the capillary origin b It will give rise to it39s own capillaries which will anastomize with the capillaries branching off ofthe terminal arteriole 1 The ow of blood into these capillaries is also regulated by precapillary sphincters c The metarteriole will also drain directly into a venule 4 Atriovenous Anastomes these are shunts between arterioles which do not branch into capillaries and venules a They function to divert blood away from capillary bed when required 1 Ex There are many atriovenous anastomes in the skin to aid in thermoregulation 3 The Capillaries a Capillaries are the smallest class of blood vessels and serve as the site of exchange between the blood and the tissues ofthe body 1 They are between 8 um to 10 um in diameter 2 To promote this exchange via diffusion the capillary wall is very thin and simple in it39s construction a It lacks the tunica adventitia media and most of the intima b The wall is composed of endothelium supported by a basal lamina and very few very scattered reticular fibers 1 To further increase the thinness ofthis wall the endothelial cell nuclei are elongated and aligned longitudinally along the vessel 2 Occasionally associated with the capillary wall is a scattered population of PericytesPerivascular Cells a Pericytes have cytoplasmic processes which will surround the capillary wall b Pericytes are multipotent stem cells capable ofdifferentiating into vascular smooth myo bers broblasts and a variety of other connective tissue cells b The Three Classes of Capillaries based on structural modi cations ofthe endothelium to facilitate the escape of the plasma ltrate into the interstitium 91 1 Continuous Capillaries a These are the most numerous and most widely distributed class of capillary 1 They are the quottypical capillaryquot b The endothelium making up the wall ofthe continuous capillary is the same type as the endothelium lining arteries and veins 1 The endothelial cells are joined to one another by fascia occludens a This incomplete junction permits the escape of uid into the interstitium b In certain specialized less permeable continuous capillaries such as those making up the blood brain barrier the endothelial cells are joined by a tight junction a zona occludens instead 1 These borders completely seal the contiguous borders of adjacent endothelial cells and so make the vessel less permeable 2 Plasmalemmal Vesicles invaginate both the lumenal and the perivascular surfaces of the endothelial cells for the transport of materials across the vascular wall 2 FenestratedDiscontinuous Capillaries a These capillaries have pores also called Fenestra in their endothelial walls 1 Fenestra are regions where the cytoplasm has been excluded from between the lumenal and perivascular cell membranes a This modi cation makes fenestrated capillaries more permeable than are continuous capillaries and so are found in areas requiring a greater degree of permeability 1 EX intestinal villi endocrine glands the kidneys b Typically the cell membranes remain and form a Diaphragm over the fenestrum 1 In some cases where even more permeability is required even this thin diaphragm is lost a These are called Modi ed Fenestrated Capillaries b EX the glomerular capillaries of the kidney where the lack of diaphragms aids in the ltration of blood and the formation of urine 3 Sinusoids a Sinusoids are odd microscopic blood vessels 1 Some considerthem not to be capillaries but a class in their own right a They have a larger lumenal diameter than do other capillaries b They are irregular in shape and lack the normal tubular arrangement ofa blood vessel 0 Along with endothelium the vascular wall also has macrophages 1 EX the Kupffer cells of the liver sinusoids b Sinusoids have large gaps between their endothelial cells 1 These gaps are guarded by macrophages 92 a The macrophages serve to remove foreign matter debris etc 2 These gaps will not only allow for the passage of dissolved substances and fluids but also formed elements in some cases 4 The Veins a Veins carry blood from the capillary beds to the heart 1 The blood is rst picked up by postcapillary venules and then flows into collecting venules muscular venules small veins medium veins and large veins 2 The veins carry blood at a much lower pressure than do the arteries and so have thinner walls with less muscle and less elastic fibers b Venules 1 Venules are the smallest class ofvein and serve to drain the capillary beds a Those venules closest to the capillary bed are structurally quite similar to capillaries 1 As the blood drains into larger venules the walls of the venules become increasingly more complex and will begin to demonstrate the typical three tunic arrangement 2 Postcapillam Venules a Postcapillary venules are the smallest venules and the simplest in construction b Postcapillary venules serve two purposes 1 They recover some of the fluid lost from the capillaries a However most of this uid is recovered by the lymph capillaries 2 They allow for the passage of leucocytes a Leucocytes will exit circulation by squeezing through the postcapillary venule wall to reach the site of infection b This can be augmented by the release of in ammatory agents such as histamine by mast cells and basophils c The wall ofthe postcapillary venule consists of 1 endothelium 2 a basement membrane 3 a very small amount of collagen 4 and frequently there will be pericytes associated with postcapillary venules a Pericytes will also be associated with the larger venules 3 Collecting Venules a Collecting venules receive blood from postcapillary venules 1 They have thicker walls due to a greater complement of collagen a They have a tunica intima of endothelium and a basement membrane b They have a tunica adventitia made up of collagen 4 Muscular Venules a Muscular venules drain the collecting venules 1 They have even thicker walls a They have a tunica intima of endothelium and a basement 93 membrane b They have a tunica media made up ofa single layer of smooth muscle cells from which they get their name 0 They have a tunica adventitia made up of collagen that is thicker than that ofthe collecting venule 0 True Veins 1 Small and Medium Sized Veins a The tunica intima is similar to that of a similar sized vein consisting of endothelium and a subendothelial connective tissue layer b Small and medium veins are more muscularthan are venules with increasingly thicker bundles of smooth muscle forming the tunica media 1 These bundles of myo bers however are interspersed with collagen bundles a So veins are less muscular than are similar sized arteries 1 Also the muscular component of veins is less consolidated than it is in arteries 0 The tunica adventitia is distinctly thicker than is the tunica media in these vessels 1 The tunica adventitia is composed of longitudinally arranged collagen bundles and lesser amounts of elastic bers d Small and medium sized veins will have valves 1 This is particularly true for veins of the limbs 2 These valves prevent the back flow of blood 3 These valves are semilunar folds ofthe tunica intima a They will lay flush against the vessel wall during normal blood ow 2 Large Veins a Structural Specializations 1 Larger veins have a somewhat thicker subendothelial connective tissue layer a They have valves also 2 The tunica media is poorly developed 3 The tunica adventitia is by far the thickest layer a The inner aspect ofthe tunica adventitia consists of longitudinally arranged smooth muscle bundles b The outer aspect consists of longitudinally arranged bundles of collagen with some elastic bers b Sinuses A sinus is a highly modi ed large vein 1 It has a tunica intima but it39s tunica media and tunica adventitia are replaced by the surrounding tissues ofthe organ it is servicing 2 EX coronary sinus intercranial sinus Unit 9 THE RESPIRATORY SYSTEM A Introduction 1 The respiratory system provides the body with oxygen and rids the body of carbon dioxide the waste product of metabolism a The circulatory system works in close association with the respiratory system by providing a medium for the transport of these gases throughout the body 2 The respiratory system proper can be divided into two portions a The Conducting Portion 1 The conducting portion consists of the nasal cavity pharynx larynx trachea and the bronchial tree 2 It serves to conduct air from the environment to the respiratory portion ofthe lungs b The Respiratom Portion 1 The respiratory portion consists ofthe alveoli a The alveoli arise off of branches of the bronchial tree called bronchioles 2 Due to their extremely thin walls it is the alveoli which serve as the site of external respiration a External respiration is the exchange of gases between the lungs and the blood 3 In addition to the conducting and respiratory portions the respiratory system also has a ventilation system designed to move air through the respiratory system proper a This ventilation system is composed of the walls of the thoracic cavity and the diaphragm B The Conducting Portion 1 General Comments a The conducting portion which extends from the nasal cavity to the bronchi is characterized by a lining of pseudostrati ed ciliated columnar epithelium with goblet cells sitting on a lamina propria rich in lymphoid cells and seromucus glands 1 As the conducting passageways branch into smaller branches there are some changes which occur a The seromucus glands gradually disappear b The height of the epithelium decreases to a ciliated simple columnar and eventually to a simple cuboidal epithelium b The lining ofthe conducting portion is designed to perform three actions on air entering the respiratory system warm lter and moisten 95 1 Secretions of the seromucus glands provide the first line of defense against pathogens humidify the air and detoxify soluble gases a Their secretions coat the surface of the epithelium and so can trap contaminants 1 These debris laden mucus us removed by means of Mucociliam Clearance a The cilia of the columnar cells beat and move the debris laden mucus towards the pharynx for elimination 2 The lamina propria is well vascularized a These blood vessels will radiate heat so as to warm the nasal cavity and thereby the air b The lamina propria is also rich in lymphoid cells 1 These lymphoid cells form diffuse aggregates which can often permeate the above lying epithelium 2 Some ofthese lymphoid cells will produce lgA which will be transported across the epithelium to kill bacteria and viruses c Changes in air pressure occur within the conducting portion during inspiration and expiration 1 Bone and cartilage will provide a degree of rigidity so as to withstand pressure differences a The bone is found in the walls of the nasal cavity b The cartilage is found in the larynx trachea bronchi and bronchioles 2 Elastic fibers provide a degree of exibility to allow for changes in the lengths of conducting tubes during breathing 3 Smooth muscle in the walls of bronchial tree will provide forthe regulation of aeration volume in response to the body39s needs a Parasympathetic stimulation causes the muscle to contract which decreases lumenal diameter b Sympathetic stimulation causes the muscle to relax which increases lumenal diameter 2 The Nasal Cavity a General Comments 1 The nasal cavity is divided into the right and left nasal cavity by the nasal septum a The right and left nasal cavities are further divided into three groovelike passageways called the nasal meatuses by bony projections called conchae 2 The nasal cavity is separated from the oral cavity by the hard and soft palates 3 The nasal cavity communicates with the external environment by the External Nares and with the pharynx by the Internal Nares 4 The nasal cavity is lined by two types of mucosa respiratory mucosa and olfactory mucosa b The Respiratom Mucosa 1 The respiratory mucosa lines the bulk ofthe internal nasal cavity 96 2 The respiratory mucosa is composed of pseudostratified ciliated columnar epithelium with goblet cells sitting on a well developed lamina propria a The goblet cells are abundant and randomly distributed throughout the epithelium b Numerous branched tubuloalveolar seromucus glands extend into the lamina propria 1 These glands will resemble small seromucus salivary glands 3 Small blood vessels are abundant in the lamina propria a These blood vessels will warm the inhaled air and allow for the passage and transport of various leucocytes b The network of capillaries directly beneath the epithelium is made up of fenestrated capillaries so as to allow for the rapid diffusion of materials 1 This is generally the case throughout much of the respiratory passageways c Veins and arteries are in close association and atriovenous anastomoses are common in this region 4 Lymphatic drainage is well developed c The Olfactory Mucosa 1 The olfactory mucosa is limited in humans to the roof ofthe nasal cavity and a small portion of the superior septum 2 The glands of the olfactory mucosa are only serous a The serous uid serves as a solvent for olfactory chemicals 1 Olfaction is chemoreceptive sense and requires the chemicals to be in solution for detection 3 The pseudostrati ed ciliated columnar epithelium of this region is thicker than it is in respiratory mucosa a The specialized cells ofthe olfactory mucosa are the Olfactom Cells 1 Olfactory cells are bipolar neurons which will detect olfactory stimuli a The dendrite extends to the surface ofthe epithelium and terminates in a swollen structure called the Olfactom Knob 1 Several long stereocilia will extend from the knob onto the surface of the epithelium so as to pick up olfactory stimuli b The axon will extend basally and penetrate the basement membrane into the lamina propria 1 In the lamina propria axons will join togetherto form fascicles 2 In turn these fascicles will join forming about 20 bundles of olfactory nerves called the Filia Olfactoria 3 The lia olfactoria will pass through the olfactory foramina of the cribiform plate to enterthe brain b Surrounding the olfactory cells are ciliated columnar epithelial cells called SustentacularSupporting Cells 1 These cells have apically oriented nuclei c The olfactory mucosa possess a third type of cell the Basal Cell 1 Basal cells are short round cells located at the basal lamina 2 Basal cells are undifferentiated and can give rise to ciliated columanr 97 cells of goblet cells 3 The Phamnx a The pharynx is divided into three portions nasopharynx oropharynx and laryngeopharynx b Nasophamnx 1 The nasopharynx extends from the internal nares to the soft palate 2 Unlike the other two portions of the pharynx the nasopharynx is normally only a respiratory passageway and it39s histology reflects that fact a The mucosa of the nasopharynx is similar to the respiratory mucosa of the nasal cavity 1 It is a pseudostrati ed ciliated columnar epithelium with numerous goblet cells sitting on a lamina propria rich in lymphoid cells and seromucus glands c Orophamnx 1 The oropharynx extends from the level of the soft palate to the level ofthe hyoid bone a It serves as both a passageway for air and for ingested foods 1 The oropharynx will communicate with the oral cavity through the fauces 2 Due to it39s dual role the oropharynx has a lining more similar to that of the oral cavity a It is lined by a stratified squamous nonkeratinized epithelium sitting on a lamina propria rich in lymphoid cells 1 It will also have associated mucus glands d Lamngeophamnx 1 The laryngeopharynx extends from the level ofthe hyoid bone to the opening into the larynx the Glottis 2 It also serves as both a passageway for air and for ingested foods and so it39s histology is identical to that ofthe oropharynx 4 The Lamnx a The larynx serves a purely respiratory role and is located anterior to the upper esophagus b The larynx is a hollow bilaterally symmetrical structure formed by plates of cartilage and by muscle 1 The cartilage component is made up of nine pieces three singular and three paired a The singular cricoid cartilage attaches the larynx to the trachea by means of the cricoid ligament b The thyroid cartilage is quite large and makes up most of the anterior wall c The paired arytenoid corniculate and cuneiform cartilages are held together by skeletal muscle d The epiglottis is the most specialized of the laryngeal cartilages 1 It guards the opening into the respiratory tree the glottis 2 It is composed ofelastic cartilage covered by loose connective tissue and an epithelium which varies based on location 98 a On the external surface where abrasion may come from food the epithelium is a stratified squamous nonkeratinized b On the internal surface which should come in contact only with air the epithelium is a pseudostratified ciliated columnar epithelium with numerous goblet cells 2 The muscular component is divided into extrinsic and intrinsic muscles a The intrinsic muscles along with dense connective tissue attach the paired cartilages together b The extrinsic muscles attache the larynx to other structures ofthe throat 0 They are skeletal muscles of pharyngeal arch derivation c The Lamngeal Mucosa 1 The laryngeal mucosa is arranged into three pairs of lateral folds reinforced by dense connective tissue cores which project into the lumen a The three pairs of folds are 1 Amepiglottic Folds a They are the superiormost pair 2 Ventricular Folds a They are the middle pair and are often referred to as the quotfalse vocal cordsquot 3 True Vocal Cords a these are the inferiormost pair and have a core of skeletal muscle along with dense connective tissue so as to vary the size and shape of the passage b Between the true and false vocal cords is a recess called the Lamngeal Ventricles 2 The mucosa is primarily a pseudostratified ciliated columnar epithelium with numerous goblet cells sitting on top ofa lamina propria rich in lymphoid cells a However there are some areas where it is a stratified squamous nonkeratinized epithelium sitting on top ofa lamina propria rich in lymphoid cells 1 These areas are the external surface of the epiglottis and the aryepiglottic folds 5 The Trachea a The Trachea is a rigid tube composed of 16 to 20 segments each segment containing a cshaped piece of hyaline cartilage 1 The cartilage free regions are on the posterior trachea facing the esophagus 2 The cartilages are joined to one another by bands of smooth muscle called the Trachealis Muscle 3 Also joining the cartilaginous rings together is the lntersegmental Dense Connective Tissue composed of dense regular connective tissue rich in collagen and elastic fibers a These collagen bers are continuous with the perichondrium ofthe rings 4 The trachea will extend from the larynx into the thoracic cavity where it will branch 99 a The two branches coming off ofthe trachea are the right and left primary bronchi b The branching point is called the carina b Histology The trachea is made up ofthree tunics the tunica mucosa the tunica submucosa and the tunica adventitia 1 Tunica Mucosa the tunic in contact with the tracheal lumen a The mucosa has a pseudostrati ed ciliated columnar epithelium with numerous goblet cells resting on a thick basement membrane 1 There are actually ve cell types in the tracheal epithelium three are observable under light microscopy two are observable only under electron microscopy a ciliated columnar cells b goblet cells c basal cells d Brush Cells 1 Unlike ciliated columnar cells brush cells possess microvillae 2 Their exact nature is still unknown a They may be immature columanr cells or degraded goblet cells b A small number of brush cells may even serve as sensory receptors 1 These cells are synapsed with the dendrites of sensory neurons e Granule Cells 1 Granule cells are small basally oriented cells lled with numerous small granules 2 There are two classes ofgranule cells recognized based on histochemical properties a Neurosecretom Granule Cells produce catecohlamines such as epinepherine and norepinepherine b ProteinHormone Secreting Cells produce hormones 2 These ve types of cells are also found in other portions of the respiratory mucosa a Due to their similarities the epithelial of the larynx trachea and bronchi are collectively called Lamngobronchial Epithelium b The lamina propria of the mucosa will be the typical loose connective tissue rich in lymphoid cells having reticular and elastic fibers c An elastic membrane separates the tunica mucosa from the tunica submucosa 1 This elastic membrane extends into the primary bronchi a At the distal portion ofthe primary bronchus it is replaced by the Muscularis Mucosa a ring of smooth muscle 2 Tunica Submucosa the middle tunic a The submucosa is a loose connective tissue tunic containing numerous seromucus glands 100 1 These seromucus glands are called Submucosal Glands 2 The ducts of these submucosal glands will pass through the submucosa and mucosa to open up in the lumen a Myoepithelial cells are associated with the acini and with certain ducts of these glands 3 Tunica Adventitia the outermost tunic a The adventitia consists ofthe cartilaginous rings the trachealis muscle and the intersegmental dense connective tissue 1 In the cartilage free posterior surface ofthe trachea the submucosal glands will partially penetrate into the adventitia and be interspersed with smooth muscle bers of the trachealis 6 The Lungs a The lungs are paired conical shaped organs located in the pleural cavities ofthe thorax 1 The lungs are covered by a thick elastic serous membrane called the Visceral Pleura a The visceral pleura produces pleural uid b The visceral pleura is contiguous with the serous inner layer of the parietal pleura 2 The connective tissues within the lungs is rich in elastic bers and smooth muscle cells to allow for the expansion and reduction of the lungs during breathing 3 The mediastinal surface of each lung has a concave depression called the hilus a The hilus is the point where the pulmonary vessels and the primary bronchus enters the lung 4 The lung is divided into Lobes a The right lung has three lobes and the left lung has two lobes b The primary bronchus will send a branch into each lobe 1 These branches are called LobarSecondam Bronchi c Each lobe will be further divided into Segments 1 The segments are each separated from one another by a connective tissue septum containing blood vessels 2 Smaller branches come off of the secondary bronchi to service each segment called TertiamSegmental Bronchi d The segments are divided into Subsegments e The subsegments are divided into Lobules 1 Each lobule is separated from the adjacent lobules by a connective tissue septum b The internal structure ofthe lung consists of a branching system of conducting passages called the Bronchial tree 1 The Primam Bronchi a There are two primary bronchi one going into each lung at the hilus b The primary bronchus is almost identical to the trachea in it39s histology 101 particularly in the proximal portions 1 The major difference is that the hyaline cartilage rings are complete circles they are not cshaped 2 The SecondamLobar Bronchi a The secondary bronchi branch off of the primary bronchi 1 One secondary bronchus will enter each lobe of the lung a Since they are actually within the lung secondary bronchi could be considered to be the rst ofthe intrapulmonary conducting passageways 1 The previously mentioned structures are part of the extrapulmonary conducting system b Secondary bronchi are histologically quite similar to primary bronchi 1 One major difference is that instead of having a continuous piece of cartilage they have a cartilaginous ring composed of several smaller discontinuous plates 2 Another difference is that where the secondary bronchus lacks plates the submucosal glands will bulge outwards obscuring the boundary between the adventitia and the submucosa 3 Also the elastic membrane is lacking having been replaced by the muscularis mucosa a This will be the case in the smaller branches as well 1 It will increase in thickness in the smaller branches b The smooth muscle cells of the muscularis mucosa will be interspersed with some elastic bers c When the muscularis mucosa contracts the mucosa displays longitudinal folding 4 The secondary bronchi will have a pseudostratified ciliated columnar epithelium but as the bronchial tree continues to branch this epithelium will diminish in height 5 The lamina propria is rich in lymphoid cells and has all three fiber types 3 TertiamSegmental Bronchi a Tertiary bronchi will branch off of the secondary bronchi and enter into each segment 1 They will travel in the connective tissue ofthe intersegmental septa b These smaller vessels show some modi cations from secondary bronchi 1 They show decreasing cartilaginous plates 2 They show decreasing epithelial height although its the same type 3 They show an increase in the muscularis mucosa 4 The Bronchioles a The tertiary bronchioles give rise to several orders of smaller vessels collectively called bronchioles 1 The lumenal diameters ofthese passageways become progressively smaller and the walls become increasingly simpli ed a They lack cartilage plates and very few have seromucus glands in 102 the submucosa 1 What remains is simply the mucosa surrounded by a bit of loose connective tissue making a up the adventitia 2 The bronchioles are named in order of branching Primary Secondam Tertiam and Terminal Bronchioles a The terminal bronchioles will enter into the respiratory portion of the respiratory system proper b Histology 1 The epithelium is a simple ciliated columnar a It will have some goblet cells but the number of goblet cells will progressively diminish 1 The goblet cells will be replaced by ClaraBronchiolar Secretom Cells a Clara cells are dome shaped secretory cells 1 Their secretions may reduce surface tension on the walls of more distal bronchioles 2 The muscularis mucosa reaches it39s greatest thickness in the tertiary bronchioles and begins to thin more distally 3 Bronchioles especially terminal bronchioles can alter their lumenal diameter C The Respiratory Portion 1 The Passaqewavs ofthe Respiratorv Portion a The terminal bronchioles will give rise to two orders of Respiratory Bronchioles 1 Histologically respiratory bronchioles resemble the other types of bronchioles a The major difference is that they show thin walled outpocketings called Alveoli 1 The number ofalveoli will increase with each branching b The portions that lack alveoli show 1 a simple cuboidal epithelium 2 a thin lamina propria 3 and a muscularis mucosa which is heavily laced with elastic bers b The respiratory bronchioles will give rise to smaller passageways called Alveolar Ducts 1 The walls of the alveolar ducts have numerous alveoli a There are so many alveoli that there is little space between them and the portions of the walls between adjacent alveoli lack the simple cuboidal epithelium 1 Instead the wall is lined by a simple squamous epithelium like that of the alveoli resting on top of a thin connective tissue containing some smooth muscle fibers 2 The alveolar ducts terminate in Alveolar Sacs a Alveolar sacs are thin walled structures that contain elastic and reticular 103 bers 1 Alveolar sacs lack smooth muscle 2 It39s epithelium is a simple squamous epithelium b Each alveolar sac is composed of several alveoli which will open into a common chamber called the Atrium 2 The Alveoli a Alveoli are the site of external respiration b Alveoli are delicate cupshaped structures that are lined by an extremely attenuated simple squamous epithelium 1 A single wall termed the lnteralveolar Septum is formed between two adjacent alveoli a The interalveolar septum is composed ofthe cells of the two alveoli and a thin connective tissue component between them 1 The connective tissue contains many elastic and reticular bers a few mast cells leucocytes broblasts and an anastomizing capillary network b The interalveolar septum also contains a specialized cell called the Septal Cell 1 Septal cells super cially resemble broblasts 2 Septal cells contain bundles of actin and myosin which allow them to contract due to hypoxia a However their function is still unknown 2 Alveoli communicate through openings called Alveolar Pores a Alveolar pores allow for the equilibration of air pressure within a lung lobule c The alveoli are lined by two cell types 1 Alveolar Epithelial CellsTvpe 1 Alveolar Cells a These are extremely attenuated squamous cells that form the bulk of the alveolar wall b Type 1 cells share a common basement membrane with the endothelial cells of the pulmonary capillaries 1 The fusion of type 1 cells and endothelial cells forms the Alveolar MembraneRespiratorv quot 39 e a This is an extremely thin structure at 05 um in thickness to maximize the diffusive exchange of gases 2 The pulmonary capillaries are continuous capillaries although they are very thin walled 2 Secretorv CellsTvpe 2 Alveolar Cells a These are short somewhat rounded cuboidal cells b They contain the normal host oforganelles found in a secretory cell 1 Especially prominent are CytosomesMultilamellar Bodies 2 These organelles produce surfactant a phospholipid rich secretion which reduces surface tension d Local Defense Mechanisms ofthe Alveolus 1 Alveolar MacrophagesDust Cells 104 a Alveolar macrophages are commonly found in the interalveolar septum or on the alveolar cells b In respiratory passages these macrophages dispose of particulate matter and pathogens 1 Bacteria and viruses are also hindered by the uids of the alveolus which will contain lgA and interferon c Alveolar macrophages will also play a role in augmenting speci c immune reactions ofthe local lymphoid cell population and of regional lymph nodes 1 Regional lymph nodes are at their most abundant nearthe hilus 2 Both humoral and cellmediated immunity play a prominent role in the lungs39 defense against infection a B cells T cells macrophages mast cells and other leucocytes produce various antibodies lymphokines and other mediators that will promote and regulate in ammation and the immune response 105 Unit 10 THE DIGESTIVE SYSTEM Subunit 10A The Oral Cavity and the Alimentary Canal A An Introduction to the Digestive Tract 1 The digestive system is involved with the intake mechanical and chemical breakdown and absorption of food a It also eliminates the undigestible residue through defecation b The end products of digestion are carried through the blood stream and lymphatics to all ofthe tissues ofthe body 2 Digestion involves the digestive tract and accessory organs a The accessory organs are located outside of the alimentary canal and secrete their products into the tract via ducts 1 Their products are a variety ofenzymes for the chemical break down of food 2 The accessory glands include the salivary glands liver gall bladder and pancreas b The digestive tract is also known as the quotGI tractquot or quotalimentam cana 1 The entrance to the G l tract is the oral cavity where food is masticated mixed with saliva and formed into a bolus 2 The bolus is passed to the phamnx which will convey it to the esophagus the start ofthe digestive tube proper 3 This muscular tube will conduct the bolus to the stomach where muscular contractions mix it with gastric enzymes and HCI to form the semifluid chyme 4 The chyme is delivered to the small intestine where the bulk of digestion and the absorption of nutrients occurs a The small intestine is divided into three segments duodenum 39e39unum and ileum b To assist in the chemical breakdown of food there are a variety of chemical agents produced by the pancreas liver and the small intestine itself 5 The residue is passed on to the large intestine a In the large intestine there is a slight degree of nutrient absorption but the main function is to prepare and eliminate the feces b The large intestine is divided into the colon cecum rectum and anal canal 1 The colon is the longest segment and is subdivided into the ascending colon transverse colon descending colon and sigmoid colon 2 The vermiform appendix is attached to the cecum 106 B The Oral Cavity 1The oral cavity is also known as the buccal cavity or mouth and includes thelips cheeks roof of the mouth being composed of the hard and soft palates tongue gingivae teeth and the salivary glands a The oral cavity proper extends from the vestibule the space between the lip cheeks and the gingivae teeth to the fauces the opening into the oropharynx b Although histologically distinct from the digestive tube the oral cavity is the entrance to the alimentary canal The Lining of the Oral Cavity a The lining ofthe oral cavity is a mucus membrane consisting ofan oral mucosa resting on a submucosa 1 The other layers will depend on the location a Ex deep to the submucosa on the soft palate is skeletal muscle b The oral mucosa consists ofa stratified squamous epithelium resting on a relatively cellular lamina propria 1 The strati ed squamous epithelium will range from nonkeratinized to parakeratinized depending on location and the degree ofwear and tear that it has been exposed to 2 The lamina propria will be well vascularized and rich in lymphoid cells c The oral submucosa is a loose connective tissue layer 1 It is lacking in the region of the hard palate where the lamina propria fuses directly on to the periosteum of the palatine and maxillary bones 2 The submucosa will contain groups of intrinsic salivary glands called Buccal Glands whose ducts penetrate the mucosa a The secretions of these minor salivary glands can be serous mucus or mixed in nature 3 The Tongue a The tongue consists of a core of skeletal muscle bundles covered by a mucus membrane 1 The anterior two thirds of the tongue is separated from the posterior third by the terminal sulcus 2 The paired muscle bundles ofthe tongue are separated by the median septum which divides the tongue into right and left portions b The dorsal surface ofthe anterior tongue will have numerous small protuberances ofthe mucus membrane called Papillae There are four classes of papillae 1 Filiform Papillae slender projections that are 2 to 3 mm in length a They consist ofa connective tissue core covered by strati ed squamous epithelium as is the case for all papillae 1 The surface epithelial cells are parakeratinized a This causes them to appear whitish b These cells are alive 1 The nucleus is present although reduced 107 l 2 The cells are rich in keratin but there isn39t enough to kill the cell 2 Foliate Papillae very rare leaf shaped papillae found along the margin of the tongue 3 Fungiform Papillae mushroomshaped papillae a Their connective tissue core is well vascularized and their epithelium is nonkeratinized 1 So these papillae appear to be red b Fungiform papillae will often contain taste buds 4 Circumvallate Papillae the largest most complex and least numerous type a There are between 10 to 12 circumvallate papillae along the terminal sulcus forming a b Each papillae is partially sunk into the mucosa and is surrounded by a circular trench c Their lateral surfaces are covered by taste buds 1 The ducts of serous glands called von Ebner39s glands open into these trenches to provide a solvent for the detection of taste c The Taste Buds 1 The taste buds are small pale ovoid bodies located within the darker staining mucosal epithelium a They will extend from the basement membrane to open on to the surface of the tongue by Taste Pores 2 Taste buds are distributed on both fungiform and circumvallate papillae soft palate glossopalatine arch epiglottis and posterior pharyngeal wall 3 Histology a There are two cell types present in the taste bud They are arranged like segments of an orange 1 Taste CellsGustatorv Receptor cells these are neuroepithelial cells a They have elongate stereocilia that extend into the taste pore so as to detect gustatory stimuli 1 Gustation is a chemoreceptive sense and so the stimulus must be in solution for detection 2 Associated with these neuroepithelial cells are sensory neurons which will pick up the impulse and conduct it towards the brain 2 Supporting CellsSustentacular Cells columnar epithelial cells that surround protect and aid the taste cells 4 There are only four taste sensations sweet sour bitter and salty 4 The Extrinsic Salivam Glands a In addition to the minor salivary glands the mouth also has three pairs of major salivary glands the parotid the submandibular and the sublingual glands 1 These are compound glands consisting of several lobes that are further divided into lobules by connective tissue septa a The secretory portion will be either alveolaracinar or tubuloalveolartubuloacinar 2 These extrinsic glands are located outside ofthe oral cavity and release their products into the mouth by means of ducts 108 a The duct system is least developed in the sublinguals and most developed in the parotids b The secretion will be moved from the secretory portion to the duct system by contraction of myoepithelial cells c The Basic Duct System 1 Intralobular Ducts are the smallest branches and will receive the product directly from the secretory portion a They are microscopic and located within the lobules b There are two types of intralobular ducts one leading into the other 1 lntercalated Ducts the rst to receive the product a At the acinus or tubule they are lined by a simple squamous epithelium b Closer to the second type of intralobular duct they become progressively taller and are lined by a simple cuboidal epithelium 2 Striated Ducts receive the product from the intercalated ducts a The striated ducts are lined by a simple columnar epithelium that demonstrates basal infoldings 1 These striations are due to infoldings of the basal cell membrane which yield energy for the ion pumps which will actively modify the passing products of the salivary lands 2 lnterlobular Ducts arise from the union of intralobular ducts and run between the lobules a They are lined by a tall simple columnar epithelium and also contain connective tissue from the interlobular septa 3 Lobar Ducts arise from the union of several interlobular ducts a They are lined by epithelium ranging from high simple columanr to stratified cuboidal b One lobar duct will drain an entire lobe 4 Ma39or Excretom Duct is formed by the fusion of the lobar ducts and opens onto the oral mucosa a It is lined by a stratified epithelium the exact nature depending on the duct and the location b The Three Extrinsic Salivary Glands 1 The Parotid Glands a The parotids are the largest salivary glands 1 They are located subdermally super cial to the masseter muscle immediately anterior to the pinna b In the adult the parotid gland contains only serous acini which secrete a watery substance 1 The secretions include water salts protein the enzymes lysozyme to destroy bacteria and amylase which will hydrolyze carbohydrates to yield disaccharides 109 c Each parotid gland will have one major excretory duct the parotid duct which will be lined by strati ed squamous epithelium 1 It opens into the cheek at about the level of the second upper premolar 2 The Submandibular Glands a These glands are located nearthe posterior portion ofthe mandibular angle b These glands are compound tubuloacinar glands with primarily a mixed seromucus product 1 Their product is intermediary between the parotid and sublingual glands 2 Some ofthe secretory portions ofthe gland are mucus tubules some are serous acini but most are mixed being a mucus tubule capped by a serous demilune c They will have several major excretory ducts per glands called the submandibular ducts which will open onto the oor of the oral cavity 1 The ducts histology runs from strati ed cuboidal at the fusion ofthe lobar ducts to strati ed columnar and in some cases to stratified squamous at the opening 3 Sublingual Glands a These glands are located in the oor ofthe mouth below the tongue b They are compound tubuloacinar glands 1 The secretion is mixed seromucus 2 In these glands the mucus tubules predominate a Most ofthe mucus tubules are independent but some will be capped by serous demilunes b Pure serous acini are extremely rare in these glands c Sublingual glands have an abbreviated duct system 1 The intralobular ducts are difficult to locate 2 The lobar ducts often open directly onto he oor ofthe mouth d The product is a thick mucus rich secretion with a low enzyme content 5 The TeethDentes a Humans have a variable or heterodontic dentitiion meaning that we have teeth of differing shapes designed to do different tasks b Humans have two sets ofteeth deciduous 20 and secondam 32 c The various shapes and sets ofteeth have the same basic structure 1 Crown the exposed portion of the tooth which projects above the gingivae a The crown is covered by two extremely hard materials Enamel and Dentin that surround a loose connective tissue lled cavity called the Pulp Cavity 1 Enamel is restricted to the crown but dentin covers the entire tooth 2 m one to three in number these are conical projections which penetrate into the sockets of the mandible and maxilla a The root has narrow channels to allow for the passage of blood vessels and nerves to and from the crown called Root Canals 110 1 At the apices of the roots are openings called the Apical Foramen b Covering the dentin ofthe root is an adhesive material called Cementum that attaches to periodontal ligaments to the tooth 3 CervixNeck a narrow portion between the roots and crown C The Basic Histology ofthe Digestive Tube 1 The histology ofthe entire digestive tube demonstrates a basic pattern a The wall is composed of four concentrically arranged layers enclosing the lumen tunica mucosa tunica submucosa tunica muscularis and tunica serosa 2 The Tunica Mucosa is the innermost layer being in direct contact with the lumen a It consists of three layers 1 A lining ofepithelium which may be specialized for protective functions ex esophagus or for absorption and secretion ex stomach a Typically the epithelium is a simple columnar 2 A lamina propria surrounds the epithelium a The lamina propria will be rich in reticular bers and lymphoid cells b It will support the epithelial layer and functions immunologically 3 The third layer is a sheet of smooth muscle called the Muscularis Mucosa a Contraction of the muscularis mucosa causes folding ofthe mucosa 3 The Tunica Submucosa is the second layer a The tunica submucosa is a relatively thick layer of loose connective tissue containing lymph vessels blood vessels lymphoid nodules and a nerve plexus 1 The nerve plexus is an autonomic plexus called the Submucosal Plexus which controls the muscularis mucosa 2 Although a loose connective tissue the ber content is fairly high and variable in this tunic 4 The Tunica MuscularisMuscularis Externa is the third tunic a This tunic will typically consist of two sheets of smooth muscle 1 The inner sheet is arranged circularly the outer sheet is arranged longitudinally 2 Between the two sheets is a sparse connective tissue layer containing an ANS nerve plexus called the Myenteric Plexus a This nerve plexus controls the tunica muscularis b The tunica muscularis is greatly modi ed in the upper esophagus colon and stomach 5 The Tunica Serosa is the outermost tunic a Typically the tunica serosa is a serous membrane being composed of mesothelium resting on a thin layer of loose connective tissue 1 It is also known as the Visceral Peritoneum and is contiguous with the Parietal Peritoneum which lines the walls ofthe abdominopelvic cavity 2 This is the case for all of the organs of the digestive tube located inferior to 111 the diaphragm a Most of the esophagus is superior to the diaphragm and so is not covered by the visceral peritoneum 1 Instead those portions superior to the diaphragm are covered by a loose connective tissue which attaches the esophagus to surrounding structures a The outermost tunic of these portions of the esophagus is called the Tunica Adventitia D The Esophagus 1 The esophagus is a muscular collapsible tube which conveys the bolus from the pharynx to the stomach by means of peristaltic contractions a It can be considered to be the start of the digestive tube proper since it is the first organ to have the four tunic arrangement 2 Tunica Mucosa a The tunica mucosa of the esophagus has an epithelium of strati ed squamous nonkeratinized type b The lamina propria is rich in lymphoid cells and will have occasional lymph nodules 1 At the lower margins ofthe esophagus Esophageal Cardiac Glands may be present in the lamina propria a The occurrence of these glands is subject to a high degree of individual variation ranging from abundant to absent b They are located near the GastroesophagealCardiacLower Esophageal Junction 1 The esophageal cardiac glands are compound tubular glands which secrete mucus to protect the esophagus from acid reflux c The muscularis mucosa is formed by longitudinally oriented smooth muscle cells supported by delicate elastic bers 3 Tunica Submucosa a The submucosa consists of a loose connective tissue having a dense collagenous and elastic fiber framework that is very plastic and resilient b Esophaqeal Glands properDeep Esophaqeal Glands are located in the submucosa and have ducts which open onto the epithelial surface 1 They are mucus secreting glands which lubricate the passage ofthe bolus c The submucosa along with the mucosa will be arranged into folds to allow for greater distension during swallowing 4 Tunica Muscularis a The tunica muscularis ofthe esophagus is unique being composed of both skeletal and smooth muscle fibers under ANS innervation 1 The upper third ofthe esophagus has both the circular and longitudinal layers ofthe muscularis externa composed of skeletal muscle 2 In the middle third smooth muscle begins to replace skeletal muscle 112 3 The lower third of the esophagus has both the circular and longitudinal layers ofthe muscularis externa composed of smooth muscle b Innervation ofthe tunica muscularis comes from the vagus nerve parasympathetic and both the cervical and thoracic trunks sympathetic 5 Tunica Adventitia a The outermost tunic for most of the esophagus is not a tunica serosa but instead a tunica adventitia 1 The tunica adventitia is a loose connective tissue sheath that connects the esophagus to neighboring structures b A short portion ofthe esophagus does penetrate the diaphragm and enter into the abdominopelvic cavity 1 This terminal portion is covered by a tunica serosa as is all of the digestive tube in the abdominopelvic cavity E The Stomach 1 The stomach is an expansion of the digestive tube that originally served to store food a In mammals it serves both to store and to digest food 1 The food enters the stomach as the semisolid bolus and is exposed to chemical and mechanical forces which reduce it to a partially digested uid mass called Chyme a Mechanical digestion is controlled by the myenteric nerve plexus 1 It causes mixing ofthe food with digestive enzymes and HCI to promote chemical digestion 2 It also churns the food to further break it down mechanically a process called maceration b The stomach produces a variety of secretions collectively called Gastric Juice to allow forthe chemical digestion ofthe bolus 1 These secretions include the protease pepsin the glycoprotein gastric intrinsic factor mucus and HCI b The primate stomach is divided into four regions 1 Cardiac Region the portion of the stomach to receive the bolus from the esophagus and guarded by the Cardiac Sphincter 2 Fundic Region the dome shaped region superior to the cardia which serves to store food for up to one hour 3 Pyloric Region a narrow segment connecting the stomach to the duodenum and guarded by the Pyloric Sphincter 4 The Body the bulk ofthe stomach it is located between the cardia and pylorus c When it is empty the stomach39s mucosa is arranged into folds called Rugae which allow for more room for distension 2 Tunica Mucosa a The mucosa of the stomach is divided by a pattern of shallow furrows into small 113 slightly elevated areas between 1 to 6 mm in diameter which have many small Gastric Pits also called Foveolae 1 The gastric glands will open into these gastric pits b The mucosa of the stomach is composed of 1 tall simple columnar epithelium a This epithelium lines the gastric glands b These cells are characterized by an apical mucus cap a This mucus cap is secreted by mucus cells onto the epithelial surface so as to form a barrier against the acidic contents of the stomach c This is the typical epithelium of the digestive tube 2 a lamina propria which is typically lymphoid a It supports both the surface epithelium and the numerous gastric glands which open into the gastric pits 1 These glands are simple or branched tubular glands and vary in cellular composition and function depending on their location in the stomach a Three types of gastric glands can be identi ed cardiac pyloric and fundic glands 2 The gastric pits and the gastric glands are invaginations into the lamina propria from the surface epithelium 3 and the standard muscularis mucosa although it is well pronounced c The Glands of the Stomach 1 Fundic GlandsGastric Glands a These glands are located in the body and fundic regions ofthe stomach 1 They are the most important secretors of gastric juice 2 Several fundic glands may open into the same gastric pit 3 Each gland may divide into two orthree branches at it39s terminal end b There are four cell types in the fundic gland 1 Chief CellsZymogenic Cells a Chief cells are simple cuboidal or low columnar epithelial cells found in the lower third or lower third ofthe gastric gland b Chief cells produce Pepsinogen the precursorto Pepsin 1 Pepsin is a protease which breaks polypeptides down into peptide subunits 2 Pepsin is activated by the low pH ofthe stomach a During digestion the stomach has a pH of 18 2 Parietal CellsOxyntic Cells a Parietal cells are large pyramidal shaped cells found lining the upper half ofthe gastric gland b Under the electron microscope parietal cells display secretory canaliculi 1 Secretory canaliculi are invaginations of the cell membrane lined by microvilli for the secretion of cellular products 0 Parietal cells produce hydrochloric acid and gastric intrinsic factor 1 HCl causes the low pH ofthe stomach which activates pepsin 114 2 Gastric intrinsic factor is a glycoprotein which facilitates the absorption of vitamin B12 by the small intestine 3 Mucus Neck Cells a Mucus neck cells are columnar epithelial cells found among the parietal cells b They produce mucus 4 Argentaf n CellsEnteroendocrine Cells a Argentaf n cells are small granulated cells ofwide distribution 1 They are particularly prevalent in the pyloric region of the stomach but they are also found in the large intestine small intestine pancreas and liver b Argentaf n cells are hormone producing cells 1 They are part of the APUD the amine precursor uptake and decarboxylation system 2 They are part of the diffuse endocrine system a group of endocrine cells distributed throughout several organ systems a They produce a variety of hormones and vasoactive amines such as 1 Gastrin which increases gastric motility and the production of HCL and pepsinogen 2 Somatostatin which inhibits the function of gastrin and several other gastric hormones 3 lnsulin promotes the storage of glucose as glycogen 4 Glucagon promotes the release of glucose from glycogen 5 Histamine which stimulates gastric secretion and increase vascular permeability 6 Serotonin which inhibits gastric secretion and is a powerful vasoconstrictor 2 Cardiac Glands a These glands are located in the cardiac region 1 These glands are relatively short as are the gastric pits which they open into b Cardiac glands contain primarily mucus neck cells but will also have some argentaf n cells 1 Chief and parietal cells are absent in cardiac glands 3 Pyloric Glands a These glands are located in the gastric pits of the pyloric region 1 These pits are deeper and their glands are longer and more branched and coiled than are the cardiac glands b The cellular content of the pyloric glands is similar to that of the cardiac glands except for the higher argentaf n cell content 3 Tunica Submucosa a The tunica submucosa is similar to that ofthe esophagus 115 4 Tunica Muscularis a The tunica muscularis ofthe stomach is modi ed to increase it39s role in mechanical digestion 1 It has three sheets of smooth muscle as opposed to the normal two sheets a The outer sheet is longitudinally arranged b The middle sheet is circularly arranged c The inner sheet is obliquely arranged 5 Tunica Serosa a The tunica serosa is the visceral peritoneum an d the typical outer tunic of the digestive tube F The Small Intestine 1 General Contents a Chyme arriving in the small intestine from the stomach is subject to a very different environment 1 The digestive juices of the small intestine are alkaline and contain a wide array of hydrolytic enzymes a The bulk of chemical digestion occurs in the small intestine 1 Prior to entering the small intestine a Some carbohydrates have been reduced to disaccharides 1 But the disaccharides must still be reduced to monosaccharides to cross the intestinal epithelium and polysaccharides remain in the chyme b Some proteins have been broken down into peptides 1 But the peptides must be degraded into amino acids and polypeptides remain in the chyme c Lipids have not begun to be digested d Nucleic acids have not begun to be digested b The alkaline nature of intestinal juice provides the correct pH for the enzymes 78 and helps to neutralize the acidity of the chyme 2 The small intestine also performs the bulk ofthe absorption of nutrients a Only simple sugars salts and water will have been absorbed priorto the small intestine b This is why the small intestine is the longest segment of the digestive tube at 7 m and has numerous modifications of the lumenal surface b The small intestine can be divided into three segments duodenum jejunum and ileum running from the pyloric sphincter to the ileocolic sphincter c The wall ofthe small intestine will have modifications of the mucosa and submucosa designed to increase surface area so as to increase the absorption of nutrients 1 Plicae Circularis crescenteric folds of the mucosa and submucosa which causes the chyme to spiral through the lumen 2 M finger like folds of the mucosa 116 a They range from 05 to 15 mm in length b They are very numerous averaging between 10 to 40 villi per square mm c They will have a core of loose connective tissue rich in capillaries and containing a lymph vessel called the Lacteal d They greatly increase the surface area for the absorption of nutrients 3 Microvilli finger like extensions ofthe apical cell membrane ofthe intestinal epithelial cells a They greatly increase the surface area for the absorption of nutrients 4 Intestinal CrvptsCrvpts of Leiberkuhn invaginations ofthe intestinal epithelium into the underlying lamina propria to the level ofthe muscularis mucosa a The intestinal crypts form tubular glands also called Intestinal Glands 2 Tunica Mucosa a The tunica mucosa of the small intestine is lined by a tall simple columnar epithelium The cell types present vary in the different portions of the intestinal epithelium 1 Epithelial Cells ofthe Villi there are three cell types present in the villus a Columnar Absorptive Cells these columnar epithelial cells have a well developed quotstriated borderquot of microvilli 1 The plasma membrane is covered by a surface coat of branching glycoproteins a This glycoprotein surface coat has two functions 1 to protect the cell from proteolytic and mucolytic agents 2 and to provide binding sites for the absorption of specific nutrients 2 The enzymes catalyzing the final breakdown of proteins peptidases and carbohydrates disaccharidases are located in the membrane of the microvilli a Their end products will be absorbed across the intestinal epithelium b Goblet Cells mucus producing cells considered to be unicellular mucus glands 1 The mucus serves to lubricate and protect the epithelium c Argentaf n CellsEnteroendocrine Cells hormone secreting cells similar in appearance and function to those found in the stomach and large intestine 1 Argentaf n cells produce a variety of peptide hormones a Enteroglucagon causes glucagon secretion b Serotonin inhibits gastric secretion and is a vasoconstrictor c Secretin controls the secretion ofthe accessory digestive organs d PancreozyminCholecystokinin controls the secretion ofthe accessory digestive organs e Motilin increases smooth muscle contractions f Neurotensin decreases smooth muscle contractions and modulates glucoregulatory systems 117 2 Epithelium Lining the Intestinal Cmpts is continuous with the epithelium lining the villi and includes the above mentioned three types of cells plus two additional types found at the bottom ofthe cry ts l0 a Undifferentiated Stem Cells located at the base ofthe crypts these cells will undergo mitosis to replace cells lost from the villi 1 They will migrate from the bottom ofthe crypt to the villus a During their normal life cycle they will later be shed 1 The life cycle lasts about ve days 2 They are pleuripotent cells b Paneth Cells these columnar cells are found in groups at the base of the crypts 1 They are large cells with abundant secretory granules a These granules are eosinophilic b The functions ofthese granules are still undetermined but one product which has been identi ed is Lysozyme b The lamina propria lls the spaces between the crypts and forms the cores ofthe villi 1 It resembles the reticular connective tissue ofthe lymphoid organs in ber content and in it39s high cellular content a V thin the fiber framework are found large numbers of eosinophils plasma cells lymphocytes and macrophages 1 Many of the plasma cells have been shown to produce lgA which passes onto the lumenal surface for protection against bacteria 2 Isolated lymph nodules are found throughout the lamina propria of the small intestine a They reach their greatest numbers in the ileum where groups of nodules form Peyer39s Patches 1 Each Peyer39s patch consists of between 30 to 40 nodules c The muscularis mucosa is the typical but also is re ected into thin strands of smooth muscle in the lamina propria of the intestinal villi 1 The smooth muscle ofthe villus will surround a lacteal a Contractions ofthis smooth muscle facilitates the movement of lymph in the lacteal 3 Tunica Submucosa a The submucosa is high in ber content being almost a dense irregular ct b In the duodenum the submucosa contains large mucus glands called Submucosal GlandsBrunner39s Glands 1 These glands secrete an alkaline mucus which neutralizes the acidity of the chyme a 80 they are particularly prevalent closer to the pyloric sphincter b The alkaline nature ofthis mucus also helps in digestion 2 The submucosal glands are actually deep invaginations ofthe surface epithelia 4 Tunica Muscularis 118 a The tunica muscularis is made up of the typical two smooth muscle sheets as is standard for the digestive tube 5 Tunica Serosa a The tunica serosa is made up ofthe typical mesothelium resting on a thin layer of loose connective tissue as is standard for the digestive tube G The Large Intestine 1 General Contents a The main functions of the large intestine is the absorption ofwater and minerals converting the residual chyme to feces and eliminating the feces 1 Certain vitamins are absorbed by the large intestine a In particular vitamin Kwhich is produced by the resident E coli 2 Although the colon can absorb amino acids nearly all ofthe absorption of nutrients occurs in the small intestine a As a result many of the modi cations to increase surface area found in the small intestine are lacking in the large intestine 1 It lacks the plicae circularis and villi 2 It has a very reduced striated border due to having much shorter microvilli on the columnar epithelial cells b The intestinal crypts however are well developed and are quite deep 1 Although they lack paneth cells and have more goblet cells than do the crypts of the small intestine b Secretion in the large intestine is limited to a viscous uid with a high mucus content which acts as a lubricant for the passage ofthe feces c The large intestine is much shorter than is the small intestine being 15 m long 1 It is named for it39s greater diameter due to the decreased need for absorption d There are four parts to the large intestine beginning at the ileocolic sphincter and extending to the anal sphincter 1 Cecum a blind pouch located at the ileocolicjunction a It has the vermiform appendix attached to it 2 Colon the longest segment extending from the leocolicjunction to the rectum 3 Rectum a short segment that stores the feces prior to elimination 4 Anal Canal a short passageway for the elimination ofthe feces 2 The Histology of the Colon a Tunica Mucosa 1 The epithelium of the mucosa is a simple columnartype a It has columnar absorptive cells goblet cells argentaf n cells and undifferentiated stem cells 1 The undifferentiated stem cells are limited to the bases ofthe crypts 2 The number of goblet cells increases in the lower portions ofthe colon to lubricate the passage of the increasingly dehydrated feces 2 The lamina propria and muscularis mucosa are structurally similar to that of 119 the small intestine a Lymphatic nodules will continue to occur and will even push into the submucosa through the muscularis mucosa b Tunica Submucosa 1 The tunica submucosa is similar to that ofthe small intestine c Tunica Muscularis a The tunica muscularis ofthe colon does show some modi cation 1 The outer longitudinal sheet is arranged into three at bands instead of as a continuous sheet called the Teniae Coli a This causes the colon to be organized into segments called Haustra b The teniae coli is lost at the level of the rectum and the continuous longitudinal sheet resumed d Tunica Serosa 1 The tunica serosa is similar to that ofthe small intestine and stomach 3 The Rectum and the Anal Canal a The rectum and anal canal are modified portions ofthe large intestine designed for the storage and elimination of the feces 1 In the rectum the crypts are fewer and deeper a The crypts will hold mucus producing glands called Anal Glands b Also the teniae coli is lost 2 In the anal canal the mucosa assumes longitudinal folds called the Columns of MorgagniAnal Columns a The epithelium is a strati ed squamous nonkeratinized type b The anal crypts are shallower and the number ofanal glands increases b The anus is actually two sphincters 1 The internal sphincter is composed of smooth muscle 2 The external sphincter is composed of skeletal muscle 120 Subunit 108 The Accessory Organs ofthe Small Intestine A The Pancreas 1 The pancreas has both an exocrine and an endocrine nature a These two functions are handled by histologically distinct regions of the pancreas 1 The exocrine pancreas accounts for the bulk of the parenchyma 2 The exocrine pancreas is punctuated by small discrete islands ofendocrine tissue called the Islets of Langerhans 2 The Exocrine Pancreas a Histology 1 General Comments a The exocrine portion of the pancreas is a compound acinar gland 1 It is composed of secretory acini deployed at the ends ofa branching duct system a These ducts will eventually coalesce to form the Main Pancreatic Duct and the Accessory Pancreatic Duct if present which will drain into the lumen of the duodenum via the Hepatopancreatic Am ulla 2 Although the pancreas is retroperitoneal there is a peritoneal component covering a portion of the pancreatic surface 3 Deep to the peritoneum will be a thin incomplete capsule formed of dense irregular connective tissue a This capsule will extend into the pancreas as the Septa 1 These connective tissue septa will divide the pancreas into incomplete lobules of varying size 4 The secretom acinus of the pancreas is composed of pyramidal epithelial cells arranged around the terminus ofan lntralobularlntercalated Duct a These epithelial cells are joined to each other by junctional complexes on their lateral surfaces 1 They rest on a basement membrane which is rich in reticular fibers b The apical and basal portions ofthe acinar cells demonstrate two distinct staining patterns 1 The apical portion is acidophilic due to the presence of many membrane bound secretory vesicles containing digestive enzymes 2 The basal portion is basophilic due to the presence ofabundant amounts of rough endoplasmic reticuli c The nucleus of the acinar cell is basally oriented 5 The Ducts ofthe Pancreas a lntralobularlntercalated Ducts are composed of low simple cuboidal epithelium to typical simple cuboidal epithelium 121 1 These cells are easily distinguished from the acinar cells by their poorer staining b lnterlobular Ducts which drain the intralobular ducts are composed ofa tall columnar epithelium 1 they travel through the septa 2 They increase in size as more ducts join them a The larger interlobular ducts will have a few argentaf n and goblet cells interposed among the columnar cells c Main Pancreatic DuctDuct of Wirsunq is composed of stratified columnar epithelium lined by dense collagenous connective tissue 1 It travels the length ofthe pancreas and opens into a common passageway for the products ofthe pancreas and liver called the Hepatopancreatic AmpullaAmpulla of Vater a The main pancreatic ductjoins the Common Bile Duct to form the hepatopancreatic ampulla 1 The hepatopancreatic ampulla opens onto an elevation ofthe duodenal mucosa called the Duodenal Papilla b The terminal portion ofthe main pancreatic duct like the the common bile duct will have longitudinally and circularly arranged smooth muscle which is part of the Sphincter of Oddi 1 The sphincter of Oddi controls the ow ofdigestive enzymes from the pancreas gall bladder and liver b Functional Considerations 1 The exocrine pancreas synthesizes a variety of digestive enzymes required for the breakdown of proteins carbohydrates fats and nucleic acids including trypsin chymotrypsin pancreatic amylase procarboxypeptidae proelastase various lipases deoxyribonuclease and ribonuclease a These enzymes are stored in zymogenic granules or secretory vesicles in the apical portion of the acinar cells 2 Secretion of the pancreaticjuice can be stimulated nervously or hormonally a Nervous stimulation is provided by the vagus nerve b Hormonal stimulation is provided by two hormones both ofwhich are produced by the duodenal mucosa 1 PancreozyminCholecystokinin initiates the release of enzymes by the pancreas 2 Secretin initiates the release ofan aqueous bicarbonate solution by the exocrine pancreas a This aqueous bicarbonate solution is the medium for the conveyance of the enzymes to the intestine b This aqueous bicarbonate solution also provides an alkaline environment to both neutralize stomach acids and to activate the pancreatic enzymes 3 The Endocrine Pancreas a Histology 1 The endocrine pancreas is arranged into small spherical aggregates of cells 122 known as the islets of Langerhans scattered amid the exocrine acini and ducts a The cells ofthe islets of Langerhans are arranged into compact anastomizing cords b The isles of Langerhans use the blood stream not ducts to deliver their products 1 So the islets are extensively vascularized by fenestrated capillaries 2 The Cells of the lslets of Langerhans each islet can hold several different types of cells all of which are epithelial a Alpha Cells 1 Alpha cells typically represent 20 of the cell population in an islet 2 They produce the hormone glucagon b Beta Cells 1 Beta cells typically represent 75 ofthe cell population in an islet 2 They produce the hormone insulin c C Cells 1 These are clear staining cells lacking stainable granules a They may represent cells which have already released their products 2 They are low in number d Delta Cells 1 Delta cells are also low in number making up about 5 ofthe cell population in an islet 2 Delta cells produce the hormone somatostatin which regulates nutrient homeostasis a Delta cells are identical in appearance to somatostatin producing argentaffin cells of the duodenum e There is an unnamed fth type of cell in the islet which produces pancreatic polypeptide 1 They are located mostly in the periphery ofthe islets located in the head ofthe pancreas a Some individual pancreatic polypeptide producing cells are found throughout the exocrine tissue of this area 2 Pancreatic polypeptide is released in response to the ingestion of food a It stimulates the secretion of gastric enzymes and inhibits bile secretion and intestinal peristalsis b Functional Considerations 1 Glucagon an insulin exert a strong in uence on carbohydrate metabolism especially in adipose tissue muscle and the liver a Glucagon is released in response to a decrease in bloodsugar levels 1 It stimulates glycogenogenolysis the breakdown ofglycogen to release glucose into the blood a lnsulin is released in response to an increase in bloodsugar levels 1 It stimulates glucose to be stored as glycogen 123 B The Liver 1 Introduction a The liver is a large glandular organ lling most of the right upper quadrant ofthe abdominopelvic cavity 1 It is immediately below the diaphragm and above the stomach 2 It serves to process nutrientrich blood from the digestive tract lter out harmful materials from this blood and to produce m which will assist in the digestion of materials in the small intestine 2 Histology a The liver has four incompletely separated lobes in humans 1 The lobes are invested with an extremely thin dense irregular connective tissue capsule called Glisson39s Capsule a Glisson39s capsule partially divides the four lobes 2 Visceral peritoneum covers portions of the liver and is external to Glisson39s capsule where present a It covers all of the external liver except where the liver39s ct capsule attaches to the epimysium ofthe diaphragm 3 The liver has a hilus called the Porta Hepatis for the passage of blood vessels and the bile carrying common hepatic duct a The liver receives a dual blood supply 1 It receives oxygenrich blood from the hepatic artery 2 It receives nutrient rich blood from the hepatic portal vein which is the nal portion ofthe hepatic portal system a The hepatic portal system is a system of blood vessels which carries nutrientrich blood from the capillary beds servicing the digestive tube to the liver b Organization ofthe Parenchyma 1 The liver39s parenchyma is not divided up into discrete compartments or lobules by connective tissue partitions a Instead the hepatocytes are organized into anastomizing plates or laminae deployed about two axises serving as histological landmarks 1 These two axises are the Portal Triad and the Central VeinVenule a The Portal Triad 1 The portal triad consists ofthree structures a a branch ofthe Portal Vein b a branch ofthe Hepatic artem c and a branch ofthe Bile Duct 2 These three structures are bound together by a sheath of collagen called the Portal Canal which also contains small lymphatics and nerves 2 In a histological section an imaginary line can be drawn connecting these triads breaking the parenchyma up into hexagon shaped lobules 124 M This is one way and the most common way of break779 up the liver pare70h yma a In the center of each lobule is the central vein b The sinusoids are radially arranged within the lobule running from the portal triad to the central vein 0 The hepatocytes are arranged into plates between the sinusoids 3 The direction of blood ow in the liver lobule proceeds from the periphery to the center a The branches of the hepatic artery found in each triad at each corner deliver oxygenrich blood to the sinusoids b Branches of the portal vein also found at each corner will deliver nutrient rich blood to the sinusoids c This blood will flow towards the center ofthe lobule delivering oxygen and nutrients to the cells and picking up wastes d The sinusoids will form an anastomizing network that allows blood from a few terminal portal veins and hepatic arterioles to perfuse a large network of sinusoids within the lobule 1 The sinusoids will carry blood to the center of the lobule where they are drained by the central vein a The central vein runs along the long axis ofthe Iobule it is perpendicular to the sinusoids b The quotcentral Veinquot is really a venule 1 The central venules of neighboring lobules will join together and eventually form the Hepatic Vein which will exit the liver at the porta hepatis c The Hepatocyte 1 The primary cellular constituent ofthe liver is the Hepatocyte which makes up about 80 of the cellular population a The hepatocytes are in close association with a duct system for the secretion and delivery ofm b The hepatocytes are in close association with a sinusoid system for the secretion ofsubstances into the blood 1 This association with the sinusoids also allows for the recovery of nutrients arriving from the digestive tract through the hepatic portal system a The hepatocytes store these nutrients and will release them into the blood as needed 1 Ex glucose is stored as glycogen fats are stored as triglycerides c Hepatocytes will synthesize most ofthe circulating plasma proteins detoxify many lipidsoluble poisons and remove certain waste products from the blood 2 The hepatocyte is a polyhedral cell with a central often polyploid nucleus a On average 65 ofthe hepatocyte population is mononucleated but 35 are binucleated b Rough endoplasmic reticuli are abundant in these cells and accounts in 125 part for the basophilic nature of these cells 0 Smooth endoplasmic reticuli are also abundant d The ultrastructural relationship between the hepatocyte and the sinusoidal endothelium is designed to maximize the easy exchange of substances between the hepatocytes and the blood 1 The sinusoidal endothelium is discontinuous with open fenestrations 2 A Perisinusoidal SpaceSpace of Disse separates the endothelium from the hepatocytes a Plasma freely percolates through the sinusoidal endothelium into the perisinusoidal space b The perisinusoidal space contains plasma but the formed elements can not enter due to the size of the sinusoidal fenestrae 1 This allows for liver cells to receive nutrients and to dispense substances into circulation c The perisinusoidal space contains only a few reticular bers an incomplete or even absent basal lamina and short microvilli which project into the space from the hepatocytes 3 The lining ofthe hepatic sinusoids consists of two cell types a The more abundant cell is the endothelial cells which form a discontinuous lining 1 These endothelial cells are flat and have dense nuclei b The stellate shaped Kupffer Cells guard the fenestrae ofthe sinusoids 1 Kupffer cells are stellate shaped cells having vesicular nuclei 2 Kupffer cells are macrophage which line the liver sinusoids a They phagocytize bacteria and foreign particulate matter assist in the secretion of bilirubin and assist in the conservation of iron from deteriorating erythrocytes 1 They filter the blood perfusing the liver 2 They are involved in the metabolism of hemoglobin recovered from senescent erythrocytes a They produce bilirubin and store iron in the cytoplasm as hemosiderin 1 Bilirubin is released into circulation and retrieved by the hepatocytes which conjugate it and excrete it in the bile 4 Another cell associated with the sinusoids is the Lipocyte a Lipocytes are fat storing cells 1 They will also store the fat soluble vitamin vitamin A b Lipocytes are found in small numbers within the perisinusoidal spaces e The Flow of Bile 1 The intralobular passageways which carry bile from it39s point oforigin to the bile ducts at the periphery of the lobule are the Bile Canaliculi a The walls of the bile canaliculus is composed of the indented cell membranes of contiguous hepatocytes 1 Tight junctions along the lateral walls of the adjacent hepatocytes form an impermeable seal along the length of the bile canaliculus 126 a This structural arrangement segregates bile ow from blood flow 1 Bile ow is opposite to blood flow going towards the periphery ofthe lobule 2 So a bile canaliculus is in essence the space between two plates of hepatocytes into which bile is secreted by the hepatocytes 2 At the periphery of the lobule the bile canaliculus joins with the small Terminal Bile DuctulesCholangioles a The terminal bile ductules have a lining of low simple cuboidal epithelium b The terminal bile ductules will often have short extensions called the Canals of Hering 1 Their walls are lined by a simple squamous epithelium 2 The canals of Hering will extend into the lobule and carry bile from the bile canaliculi into the terminal bile ductule proper 3 The terminal bile ductules will empty into larger vessels called lnterlobular Bile Ducts a lnterlobular bile ducts are lined by a simple cuboidal epithelium b These are the bile duct branches found in the portal triad 4 lnterlobular bile ducts drain into the Portal Canals a Portal canals are lined by a simple columnar epithelium 5 Portal canals will drain into the Right and Left Bile Ducts which will drain into the Common Hepatic Duct which drains the entire liver 0 The Gall Bladder 1 Introduction a Since the liver is continuously producing bile and bile is only needed when food has been ingested the gall bladder serves to store and to concentrate bile until it is needed 1 To facilitate this the gall bladder is distensible and contractible 2 Because it does not produce it39s own products the gall bladder is not organized like the pancreas or liver as a secretory organ b The gall bladder is a small pear shaped organ located in a depression on the inferior surface of the right hepatic lobe 1 It will release bile into the Cystic Duct which will join the common hepatic duct to form the common bile duct 2 The Histology of the Gall Bladder a The gall bladder39s histology more closely resembles that of the digestive tube than that ofthe other accessory organs 1 It is not principally epithelial or organized into lobes and lobules 2 Instead it is a hollow organ with tunics making up it39s walls b The Wall of the Gall Bladder 1 The lumen of the gall bladder is lined by a Tunica Mucosa a The tunica mucosa is arranged into folds called Rugae which will be lost as the gall bladder fills with bile 127 b The epithelium of the tunica mucosa is a simple columnar type 1 These columnar cells have central elongate nuclei 2 Their apical surface have microvilli 1 These microvilli increase surface area for the better absorption of water from bile so as to concentrate the bile 3 The mucosa lacks glands except in the neck of the gall bladder where invaginations ofthe epithelium will extend into the lamina propria forming simple tubuloacinar glands c The lamina propria ofthe tunica mucosa is a loose connective tissue d The tunica mucosa of the gall bladder does not have a true muscularis mucosa Instead it has a Muscular Lamina 1 The muscular lamina consists of irregularly anastomizing bundles of smooth muscle separated by dense connective tissue rich in collagen elastic and reticular bers 2 External to muscular lamina is the Perimuscular Layer a The perimuscular layer is a relatively thick connective tissue layer external to the tunica mucosa b The perimuscular layer is a dense irregular connective tissue consisting mostly of collagen bers and some elastic bers 1 It will also have broblasts and a variable number of adipocytes c Lymphatics and blood vessels will traverse the perimuscular layer on their way to the mucosa 3 The outermost layer of the gall bladder is a Tunica Serosa a The perimuscular layer is covered for the most part by the same visceral peritoneum covering the liver 3 The Duct System ofthe Gall Bladder a The neck ofthe gall bladder connects to the cystic duct b The cystic duct joins with the common hepatic duct to form the common bile duct 1 The common bile duct joins with the main pancreatic duct to form the hepatopancreatic ampulla 2 The walls of the common bile duct will be lined by a tall simple columanr epithelium and will include scattered bundles of smooth muscle a The smooth muscle bundles are not prominent until the terminus of the duct near the duodenum 1 Here the smooth muscle component is well developed to form the sphincter of Oddi a The sphincter of Oddi is located around the terminus of the common bile duct and often around the terminus of the main pancreatic duct b The sphincter of Oddi prevents the ow of bile into the common bile duct when the stomach is empty 1 This causes the bile to back up the cystic duct into the gall bladder of r storage c The sphincter of Oddi is composed of circular and longitudinal smooth muscle It generally consists of four parts 128 1 Sphincter Choledochus an annular sheath around the common bile duct a lt39s contraction prevents bile ow 2 Sphincter Pancreaticus an annular sheath around the terminus ofthe main pancreatic duct a lt39s contraction prevents the flow of pancreatic juice 3 Sphincter Ampullae an annular sheath around the opening of the hepatopancreatic ampulla 4 Fasciculi Longitudinales a longitudinally arranged smooth muscle a It facilitates the ow of bile when contracted 4 Functional Aspects of Bile a Bile is an aqueous solution composed of pigments such as biliverdin and bilirubin water salts lecithin and cholesterol 1 Bile salts emulsify fats to increase their solubility a Bile salts will often be reabsorbed by the small intestine to be used again b Bile is stored in a concentrated form in the bladder due to the ability ofit39s epithelium to absorb water and inorganic salts 5 Hormonal Control ofthe Gall Bladder a The hormones secretin and cholecystokininpancreozymin are released from the duodenal mucosa 1 This release is stimulated by the presence of fatty substances in the chyme entering the duodenum from the stomach 2 Cholecystokininpancreozymin activates contraction ofthe gall bladder which causes it to release bile a It also induces the relaxation ofthe sphincter of Oddi 3 Secretin acts on the bile ducts causing them to release a watery bicarbonate solution that mixes with and dilutes the bile 129 l Unit 11 THE INTEGUMENT A General Features and Functions The integumentary system consists of the skin hypodermis and a series of suborgans or skin derivatives a The skin consists of two tissue layers 1 The Epidermis a superficial epithelial component 2 and the Dermis an underlying connective tissue layer b Deep to the dermis of the skin is a second connective tissue layer rich in adipose called the Hypodermis 0 Together the skin and hypodermis invest the entire external surface ofthe body d The appendages ofthe skin develop from the epidermis and so are of epithelial derivation Features and Functions ofthe lntegument and It39s Derivatives a The skin is the heaviest and most versatile ofthe body39s organs 1 It accounts for approximately 16 of one39s body weight b The lntegument is an effective protective shield against a wide range of chemical physical and biological insults 1 Ex The skin is an effective screen against ultraviolet radiation 0 The skin is highly regenerative 1 This allows it to heal quickly 2 This also allows it to synthesize a variety of substances such as vitamin D d The skin shows great topographic diversity 1 It ranges from thick to thin from rough to smooth from noticeably hairy to less so e The skin is an organ of personal recognition ex fingerprints and sexual attraction f The skin is a very plastic resilient and mobile organ 1 It will conform to the body39s contours 2 It will readily adapt to constant body movements g The skin is an important thermoregulatory organ 1 Body temperature is controlled by vasoconstriction and vasodilation of the cutaneous vasculature 2 The skin also allows for cooling ofthe body through evaporational cooling provided by sweating h The lntegument is a very vascular structure 1 It can store up to 45 ofthe total blood volume a This vasculature can also exert a control over blood pressure 1 Capillary shunts in the lntegument can lower blood pressure i The skin can be considered to be the largest sense organ in the body 1 It detects pain pressure heat cold and touch 130 j The integument is continuous with internal mucus membranes at the eyelids nostrils lips prepuce vulva and anus 1 These transitional zones are termed Mucocutaneous Junctions k The integument is involved in the immune system 1 This is due in part to the Langerhans Cells the macrophages of the integument a They monitor the external and internal integumentary environments b They also sensitize lymphocytes against antigens by acting as quotpresenter cellsquot B The Egidermis 1 Introduction a The epidermis consists of stratified squamous epithelium keratinized type 1 It is composed of four different cell types or lines keratinocytes melanocytes Langerhans cells and Merkel cells a These four groups of cells can be considered to make up a series of epidermal minisystems 1 the keratinizing or malpighian system 2 the pigmentary or melanocyte system 3 the reticuloendothelial or Langerhans system 4 and the Merkel system b Being an epithelium the epidermis lacks blood and lymph vessels 1 So it39s cells must receive oxygen and nutrients via diffusion from the dermis c The epidermis does contain sensory nerve endings 1 However the bulk of integumentary innervation occurs in the dermis 2 The Layers ofthe Epidermis a The strati ed epithelium of the epidermis is divided into layers based upon the life stages ofthe keratinocytes the predominant cell type 1 The layers progress from the deepest most basal to the super cialmost a The deeper living layers are collectively called the Stratum Malpighi 1 The stratum malpighi will always be composed ofthree layers stratum basale stratum spinosum and stratum granulosum b The super cial dead layers are collectively called the Stratum Corneum 1 The stratum corneum will be of two layers in thick skin and one layer in thin skin a In thick skin the two layers are stratum lucidum and stratum corneum proper 1 Thick skin is restricted to the palms and soles in humans b In thin skin the stratum lucidum is absent 1 Thin skin is much more widely distributed than is thick skin 13 b The Histology of the Epidermal Layers 1 The Stratum Malpighi The viable cell layers a The Stratum BasaleStratum Germinativum 1 The stratum basale is the deepest layer and rests on the basement membrane 2 It is a single layer of cuboidal or columanr shaped cells 3 This is a layer of great mitotic activity a It serves as a source of stem cells for new keratinocytes b The Stratum Spinosum 1 The stratum spinosum is actually several layers of polyhedral shaped cells 2 The term quotspinosumquot refers to the spinelike appearance of these cells under early light microscopic studies a Today these quotspinesquot are known to be an artifact due to excessively dehydrating the tissue 3 The stratum spinosum also displays some mitotic activity and the beginning of keratin deposition in the keratinocytes slight c The Stratum Granulosum 1 The stratum granulosum is made up of several layers of attened cells 2 These keratinocytes have conspicuous lamellated granules of keratohyaline a It is in this stratum that keratinization begins in earnest and the deposition of keratin achieves it39s greatest rate 3 There is no mitotic activity in this layer or in any layers superficial to it 2 The Stratum Corneum The horny cell layers a The Stratum Lucidum 1 The stratum lucidum is made up of several layers of attened anucleate cells a These keratinocytes are dead 1 Their living material has been replaced by keratin 2 As it39s name indicates the stratum lucidum has a clear appearance 3 It is found only in thick skin b The Stratum CorneumStratum Corneum Proper 1 The stratum corneum proper is composed of many layers of dead attened keratinrich cells a The number of layers will vary based on the degree of mechanical abuse 3 The Cells of the Epidermis a The Keratinocytes 1 Keratinocytes are the most abundant cell type of the epidermis a They are named for the rich amounts of keratin which will accumulate in the cell as it matures 1 The production of keratin is the main function ofthese cells 2 The basal keratinocytes along with producing new keratinocytes are the 132 developmental source of the cutaneous appendages 3 It is the morphology ofthe various keratinocyte life stages which is the basis for naming the layers ofthe epidermis a The phases of cytomorphosis of the keratinocyte are proliferation differentiation and exfoliation 1 Proliferation ie mitosis the production of new keratinocytes a The stratum basale is an inexhaustible source of keratinocytes b Due to it39s high mitotic activity the epidermis is continually renewing itself to compensate for cell loss 1 Normally the rate of proliferation is equal to the rate of exfoliation 2 Differentiation the process of keratinization and the changes which the keratinocytes undergo towards this end a As the keratinocytes migrate super cially and lose their mitotic potential they begin to synthesize amorphous proteins and increasing amounts of fibrillar proteins to form the fibrilmatrix complex or quotkeratin patternquot that lls the corni ed cells 1 In the stratum granulosum keratohyaline granules appear a These granules will become part ofthe keratin pattern b Also in the stratum granulosum the keratinocytes contain smaller membranebound organelles called KeratinosomesMembraneCoating granules 1 The products of the keratinosomes will be discharged extracellularily and will serve to bind the keratinocytes to one another in the stratum corneum 3 Exfoliation the lose of keratinocytes from the surface layers of the epidermis a This is the nal stage in the keratinocyte life cycle b It is a continuous process whereby the outermost surface layers of keratinrich cells are shed away by the body b The Melanocytes 1 The melanocytes are scattered among the keratinocytes throughout the stratum malpighi hair follicles and dermal connective tissues a They produce the protective pigment Melanin b Melanocytes are derived from neural crest ectoderm 1 Morphologically they are dendrite bearing cells with a prominent perinuclear cytoplasm a They also have cytoplasmic processes which extend to neighboring keratinocytes 2 Melanocytes produce melanin through the process of Melanogenesis a Melanogenesis occurs in the Melanosomes ofthe melanocytes 1 Melanosomes are membrane bound pigment forming organelles a They contain the amino acid Tyrosine which undergoes oxidation to produce melanin 2 Melanin will accumulate in the melanosomes as melanogenesis 133 progresses 3 Once melanogenesis is completed melanin bearing melanosomes are transferred to the adjacent keratinocytes by the cytoplasmic processes of the melanocytes b There are no sex or racial difference in the approximate number of melanocytes 1 The differences are due to the size and number of the melanosomes as well as the distribution ofthe melanocytes within the keratinocytes c The Langerhans cells 1 These cells are located throughout the stratum malpighi 2 Appearance a Like the monocytes they are derived from Langerhans cells have an indented nucleus b They have a clear cytoplasm containing modest amounts ofendoplasmic reticuli Golgi and mitochondria c They also have rodshaped granules called Langerhans Granules 1 The function of Langerhans granules is still undetermined 3 Langerhans cells clear antigens from the integument a This is done both by phagocytosis and by sensitizing lymphocytes to the antigen 1 The Langerhans cell will bind to antigens and migrate to the lymph nodes so as to stimulate production of antibodies specific for the antigen 4 Today Langerhans cells are known to exist in other stratified squamous epithelia a Ex the oral mucosathe esophagus the cervix and the vagina b They are also found in hair follicles sebaceous glands and their ducts apocrine gland ducts the dermis the thymus lymph nodes and dermal lymph vessels c 80 Langerhans cells may prove to be circulating cells of a greater immunological function than had been previously expected d The Merkel Cells 1 They are distributed among the keratinocytes in or near the stratum basale 2 Merkel cells have a distinctive clear appearance a They have an irregularly shaped nucleus and many scattered granules 1 These granules may produce polypeptide hormones but their placement in the neuroendocrine system is strictly conjecture at this point 3 Merkel cells have an intimate association with dermal and epidermal nerve bers that are believed to be sensory in nature a As a result they are believed to be involved in touch reception 134 C The Dermis 1 Introduction a Like all epithelia the epidermis rests on a basement membrane which sits on an underlying loose connective tissue 1 The dermis is this connective tissue layer in the skin a The interface between the dermis and epidermis is called the Dermoepidermal Junction 1 It varies throughout the body from relatively smooth in thin skin to highly corrugated in thick skin 2 Projections ofthe dermis called Papillae interdigitate with the epidermis to better establish this interaction b Like other connective tissues the dermis is composed of various morphologic and functional frameworks collagen reticular and elastic bers embedded in an amorphous ground substance 1 It also includes the customary nerves blood vessels and lymph vessels 2 The dermis will also contain various cutaneous appendages which have invaginated from the epidermis 3 Smooth muscle is found in the dermis a It is associated with the blood vessels b It is also associated with the hair follicles where it is known as the Arrector Pili Muscle c In some regions of the body dermal smooth muscle may also be arranged into a meshwork 1 EX the dartos muscle of the scrotum c The cellular elements are relatively sparse 1 The most abundant cell type is the broblast 1 There are also macrophages mast cells melanocytes plasma cells adipocytes and a variety of wandering leucocytes 2 The Dermis ls Divided lnto Two Lavers a The Papillam Layer 1 The papillary layer is the most superficial layer it sitsjust below the epidermis a It gets it39s name from the dermal papillae which interdigitate with undulations ofthe epidermis 2 The papillary layer comprises 20 ofthe dermis 3 The papillary layer is a loose connective tissue a It is characterized by a loose arrangement of relatively thin bers embedded in a considerable amount of ground substance b The Reticular Layer 1 The reticular layer is the deeper portion ofthe dermis it sits just above the hypodermis 2 The reticular layer comprises 80 ofthe dermis 3 The reticular layer is a dense irregular connective tissue 135 a It is characterized by a dense pattern ofthick bers embedded in lesser amounts of ground substance b There are fewer cellular elements in this portion ofthe dermis 4 The term quotreticularquot comes from the meshwork or retinaculumn of collagen bers in this layer not to the presence of reticular bers 3 Most of the nerve supply and all ofthe blood supply ofthe integument is found in the dermis and hypodermis a Remember the vasculature ofthe dermis services the epidermis as well as the dermis b There are many atriovenous anastomes to allow for atriovenous shunts past capillary beds to help control such as blood volume and heat loss 4 Functions of the Dermis a It protects and cushions the body b It provides resistance to mechanical stresses c It serves as a barrier to infection d It participates actively in wound healing and in ammation e It has an inductive effect on the epidermis f It is involved in the thermoregulation of the body D The Hypodermis 1 The hypodermis is a layer of loose connective tissue rich in adipocytes located immediately deep to the skin a There are two classes of hypodermis 1 Panniculus Adiposis a hypodermis which is massively in ltrated with adipocytes a This makes up the bulk of the hypodermis in humans 2 Panniculus Carnosus a hypodermis which contains sheets of skeletal muscle a These cutaneous muscles move the skin b The panniculus carnosus is well developed in all mammals except for humans 1 In humans the panniculus carnosus is restricted to the anterior neck and the muscle component is known as the platysma muscle b The hypodermis will vary in thickness throughout the body and in response to nutritional levels 0 The hypodermis is connected to underlying structures such as the deep fascia aponeuroses or periosteum d The hypodermis has a number of functions 1 It provides support for the above lying skin 2 It stores fat 3 It cushions the body 4 It insulates the body 136 5 It serves as a passage for the large blood vessels ofthe skin e The dermis and hypodermis contain some of the sensory receptors of the integument 1 Pacinian Corpuscles a Pacinian corpuscles are large encapsulated dendritic endings found in the hypodermis and occasionally in the dermis which monitor vibrations 1 They are found in all ofthe deep connective tissues of the body a They are especially abundant in the ngers and also in deep musculoskeletal tissues 2 Genital Corpuscles a Genital corpuscles are similar to pacinian corpuscles but are restricted to the external genitalia 3 Meissner39s Corpuscles a These are smaller encapsulated dendritic endings found in the dermal papillae of hairless skin 1 They are especially abundant in the tips ofthe ngers and toes b Meissner39s corpuscles are touch receptors E The Cutaneous Appendages 1 The Sudoriferous Glands a Sudoriferous glands are the sweat glands 1 Based on mode and nature of secretion the sudoriferous glands are divided into two classes apocrine and eccrine b Eccrine Glands 1 Eccrine glands are the most numerous and widely distributed class of sudoriferous gland a They are found throughout the integument being absent only in a very few areas 1 They are absent at the lips glans penis glans clitoris labia minora and beneath the nails b They are at their greatest numbers per unit area on the palms and soles 2 Eccrine glands are simple coiled tubular glands a Their tightly coiled secretory unity may be located in either the dermis or hypodermis 1 This causes two subclasses of eccrine glands to be recognized Shallow and Deep Eccrine Glands respectively 3 Histology a The duct is composed ofa two layered stratified cuboidal epithelium b The secretory unit is composed of a single layer of pyramidal shaped cells 1 Myoepithelial cells are located between the secretory cells and the 137 basement membrane 2 The secretory portion is composed of two cell types a Clear Cells clear staining cells having a broad base and a narrow apex 1 They secrete a solute containing watery uid through an active transportdiffusion mechanism a form of merocrine secretion b Dark Cells darkly staining cells having a narrow base and a broad apex 1 They secrete a mucinrich uid through merocrine secretion 4 Eccrine glands function primarily in thermoregulation and slightly in the removal of wastes c Apocrine Glands 1 Apocrine glands are much less widely distributed than are eccrine glands a They are found primarily in the axillary and perianal areas b They are associated with hair follicles c they become active with the onset of puberty 2 Structurally apocrine glands are similar to eccrine glands in that they are also coiled tubular glands a However the secretory portion is larger in apocrine glands 1 The secretory portion can also be located in either the dermis or hypodermis b The duct will run parallel to a hair follicle 1 It opens into the hair follicle a Typically the duct opens into the hair follicle above the opening point of the sebaceous gland 3 Histology a The duct is a two layered strati ed cuboidal epithelium b The secretory portion is composed of a single layer of cuboidal or columnar shaped cells resting on a basement membrane 1 Between the secretory cells and the basement membrane are myoepithelial cells 2 The secretory cells have basally oriented nuclei numerous mitochondria numerous Golgi and numerous granules 3 Their product is secreted by merocrine apocrine and holocrine mechanisms 4 Along with playing a role in thermoregulation and the removal of wastes apocrine glands may also release pheromones and may play a role in chemical communication 5 Specialized Apocrine Glands a Ceruminous Glands lining the external auditory canal these glands produce Cerumen ear wax b Mammary Glands 2 The Sebaceous Glands a Sebaceous glands are also widely distributed glands being found on all surfaces ofthe body except for the soles and palms 138 1 The secretory portion is located in the dermis 2 Their ducts are associated with hair follicles and they release their product into the upper hair follicle b Sebaceous glands are simple branched acinar glands c Histology 1 The duct is composed of strati ed squamous epithelium keratinized a It is continuous with the epithelium of the epidermis andor hair follicle 2 The secretory portion consists of acinar cells in various stages of differentiation a In the periphery of the acinus the cells are small and resemble basal keratinocytes b Towards the center of the acinus the cells are enlargened with accumulated lipid 1 As the amount oflipid increases in the cell the cell dies d Mode of Secretion and Secretion 1 The sebaceous glands secrete their product by holocrine mechanisms 2 The product ofthe sebaceous gland is Sebum oil a The release of sebum is under hormonal control 1 Sebaceous glands become enlarged and more active with the onset of puberty b Sebum may lubricate and soften hairs and the cornified layers of the skin 1 It may also help to prevent their drying out 0 Sebum may also contain some pheromones 3 The Hair a Hairs are filamentous strands of fused keratinocytes projecting from the surface of the skin 1 Hairs are produced by and located in Follicles a Follicles are epidermal invaginations into the dermis andor hypodermis 2 Hairs are found over most of the body a The distribution number and thickness of the follicles will vary 3 The follicles are associated with sebaceous glands and form units called Pilosebaceous Units 4 Hairs grow to a certain length enter a resting phase are shed and are then replaced periodically a This is called the Hair Growth Cycle 5 Projecting into the base of the hair follicle is an indentation of highly vascularized connective tissue known as the Dermal Papillae a The dermal papillae has an inductive influence on hair formation 1 lfthe dermal papillae is destroyed the hair can not be replaced 6 The keratin of the hair is different from the keratin of the epidermis being what is called quotHard Keratinquot b Structure 1 The hair has an exposed portion called the Shaft a The shaft has a central Medulla typically 1 The medulla may be lacking in thinner hairs 139 b Surrounding the medulla ofthe shaft is the Cortex c Surrounding the cortex is the Cuticle which is composed of overlapping plates of keratinocytes 2 The hair also has a portion embedded in the integument called the m a The root also has an expanded portion at it39s base called the w b The root contains Matrix Cells 1 Matrix cells are pleuripotent stem cells located in the hair bulb and in the dermal papillae a They have a high degree of mitotic activity and will give rise to the keratinocytes of the hair 2 Matrix cells derive from basal epidermal cells c The root also will contain some melanocytes especially in the hair bulb and even in the dermal papillae 1As in the epidermis these melanocytes will provide pigmentation to the developing keratinocytes 3 Surrounding the root and those portions of the shaft located within the skin are the Root Sheaths a The Internal Root Sheath is the inner sheath 1 It is composed ofa sleeve of cells extending from the root to about the level ofthe sebaceous pore a The cells are derived from matrix cells b The External Root Sheath is located outside ofthe internal root sheath 1 It is an extension ofthe basal layers ofthe surrounding epidermis 4 The hair follicle is made up ofthe embedded portions of the hair and the surrounding root sheaths 0 Associated with the hair follicle are a sebaceous glands b on rare occasion an apocrine gland c the arrector pili muscle d and the Root Hair Plexus 1 The root hair plexus is a meshwork of free dendritic endings located at the base ofthe hair a These nerve endings are tactile receptors d The hair plays a role in 1 thermoregulation 2 protection 3 and tactile perception 4 The Nails a The nger and toe nails are tightly packed plates of keratinocytes that cover the dorsal surfaces ofthe digits at their distal ends 1 Like hair nails are composed of hard keratin b The nail will have a Matrix located in the dermis as did the hair 1 The matrix is mitotically active and will give rise to the nail plate 2 It is derived from an invagination ofthe epidermis 3 The distal portion of the matrix can often be seen in the nail as a half moon 140 shaped white area called the Lunula c The portion of the skin that the nail covers or rest on is called the Nail Bed 1 The epidermis ofthe nail bed consists only of the stratum basale and stratum spinosum typically 141 Unit 12 THE URINARY SYSTEM A Introduction 1 The urinary system includes the kidneys ureters urinary bladder and urethra 2 The kidneys regulate the composition and volume of body fluids by constantly adjusting blood plasma volume and constituents that circulate through them a This regulation is accomplished by 1 the retention of important circulating elements 2 the excretion of certain metabolic wastes 3 the variable retention of body water 4 the differential regulation of salt content 5 and the maintenance ofthe acidbase balance b The formation of urine is basically the byproduct of those continual renal processes that preserve the body39s internal uid environment against disruptive in uences from the external environment 1 Through the regulation ofthe composition and the volume of circulating plasma the kidneys also will in uence the immediate internal environment bathing every cell B The Kidneys 1 General Organization and Function a The kidneys are paired organs located retroperitoneally high on the posterior abdominal wall 1 Each kidney is surrounded by three layers of connective tissue a The outermost layer is called the Renal Fascia 1 The renal fascia is a thin layer of brous connective tissue which anchors the kidney to the abdominal wall and to other adjacent structures b The middle layer is called the Adipose Capsule 1 As the name indicates the adipose capsule is a layer of adipose tissue designed to protect the kidney from mechanical trauma 0 The innermost layer is called the Renal Capsule 1 The renal capsule is a thin but tough collagenous capsule composed of dense irregular connective tissue a The renal capsule is continuous with the hilus ofthe kidney and with the tunica fibrosa of the associated ureter b The functional and structural subunits ofthe renal parenchyma are the Uriniferous Tubules 1 The uriniferous tubules are radially arranged around a cavity called the 142 Renal Sinus a This radial pattern causes the kidney to be quotbeanshapedquot b The renal sinus has an expanded portion of the ureter called the Renal Pelvis 1 The renal pelvis will branch into the Ma39or and the Minor Calyces a The minor calyces collect urine from the uriniferous tubules and drain into the major calyces 2 The ureter will exit the kidney at a concave depression on the medial surface called the M a The renal artery and vein will also pass through this hilus 2 The uriniferous tubules will receive a portion ofthe circulating plasma that is routinely ltered away from the incoming renal blood supply a The epithelium of these tubules will modify the ltered plasma as it passes through the various segments ofthe tubules 1 Most of the water and essential constituents will be resorbed by the tubules and returned to the blood 2 The remainder which will contain mostly toxic metabolites is concentrated and excreted as urine 3 As a result the histology ofthe renal parenchyma represents an intimate morphological relationship between the renal vasculature and the uriniferous tubules 4 Since speci c portions of the uriniferous tubules are found in speci c regions ofthe parenchyma the kidney is divided into a Cortex and a Medulla a The cortex is the outer portion ofthe kidney immediately deep to the renal capsule 1 There are two zones to the cortex a an outer zone called the Cortical Zone b and an inner zone immediately adjacent to the medulla called the Juxtamedullam Zone 2 The cortex will send extensions into the medulla a These extensions are called Renal ColumnsCortical Columns b The medulla is deep to the cortex and surrounds the renal sinus 1 It is divided by the renal columns into pyramidal shaped portions called Renal PvramidsMedullarv Pvramids a There are between 8 and 18 renal pyramids b The base of each renal pyramid faces the cortex 0 The apex of each pyramid is called the Papillae and projects into the funnelshaped opening of a minor calyx 5 The kidney is a multilobed organ a A renal lobe is defined as a renal pyramid and the renal columns flanking It b Each lobe is associated with it39s own minor calyx 1 The uriniferous tubules of each lobe open on the papilla and form a perforated structure called the Area Cribosa 143 2 An Overview ofthe Uriniferous Tubules a The uriniferous tubules are composed of two distinct functional regions 1 The two regions are a the Renal Tubule which is involved in the production of urine b and the Collecting Tubule which is involved in the hypertonic concentration ofthe urine 2 These two regions arise separately from one another in the embryo a In the mature kidney several nephrons will fuse with and empty into a single collecting duct b The Renal Tubule 1 The renal tubule is a part of the functional unit called the Nephron a The Nephron is defined as a renal tubule and it39s associated vascular components 2 The renal tubule of the nephron is a continuous tubule consisting of histologically and functionally distinct regions along it39s length composed of a simple epithelium a The renal tubule begins within the cortex as Bowman39sGlomerular Capsule 1 Bowman39s capsule is an indented blindended pouch 2 Associated with Bowman39s capsule will be specialized capillaries located within the indention called Glomerular Capillaries a Together the glomerular capillaries and the glomerular capsule form the Renal Corpuscle ofthe nephron 1 This is where blood is filtered by the kidney to begin the formation of urine a The resultant ltrate is termed the Glomerular Filtrate b The glomerular filtrate will ow from Bowman39s capsule to other portions of the renal tubule where it will be modi ed through a process of tubular resorption and tubular excretion 1 The Proximal Convoluted Tubule receive the filtrate from the glomerular capsule a The proximal convoluted tubule is also located in the cortex but travels towards the medulla 2 The Proximal Straight Tubule receive the precocial urine from the proximal convoluted tubule a The proximal straight tubule is located in the medulla and dives down towards the pelvis 3 The Distal Straight Tubule receive the precocial urine from the proximal straight tubule a The distal straight tubule is also located in the medulla but travels up towards the cortex 1 The proximal straight and distal straight segments of the renal tubule form the Loop of Henle and are also known as the Ascending and Descending Limbs respectively 1 The Distal Convoluted Tubule receive the ltrate from the distal 144 straight tubule a The distal convoluted tubule is also located in the cortex 1 This is to allow for some regulatory in uences to occur b The distal straight tubule empties into the collecting tubule c The Collecting Tubule 1 V thin the cortex several distal convoluted tubules will empty into a collecting tubule a The distal convoluted tubule is connected to the collecting tubule by short Connecting Tubules b The portion of the collecting tubule located in the cortex and receiving urine from the distal convoluted tubules is often referred to as the Collecting Duct 2 The collecting tubule will extend through the medulla and open into the minor calyx a The portion of the collecting tubule which travels through the medulla is often referred to as the Papillary Duct 1 The papillary ducts and loops of Henle are clustered into small parallel groups called Medullam Rays in the medulla 3 The Blood Supply ofthe Kidney a Arterial Flow 1 The Right and Left Renal Arteries branch off ofthe abdominal aorta one going to each kidney 2 After entering the hilus the renal artery splits into the Segmental Arteries a There are typically segmental arteries in each kidney called the dorsal and ventral branches 3 The segmental arteries branch into the Lobar Arteries 4 The lobar arteries branch into the lnterlobar Arteries which will each penetrate a cortical column a They will extend radially to the corticomedullary junction 5 At the corticomedullary junction the interlobar arteries bifurcate and give rise to the Arcuate Arteries which will arch around the base of a medullary pyramid 6 The arcuate arteries give rise to a number of smaller arteries called the lnterlobular Arteries which will travel towards the renal corpuscle 7 At the renal corpuscle the interlobular artery will branch into the Afferent Arteriole 8 The afferent arteriole will branch into the Glomerular Capillaries of the renal corpuscle 9 The glomerular capillaries will drain into the Efferent Arteriole b From this point the efferent arterioles ofthe two types of nephrons differ 1 There are two types of nephrons a One type has it39s renal corpuscle located in the cortical zone and does not extend very far into the medulla It is the Cortical Nephron b The second type has it39s renal corpuscle located in the juxtamedullary zone and does extend far into the medulla It is the Juxtamedullam 145 Nephron 1 The juxtamedullary nephron has a much longer loop of Henle segment and two associated capillary beds which is why it is responsible for the bulk of water resorption a The cortical nephron39s loop of Henle has one associated capillary bed 2 The efferent arteriole coming off of a renal corpuscle from a cortical nephron will branch into the Peritubular Capillam Bed a The peritubular capillary bed will be in close association with the renal tubule from the proximal convoluted to the distal convoluted segments 3 The efferent arteriole coming off of a renal corpuscle from a juxtamedullary nephron will branch into two capillary beds a One is the Peritubular Capillam Bed which will be in close association with the renal tubule from the proximal convoluted to the distal convoluted segments b The other is the Vasa Recta which will actually arise from peritubular capillaries 1 These are thin walled capillaries that are arranged into long recurrent loops running through the medulla 2 It acts as a countercurrent mechanism in the resorptionsecretion of salt and water 0 Venous Supply is for the most part a mirror image ofthe arterial supply 1 The capillary beds are drained by the lnterlobular Veins 2 The interlobular veins are drained by the Arcuate Veins 3 The arcuate veins are drained by the lnterlobar Veins 4 The interlobar veins are drained by the Lobar Veins 5 The lobar veins are drained by the renal vein which exits the kidney at the hilus a Some individuals will have Segmental Veins which will drain the lobar veins and drain into the renal vein b The right and left renal veins will drain into the inferior vena cava 4 The Histology of the Renal Corpuscle a The renal corpuscle or glomerulus is the portion of the nephron responsible for the ltration of blood 1 It is composed oftwo structures that are in close association the glomerular capsule and the glomerular capillaries b Bowman39sGlomerular Capsule 1 The glomerular capsule is the start of the renal tubule 2 It has two walls an outer wall and an inner wall a The outer wall ofthe glomerular capsule is called the Parietal Layer 1 It is also known as the parietal epithelium or capsular epithelium b The inner wall is called the Visceral Layer and begins where the capsule has been indented to accommodate the glomerular capillaries 1 It is also called the visceral epithelium or glomerular epithelium 0 Between the two walls is a space called the Capsular SpaceUrinam 146 Space 1 The glomerular filtrate crosses into the capsular space from across the visceral layer 2 The urinary space is continuous with the lumen of the rest of the renal tubule a This fact causes the renal corpuscle to be divided into two poles 1 Vascular Pole where the glomerular capillaries are located 2 Urinary Pole where the capsule joins the proximal convoluted tubule 3 The Epithelium ofthe Glomerular Capsule a The epithelium of the parietal layer is a simple squamous epithelium 1 At the urinary pole it ends to be replaced by the simple cuboidal epithelium of the proximal convoluted tubule 2 At the vascular pole it is continuous with the epithelium of the visceral layer b The epithelium ofthe visceral layer is a highly modified simple epithelium 1 These epithelial cells are called Podocytes a Podocytes have large primary processes which wrap around the glomerular capillaries b Branching off ofthe primary processes are numerous small secondary processes called Pedicels 1 Pedicels of adjacent primary processes interdigitate with one another a The spaces between these interdigitating pedicels are called Slit PoresFiltration Slits b Slit pores are about 25 nm wide and are covered by a thin membrane referred to as the Slit Membrane 2 The visceral epithelium is designed to have an extremely close association with the glomerular capillaries so as to better filter the blood c The Glomerular Capillaries 1 Glomerular capillaries are fenestrated capillaries lacking diaphragms over the fenestra a These fenestra serve as another means of ltering the blood 2 An unusually thick basal lamina intervenes between the podocytes and the endothelium ofthe glomerular capillaries a This basal lamina serves as another means of ltering the blood b It is actually the fusion ofthe components of the basal lamina form both the podocytes and the endothelial cells d The Filtration Membrane aka ltration barrier allows for the ltration of blood by the nephron and consists ofthree things 1 The blood will rst encounter the fenestra of the glomerular capillaries a Due to the lack of diaphragms they allow for a rapid ow of plasma across the capillary wall b Due to their size however the fenestra prevent the passage of formed 147 elements 2 The plasma will next encounterthe basal lamina a The basal lamina is selectively permeable based on molecular weight and size 1 They restrict or prevent the passage of substances over 10 nm in diameter 2 They restrict or prevent the passage of substances over molecular weight of 68000 amu a Note albumin has a molecular weight of 65000 amu 3 Smaller substances such as simple sugars amino acids metabolites and even small peptides can pass freely across the basal lamina b Some new evidence appears to indicate that the basal lamina may also be selectively permeable based on electrostatic charge 3 The nal barrier to ltration is the size ofthe ltration slits or slit pores between the pedicels 4 The driving force for ltration is pressure called ltration pressure a Filtration pressure is the net difference in the sum of glomerular hydrostatic pressure in the blood vessels and osmotic pressure ofthe glomerular capsule 5 The Histology of the Proximal Convoluted Tubule a As the glomerular filtrate passes through the uriniferous tubules most of it39s volume and much of it39s dissolved constituents are absorbed by the tubules and returned to the blood The remainder becomes more concentrated 1 In mammals most of reabsorption 80 occurs in the proximal tubule in general and most of that in the proximal convoluted tubule in particular a The materials resorbed are retrieved by the peritubular capillaries b The remaining materials to be recovered from the precocial urine are resorbed in the distal tubule c The proximal tubule will resorb amino acids small proteins glucose and electrolytes such as sodium potassium chloride and bicarbonate 1Sodium is actively transported out ofthe tubule and into the capillaries a Since this requires energy the cells ofthe proximal convoluted tubule will have many mitochondria b The active transport of sodium establishes a diffusion gradient for the passive transport of anions particularly chloride 1 This in turn establishes a diffusion gradient for osmosis 2 Nitrogenous wastes such as urea uric acid ammonia and creatine are not resorbed aThey become more concentrated in the tubule as resorption occurs b They are actively secreted into the tubule from the blood 3 Tubular resorption and tubular secretion are designed to modify the glomerular filtrate so as to create a urine which maintains homeostasis in the body b The epithelium of the proximal convoluted tubule is a simple cuboidal type and displays many adaptations to serve it39s role in tubular secretion and tubular 148


Buy Material

Are you sure you want to buy this material for

25 Karma

Buy Material

BOOM! Enjoy Your Free Notes!

We've added these Notes to your profile, click here to view them now.


You're already Subscribed!

Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'

Why people love StudySoup

Steve Martinelli UC Los Angeles

"There's no way I would have passed my Organic Chemistry class this semester without the notes and study guides I got from StudySoup."

Jennifer McGill UCSF Med School

"Selling my MCAT study guides and notes has been a great source of side revenue while I'm in school. Some months I'm making over $500! Plus, it makes me happy knowing that I'm helping future med students with their MCAT."

Jim McGreen Ohio University

"Knowing I can count on the Elite Notetaker in my class allows me to focus on what the professor is saying instead of just scribbling notes the whole time and falling behind."

Parker Thompson 500 Startups

"It's a great way for students to improve their educational experience and it seemed like a product that everybody wants, so all the people participating are winning."

Become an Elite Notetaker and start selling your notes online!

Refund Policy


All subscriptions to StudySoup are paid in full at the time of subscribing. To change your credit card information or to cancel your subscription, go to "Edit Settings". All credit card information will be available there. If you should decide to cancel your subscription, it will continue to be valid until the next payment period, as all payments for the current period were made in advance. For special circumstances, please email


StudySoup has more than 1 million course-specific study resources to help students study smarter. If you’re having trouble finding what you’re looking for, our customer support team can help you find what you need! Feel free to contact them here:

Recurring Subscriptions: If you have canceled your recurring subscription on the day of renewal and have not downloaded any documents, you may request a refund by submitting an email to

Satisfaction Guarantee: If you’re not satisfied with your subscription, you can contact us for further help. Contact must be made within 3 business days of your subscription purchase and your refund request will be subject for review.

Please Note: Refunds can never be provided more than 30 days after the initial purchase date regardless of your activity on the site.