VET HISTOLOGY VMED 7123
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Lecture Notes Veterinary Histology VMED 7123 Dr Charlotte L Ownby Fall Semester 2004 For Lecture Notes with Color Images use go to wwwcvmokstateedu instruction mm curr histolo htt stolongeferencel indexhtm Under Course Outline double click on Organ System of Interest Digestive System I lips tongue salivary glands esophagus stomach small and large intestines The digestive system includes the gastrointestinal tract as well as associated organs like the pancreas and liver Digestive System I will cover the oral cavity lips tongue major salivary glands and the gastrointestinal tract ie esophagus stomach small and large intestines The digestive system consists throughout most of its length of a series of tubular organs lined with speci c types of epithelium to ful ll speci c functions related to the digestion and absorption of nutrients from a food source and the elimination of waste products Organ Function Lips i Ingestion and fragmentation of food Teeth Fragmentation of food Tongue Fragmentation and swallowing Salivary Glands Fragmentation and moistening of food swallowing Esophagus Passage of food from oral cavity to the stomach Completion of fragmentation and beginning of digestion Digestion emulsi caton of fats by enzymes from the pancreas and bile from the liver Stomach Small Intestine duodenum Small Intestine jejunum amp Completion of digestion and absorption ileum Large Intestine cecum Absorption of water from liquid residue Large Intestine colon Absorption of water from liquid residue Large Intestine rectum Storage of feces prior to defecation Anus Route for defecation of feces outside the body IOral Cavity Organs that make up the oral cavity include the lips teeth tongue and major salivary glands These organs function to obtain and ingest food fragment it into smaller particles moisten and swallow it Teeth will not be covered in this course Lips The lips aid in obtaining food and placing it in the mouth so that the teeth and tongue can manipulate it and begin fragmenting it Lips are covered by a stratified squamous epithelium that is usually keratinized on the outer surface and contains many hairs whereas the epithelium on the inner surface is more moist and nonkeratinized Tongue The tongue is a highly muscular organ used to manipulate food in the mouth and for the sense of taste It is covered with stratified squamous epithelium that in the anterior part forms specialized structures known as papillae that are involved in the manipulation of food as well as in the sense of taste The skeletal muscle of the tongue is unique in that it runs in three different directions allowing for a wide range of movements needed to properly manipulate foodstuffs The types numbers and distribution of papillae in the tongue vary greatly among species In domestic animals there are usually five different types of papillae 1 Filiform papillae are highly keratinized sharply pointed and aid in mechanically breaking up food material They are numerous in ruminants and cats where they are used in lapping milk 2 Fungiform papillae are smooth with a rounded surface They help manipulate the food but also have taste buds on their lateral surfaces 3 Conical papillae are somewhat larger than fungiform papillae are used in manipulating and breaking down ingested food They can be distinguished from fungiform papillae by their larger size tendency to project above other papillae and they do not have taste buds 4 Foliate papillae covered with nonkeratinized stratified squamous epithelium are leaf shaped structures defined by an invagination of the mucous membrane on their sides Many taste buds on their lateral surfaces indicate their role in gustation They are absent in ruminants but well developed in the horse and dog 5 Circumvallate papillae are the largest up to 18 inch diameter papillae They are surrounded by a deep indentation of the mucous membrane and are not numerous They do not rise above the surface of the tongue Many taste buds are located on their sides Serous von Ebner s glands empty into the quotmoatquot around these papillae and help keep it free of food particles ISalivary Glands The salivary glands all empty their secretions into the buccal cavity They vary as to their distance from the buccal cavity their size and the nature of their secretory products They can also be divided into major and minor glands We will consider only the major salivary glands of which there are three parotid sublingual and submandibular These glands all have the tubuloalveolar glandular structure and all are r ie r J of secretory endpieces connected by an elaborate system of branching ducts In general saliva is a dilute hypotonic solution containing various enzymes esp amylase and lysozyme and other proteins such as antibodies glycoproteins as well as electrolytes Saliva in the buccal cavity is the combined secretion of the numerous salivary glands both major and minor The secretions of salivary cells can be either of a serous type ie watery and rich in enzymes and antibodies or mucous ie viscid containing more glycoproteins Individual salivary glands may contain mostly cells of the serous type of the mucous type or a mixture of both types The nal composition of saliva at any given time depends on the proportion contributed by speci c salivary glands and is determined in the major glands by the parasympathetic nervous system resulting from physical chemical and psychological stimuli Salivary Gland Type of Secretory Cells Parotid Serous Sublingual Mucous 1Submandibular Mixed lBasic Plan of the Digestive Tube From the esophagus to the anus the digestive system is basically a tube very similar to other tubular organs in the body All such tubular organs are composed of several tissue layers arranged around a lumen In a quotgenericquot tubular organ these layers are as follows from the lumen to the ablumenal layer Tunica mucosa This layer is composed of epithelium connective tissue and muscle These tissues can usually be found in distinct layers as follows lamina epithelialis mucosae consists only of epithelium lamina propria mucosae consists of either loose areolar or reticular connective tissue lamina muscularis mucosae consists of smooth muscle Tunica submucosa consists of loose connective tissue nerves blood vessels and glands in some organs Tunica muscularis consists of at least two layers an inner circular and an outer longitudinal with parasympathetic ganglia located between the layers Tunica adventitia 01 tunica serosa consists of loose connective tissue If the organ is surrounded by other tissues this layer is called a tunica adventitia and its connective tissue blends with that of the surrounding tissues If the organ is suspended in the body cavity this layer is called a tunica serosa and it is covered by a simple squamous epithelium that is called mesothelium I Esophagus The esophagus connects the oral cavity with the stomach allowing and aiding in the movement of food particles to the stomach It is a muscular tube having the layers described above for the typical tubular organ In the esophagus the layers are specialized for the function of further fragmenting food particles Layers of the Esophagus Tunica mucosa o lamina epithelialis consists of stratified squamous epithelium that can be highly folded in an empty organ may be highly keratinized in animals that ingest hard dry materials such as herbivores lamina propria consists of loose connective tissue which often has scattered lymph nodules esp in pigs and humans lamina muscularis mucosae consists of smooth muscle distribution and continuity is highly species variable as follows 1 continuous in human 2 separate muscle bundles that fuse in horses ruminants and cats 3 absent in cervical part in dogs 4 absent in pigs in cervical region but complete near the stomach O O Tunica submucosa consists of loose connective tissue that is very elastic allowing for expansion when food is present ties the overlying epithelium to the underlying muscle layers seromucous glands present in most species and numerous in the dog but absent in horses and cats Lymphoid nodules may be present in the pig esophagus Tunica muscularis consists of smooth andor skeletal muscle inner circular and outer longitudinal layers usually begin as skeletal muscle at the cervical end voluntary control of swallowing changing to smooth near the distal end close to the stomach skeletal muscle throughout in ruminants and the dog Tunica serosaadventitia consists of typical loose connective tissue that blends into the connective tissue of surrounding tissues IStomach The stomach connects the esophagus to the intestines and in most species serves not only to continue the breakdown of foodstuffs via the use of digestive enzymes and acid but it also as a storage depot for food Usually food remains in the stomach a few hours during which it is converted into a liquid material called chyme Stomachs are either simple or compound ie consisting of one chamber or many chambers Simple stomachs are composed primarily of glands that is the tunica mucosa is filled with glands Ruminant stomachs are compound stomachs containing both nonglandular and glandular regions The nonglandular regions include the reticulum rumen and the omasum The glandular region is the abomasum which has its own regions similar to those found in a simple stomach Regional variation in the glands of the tunica mucosa of the stomach Not all regions of the stomach mucosa have the same histological structure They vary as follows cardia contains many mucussecreting glands fundus mostly glands secreting acidpeptic gastric juices some mucussecreting glands pylorus contains two different types of mucussecreting glands also contains endocrine cells secreting gastrin Wall of the Glandular Stomach Tunica mucosa in the empty stomach this layer is thrown into deep longitudinal folds called rugae that extend from the lamina muscularis mucosae to the lumen in the full stomach the rugae are much reduced in size as a result of distension of the tunica mucosa to accomodate the presence of a large amount of food material Tunica submucosa typical loose connective tissue contains parasympathetic ganglia located in submucosal plexuses also known as Meissner s plexuses Tunica muscularis typical smooth muscle consisting of at least two layers an inner circular layer and an outer longitudinal layer parasympathetic ganglia located between the two muscle layers in the myenteric or Auerbach39s plexus Tunica serosa typical small amount of loose connective tissue with overlying simple squamous epithelium or mesothelium Layers of the Tunica Mucosa of the Stomach Fundic Region Lamina epithelialis consists of simple columnar epithelium that forms branched tubular glands organized into gastric pits that open onto the lumen and gastric glands that empty into the base of the gastric pits Lamina propria consists of loose areolar connective tissue that in the glandular stomach is minimal between gastric glands and difficult to see in sections highly vascular containing many blood and lymphatic capillaries Stratum compactum consists of dense connective tissue containing thick collagen fibers located between the lamina propria and the lamina muscularis mucosae prominent in carnivores where it probably helps prevent the perforation of the wall of the stomach by sharp objects such as bones that might be present in the lumen Lamina muscularis mucosae consists of several layers of smooth muscle oriented both longitudinally and circularly usually not very thick Four cell types in the gastric gland Surface mucous cells line the gastric pit and secrete mucous and bicarbonate ions to protect the epithelium from digestion by gastric juice contains HCl and pepsin present in the stomach lumen Neck mucous cells found dispersed between the parietal cells secret a mucus that is thinner than that secreted by the surface mucous cells mucus protects other glandular cells from action of proteases and HCl Parietal oxyntic cells found throughout the gastric gland round cells that contain distinct eosinophilic pink cytoplasm and round prominent nucleus secrete HCl and intrinsic factor needed for absorption of vitamin B12 Chief peptic zymogenic cells found mostly near the base of the gastric glands very basophilic purple containing basally positioned nucleus and prominent basophilic apical cytoplasm filled with many ribosomes secrete pepsinogen which is converted to pepsin in the acidic milieu of the stomach Neuroendocrine cells difficult to distinguish by conventional light microscopy several types are present some secrete serotonin gastrin glucagon and somatostatin among other hormones Stem cells located primarily in the neck region difficult to identify in routine HampE sections undergo mitosis to form more cells then differentiate into the other cell types present in the gland Parasympathetic Ganglia Aggregations of parasympathetic ganglion cells are found throughout the digestive tube in two locations Some are located in the submucosa and are usually called Meissner s plexus others are located between the inner circular and outer longitundinal layers of smooth muscle in the tunica muscularis The latter ones are usually called myenteric or Auerbach39s plexus Postganglionic fibers from Meissner s plexus innervate the lamina muscularis mucosae whereas postganglionic fibers from the myenteric plexus innervate the smooth muscle of the tunica muscularis The two layers of smooth muscle in the tunica muscularis inherently contract in a wave of peristalsis that helps move stomach contents toward the small intestine However contractions of the smooth muscle are regulated by the autonomic nervous system as well as other factors such as hormones released into the stomach An increase in peristalsis results from an increase in parasympathetic stimulation a decrease in peristalsis results from an increase in sympathetic stimulation Meissner s plexus and the myenteric plexus both consist of the cell bodies of parasympathetic ganglion cells that are easily identified by their large size in comparison with other cells in the area and also by the large round nucleus that contains a prominent nucleolus These cell bodies are found in the midst of unmyelinated nerve fibers and near areas of myelinated axons ICompound stomach found in ruminants and has four parts Chamber Histology Function mechanical and chemical breakdown of food breakdown of food by Rumen nonglandular keratinized stratified m39cr9bes predgculon 0f volatile fatty acrds part Of squamous ep39thehum absorption of volatile fatty forestomach lacrds lactic acrd ammonia inorganic ions and water Reticulum nonglandular keratinized stratified part of squamous epithelium forestomach Omasum nonglandular keratinized stratified part of squamous epithelium forestomach Abomasum g39a d 39a Simple celumnar glandUIar enzymatic digestion epithelium Rumen Tunica mucosa characterized by the presence of long 1015 cm long conical projections called papillae that extend in t0 the lumen o Lamina epithelialis mucosae keratinized strati ed squamous o Lamina propria typical no glands o Lamina muscularis mucosae absent NOTE It is easy to confuse a thickened layer of connective tissue that extends into the papilla with a lamina muscularis mucosae but this tissue is connective tissue not smooth muscle Tunica submucosa merges with lamina propria no glands or lymphoid aggregates Tunica muscularis typical Tunica serosa typical Reticulum Similar to rumen except as noted below Tunica mucosa when viewed from the lumen of the reticulum the mucosa looks like a quothoneycombquot or reticulum The basis of this honeycomb is a series of connected vertical 8 primary folds that give rise to secondary and tertiary papillae which project into the lumen Lamina muscularis mucosa a layer of smooth muscle extends from the tips of the papillae down to the position of the lamina muscularis mucosa although in the reticulum this layer is not quite typical However the smooth muscle in the reticulum is continuous with the smooth muscle of the lamina muscularis mucosa in the esophagus Other tunics are typical Omasum This part of the nonglandular region of the compound stomach is notable for the complexity of the foldings of its tunica mucosa These folds or laminae are covered with a highly keratinized strati ed squamous epithelium Underlying this epithelium is the sparse loose connective tissue of the lamina propria The laminae muscularis mucosa extends into the primary laminae usually in two layers In between these two layers of the laminae muscularis mucosae there is a layer of smooth muscle belonging to the tunica muscularis These three layers of smooth muscle intertwine as they extend toward the tip of the laminae and eventually fuse to form one large mass of muscle at the tip Abomasum The abomasum is the glandular part of the compound stomach and histologically it is essentially the same as a simple stomach lSmall Intestine The small intestine is a typical tubular organ in that it has all of the typical tunics and layers However the tunica mucosa is especially modi ed to ful ll the function of absorption Also the three regions of the small intestine the duodenum the jejunum and the ileum each have special modi cations to the wall to enable each region to better perform its particular function In the small intestine digestion occurs in the lumen as well as at the surface of the lining epithelial cells Pancreatic enzymes such as trypsin chymotrypsin elastase carboxypeptidases peptide hydrolases amylase and lipases are adsorbed onto the membrane surface of the epithelial cells where they mix with the chyme present in the lumen catalyzing the breakdown of proteins carbohydrates and lipids The smaller breakdown products are then absorbed by the lining epithelial cells that are called enterocytes Layers of the Small Intestine Tunica mucosa This layer protrudes out into the lumen as projections called villi and it dips down to the underlying lamina muscularis mucosae forming pockets called crypts o Lamina epithelialis mucosae simple columnar epithelium villus a villus contains enterocytes absorption goblet cells protective mucus in its upper region and neuroendocrine cells local hormones 9 crypt a crypt crypt of Lieberkuhn contains goblet cells paneth cells defensive neuroendocrine cells stem cells intraepithelial lymphocytes defensive Lamina propria loose connective tissue rich in blood and lymphatic vessels present in the core of the villi and between crypts o Lamina muscularis mucosae thin layer of smooth muscle located at the base of the crypts O Tunica submucosa This layer blends with the lamina propria and is typical In the duodenum it has coiled branched glands known as Brunner s glands the ducts of which open into the base of the crypts Tunica muscularis typical consisting of an inner circular layer and an outer longitudinal layer Tunica serosa typical Enteroendocrine cells These cells secrete hormones such as secretin somatostatin enteroglucagon and serotonin one hormone per type of cell Paneth cells These remarkable cells contain large granules that contain defensins antimicrobial peptides as well as lysozymes and phospholipase A These chemicals represent the quotfirstlinequot of defense against microbes that enter through the digestive tract Compared to the other cells present in the epithelial lining Paneth cells are longlived ie weeks versus a few days for the other cells Specializations to enhance absorption ability The small intestine has all of the quotlayersquot of a typical tubular organ but the tunica mucosa is highly specialized to perform the function of absorption To fulfill this function it uses several strategies to increase the surface area of the plasma membrane of the absorptive epithelial cells individual cells have numerous projections of their apical plasma membranes called microvilli the lamina epithelialis and lamina propria together form folds that project out into the lumen called villi the tunica mucosa and tunica submucosa together form large transverse folds into the lumen called plicae circulares the small intestine is extremely long usually several meters Regional variations in the small intestine Duodenum presence of Brunner39s glands in the submucosa o serous in pig and horse 0 mucous in ruminant and dog 0 mixed in cat presence of chyme in the small intestine induces cells of Brunner s glands to secrete alkaline mucus that neutralizes gastric acid and pepsin and further promotes digestion no plicae circulares longest villi of all three regions highest number of goblet cells Jejunum no glands in the submucosa no lymphoid nodules Ileum permanent aggregated lymphoid nodules in the submucosa shortest villi least number of goblet cells Large Intestine Unlike the small intestine there are no plicae circulares or villi in the large intestine so the surface of the tunica mucosa is more uniform and atter than that of the small intestine Tunica mucosa o lamina epithelialis simple columnar epithelium that forms straight tubular glands lined with absorptive columnar cells recovering water and salt and numerous goblet cells producing mucus to facilitate passage of dry waste material stem cells and lymphocytes are also present 0 lamina propria loose connective tissue that contains numerous blood and lymphatic vessels collagen lymphocytes and plasma cells 0 lamina muscularis mucosae present beneath the base of the crypts and prominent undergoes rhythmic contractions mixed in cat Tunica submucosa typical Tunica muscularis inner circular and outer longitudinal layers outer longitudinal layer is organized into three separate bands known as taenia coli movement of more solid waste to the rectum Tunica serosa is typical Commensal bacteria reside in the large intestine and play a role in the continued digestion of food IDigestive System II Liver gall bladder pancreas Other organs that are part of the digestive system include the liver gall bladder and pancreas I Liver The liver is the largest gland in the body it is multifunctional To understand the function of the liver it is necessary to understand the blood supply to the liver It is supplied by the hepatic artery in the typical manner but it is the only digestive organ drained by the inferior vena cava Other digestive organs such as the small intestine parts of the large intestine stomach and pancreas are drained by the hepatic portal system which takes the blood directly to the liver Thus the liver receives oxygen poor nutrient rich blood from the hepatic portal system and oxygen rich blood from the hepatic artery Functions of the liver Digestive and Metabolic Functions synthesis and secretion of bile storage of glycogen and lipid reserves maintaining normal blood glucose amino acid and fatty acid concentrations synthesis and release of cholesterol bound to transport proteins inactivation of toxins storage of iron reserves storage of fatsoluble vitamins Non Digestive Functions synthesis of plasma proteins synthesis of clotting factors synthesis of the inactive angiotensinogen phagocytosis of damaged red blood cells storage of blood breakdown of circulating hormones insulin and epinephrine and immunoglobulins inactivation of lipidsoluble drugs General organization of the liver Structurally the liver is divided into lobules by loose connective tissue septae These septae are more prominent in some domestic animals than in others the pig has the most prominent septae and they are readily apparent grossly For a long time the lobule as defined by these septae was thought to be the basic functional unit of the liver but now it seems that another unit ie the 12 hepatic acinus might better represent the functional unit of the liver Both the hepatic lobule and the hepatic acinus will be described but rst the basic histology of the liver will be described At low magni cation the liver looks relatively homogeneous and on first examination little organization can be discerned A closer look reveals the presence of quotlobulesquot or groups of hepatocytes arranged around a blood vessel the central vein and de ned by loose connective tissue in which the portal canals are found This type of organization is most easily seen in the pig liver Hepatocytes are one of the primary functional cells of the liver They are located in at irregular plates that are arranged radially like the spokes of a wheel around a branch of the hepatic vein called the central vein or central venule since it really has the structure of a venule Portal canal Three structures are found gouped together in the loose connective tissue surrounding the plates of hepatocytes These include branches of the hepatic artery the hepatic portal vein venule and the intralobular bile ductule This group of three structures has been called a portal triad but now is called a portal canal Portal canal branch of the hepatic artery branch of the hepatic portal vein venule section of an intralobular bile ductule Hepatocytes are arranged in rows that radiate out from the central vein These rows are one cell wide and are surrounded by sinusoidal capillaries or sinusoids This arrangement ensures that each hepatocyte is in very close contact with blood owing through the sinusoids ie bathed in blood The endothelial cells lining sinusoids are fenestrated and in most species lack a basal lamina Gaps are also present between the endothelial cells Taken together these two properties make the sinusoids extremely leaky and allow for the extremely close contact between the blood and the surface of hepatocytes Many materials in the blood except for whole blood cells can pass between the spaces in the sinusoidal lining Although sinusoidal endothelial cells lie very close to hepatocytes they do not actually make contact A narrow space is present between the surface of the hepatocyte and the surface of the endothelial cell This is called the space 0fDisse it is lled with numerous microvilli from the hepatocytes As in other areas of the body these structures serve to increase the surface area of the cell membrane that comes in contact with the blood facilitating exchange of molecules between hepatocytes and the blood What is the basic functional unit of the liver Hepatic lobule The hepatic lobule is de ned as having a central vein CV at its center with its edges de ned by portal canals PC This model only takes into consideration the ow of blood in one direction ie from the branch of the hepatic artery located in the portal canal toward the central vein Yet in the liver blood actually ows from branches of the hepatic artery in several directions Thus the hepatic lobule does not de ne a quotfunctional unitquot of the liver very well Hepatic acinus More recent terminology identi es the hepatic acinus as the quotfunctional unitquot of the liver This de nition recognizes both the real pattern of blood ow in the liver and the role of hepatocytes as secretory cells secreting bile The hepatic acinus has three zones 0 Hepatocytes in Zone 1 are the first to receive blood and it is high in oxygen 0 Hepatocytes in Zone 2 are the second cells to receive blood and it is lower in oxygen 0 Hepatocytes in Zone 3 are the last to receive blood from a branch of the hepatic artery and it is lowest in oxygen 0 Thus the cells with the highest metabolic potential are found in Zone 1 and those with the least are found in Zone 3 Importantly the cells in Zone 3 are the most susceptible to ischemic conditions due to the already low level of oxygen that reaches them through the blood Secretion of bile in the liver Bile is produced and secreted by hepatocytes into a special quotductquot called a bile canaliculus This quotductquot is actually just a space formed between two hepatocytes that is separated from the connective tissue space around the hepatocytes by the presence of tight junctions The bile canaliculi empty into branches of the bile ductules which eventually empty into the hepatic duct that carries the bile out of the liver to the gall bladder for concentration and storage In the duct system bile ows in the direction opposite to the ow of blood in the sinusoids Gall Bladder The gall bladder receives bile from the liver Bile is composed of bile salts that emulsify fats forming watersoluble complexes with lipids micelles to facilitate the absorption of fat Bile salts in the small intestine also activate lipases in the intestine Functions of the gall bladder storage of bile concentration of bile acidi cation of bile send bile to the duodenum in response to cholecystokinin secreted by enteroendocrine cells in small intestine horse does not have a gall bladder and bile is continuously received from the liver Gall bladder structure The gall bladder is a sac that is lined with a simple columnar epithelium and has a tunica muscularis containing smooth muscle that is innervated by both the parasympathetic and sympathetic branches of the autonomic nervous system lTunics layers of the Gall Bladder Tunica mucosa When the gall bladder is empty this layer is extremely folded When full this layer is smoother but still has some short folds o lamina epithelialis composed of simple columnar epithelial cells with numerous microvilli on their luminal surfaces and connected by tight junctions near luminal surfaces 0 lamina propria composed of loose connective tissue rich in reticular and elastic fibers to support the large shape changes that occur in the lamina epithelialis lamina propria may contain simple tubuloalveolar glands especially in ruminants May be mucous or serous depending on species 0 lamina muscularis mucosae not present Tunica submucosa present and typical Tunica muscularis contains much smooth muscle poorly organized Tunica serosa present and typical Pancreas The pancreas contains both exocrine and endocrine components that secrete digestive enzymes and peptide hormones respectively These two components are very different structurally and functionally but are intermingled within the gland However the organization of the exocrine part into acini make it fairly easy to recognize in histological sections as does the organization of the endocrine part around areas of high vascularity Organization of the pancreas The bulk of the pancreas by volume consists of exocrine cells that secrete an alkaline solution of digestive enzymes This secretion moves through a duct system that eventually leads to the pancreatic duct Only about 5 of the volume of the pancreas consists of endocrine cells These cells secrete peptide hormones that play a role in controlling carbohydrate metabolism The endocrine cells are closely associated with large numbers of blood capillaries into which they secrete the peptide hormones The exocrine pancreas The exocrine portion of the pancreas is a compound acinar gland It has many small lobules each of which is surrounded by connective tissue septa through which run blood vessels nerves lymphatics and interlobular ducts Exocrine secretion by the pancreas is controlled by hormones and nerves 0 When the hormone secretin is released from neuroendocrine cells in the duodenum of the small intestine the pancreas secretion is watery and rich in bicarbonate This quotbasicquot secretion helps to neutralize the acidic chyme as it comes into the small intestine When cholecystokininpancreozymin CCK is released by neuroendocrine cells in the duodenum the pancreas secretes a product rich in enzymes that breakdown proteins carbohydrates lipids and nucleic acids in the lumen of the small intestine When gastrin is secreted by pyloric neuroendocrine cells the pancreatic secretion is rich in digestive enzymes Two of the digestive enzymes secreted by the pancreas are trypsin and chymotrypsin they are secreted as nonactive pro or zymogen forms and are subsequently activated by enterokinase in the lumen of the duodenum to avoid digestion of the pancreatic acinar cells A compound acinar gland o Acini The secretory cells of the pancreas are arranged around a small lumen The pancreatic acinar cells produce the digestive enzymes in the typical pattern of protein synthesis However these cells are highly active in protein synthesis for export and this high activity is re ected in their bizonal staining properties with the typical dyes used for histology ie hematoxylin and eosin The basal region of these secretory cells usually stains intensely with hematoxylin re ecting the presence of large amounts of endoplasmic reticulum where the protein is being synthesized on ribosomes These proteins move from the rough endoplasmic reticulum to the Golgi apparatus where they are glycosylated then from the Golgi as secretory granules In these granules the enzymes are found in an inactive or zymogen form They are activated after release in to the duct system The presence of numerous zymogen granules containing high concentrations of protein is re ected in the intense eosin staining in the apical region of 16 the secretory cells These granules are most abundant during fasting or between meals and least abundant after a meal has been ingested Ducts The secretory product of the acinar cells is carried out of the pancreas by a duct system as in other exocrine glands o Intercalated duct The first part of the duct system is called the intercalated duct or intralobular duct It is lined with low cuboidal epithelial cells that secrete bicarbonate ion into the secretory product This duct actually extends into the acinar lumen where its walls consist of the pale staining centroacinar cells Intercalated ducts empty into the larger interlobular ducts Interlobular ducts These ducts are lined with a low columnar epithelium that may contain goblet cells Interlobular ducts empty into the main pancreatic ducts that exit the pancreas O The endocrine pancreas The cells of the endocrine portion of the pancreas are arranged either in roundtooval shaped areas rich in blood vessels known as the islets of Langerhans or they may be scattered throughout the exocrine portions of the pancreas near the acini or ducts There are several different types of cells in the islet or other regions each secreting a different peptide hormone It is not possible to distinguish among these cells with routine hematoxylin and eosin stain used for histological preparations Immunocytochemistry is necessary to identify which cells are secreting a particular peptide This is done by staining with an antibody made to the specific peptide that is combined with a label that can be visualized at the light microscopic level such as immunoperoxidase Examples of peptide hormones secreted by the endocrine pancreas insulin increases uptake of glucose by most cells reduces blood level glucagon decreases uptake of glucose increases blood level somatostatin many effects of gastrointestinal function may inhibit insulin and glucagon secretion vasoactive intestinal peptide pancreatic polypeptide motilin serotonin substance P Hematopoiesis Hematopoiesis is the process by which immature precursor cells develop into mature blood cells The currently accepted theory on how this process works is called the monophyletic theory which simply means that a single type of stem cell gives rise to all the mature blood cells in the body This stem cell is called the pluripotential pluripotent stem cell SITES OF HEMATOPOIESIS Age ofjani n ral Site of hematopoiesis Embryo yolk sac then liver 3rd to 7th month spleen 4th and 5th months marrow cavity esp granulocytes and platelets 7th month marrow cavity erythrocytes Birth mostly bone marrow spleen and liver when needed number of active sites in bone marrow decreases but retain ability for hematopoiesis bone marrow of skull ribs sternum vertebral column pelvis proximal ends of femurs Birth to maturity Adult STRUCTURE AND FUNCTION OF BONE MARROW Bone marrow has a vascular compartment and an extravascular compartment The vascular compartment is supplied by a nutrient artery which branches into central longitudinal arteries which send out radial branches that eventually open into sinuses These sinuses converge into a central vein that carries the blood out of the bone marrow into the general circulation Hematopoiesis takes place in the extravascular compartment The extra vascular compartment consists of a stroma of reticular connective tissue and a parenchyma of developing blood cells plasma cells macrophages and fat cells The high activity of the bone marrow is demonstrated by its daily output of mature blood cells 25 billion erythrocytes 25 billion platelets 50100 billion granulocytes The numbers of lymphocytes and monocytes is also very large Bone marrow is the site for other important activities in addition to hematopoiesis These include the removal of aged and defective erythrocytes and the differentiation of B lymphocytes It is also the site of numerous plasma cells THE PROCESS OF HEMATOPOIESIS The monophyletic theory of hematopoiesis states that pluripotent stem cells multiply to produce more pluripotent stem cells thus ensuring the steady and lasting supply of stem cells Some of the pluripotent stem cells differentiate into precursor cells that are at least partially committed to become one type of mature blood cell I Pluripotent stem cells I 39 I More stem cells I ltpamauy I committed Mature blood cells Pluripotent stem cells multiply slowly into one of ve possible unipotential stem cells which then multiply rapidly into the precursor of the speci c mature blood cell for which they are destined I Pluripotent stem cell CD34 I Erythrocytes I IGranulocytes I ILymphocytes I i I Monocytes I I Thrombocytes I Although the pluripotent stem cells and the unipotential stem cells cannot be distinguished from one another histologically the precursor cells can be distinguished with a trained and practiced eye General Features Understanding the general process of hematopoiesis will be extremely helpful in distinguishing and identifying the different cells in a bone marrow smear or in an intact bone marrow preparation Basically an immature precursor cell goes from a cell that is making lots of protein to a cell that is making much less protein 19 Since structure is always related to function the structure of the precursor cell changes as it goes from making more protein to making less protein Thus a cell that is making a lot of protein will have a nucleus containing dispersed or active chromatin ie that is being transcribed actively When this cell is making less protein the chromatin is condensed or clumped because it is not being transcribed Likewise a cell that is making a lot of protein will have many large nucleoli the site of ribosomal RNA synthesis and assembly as protein secretion decreases there are smaller and fewer nucleoli Cells with high protein synthetic activity have more ribosomes in their f pl m and quotJ the f pl stains more basophilic hematoxylin staining of the RNA in ribosomes Cells with lower protein synthetic activity have fewer ribosomes thus less basophilic staining with hematoxylin leaving the cytoplasm appearing more acidophilic due to the eosin staining of cytoplasmic proteins In cells with high protein synthetic activity the Golgi apparatus is highly developed occupies much of the cytoplasm thus pushing the nucleus off to one side acentric nucleus Cells with low protein synthetic activity have a smaller Golgi and the nucleus tends to be more centrally located The chart below summarized these features Cell Organelle Making lots of Protein Making less Protein Nucleus chromatin is dispersed chromatin is clumped Nucleoli more fewer Cytoplasm more ribosomes basophilic fewer ribosomes acidophilic Golgi apparatus Iarge nucleus off center smaller nucleus more centered except in RBC precursors where the nucleus becomes more off centered until it is extruded from the cell at the last stage before maturity ERYTHROPOIESIS As the cells are maturing in the erythrocytic series the cells are usually getting smaller the nucleus is becoming smaller and more condensed and is eventually lost and the cytoplasm is becoming pinker rather than blue The cells in the developing erythrocyte series are as follows Unipotent stem cell cannot be distinguished from other unipotent stem cells by histology 20 Proerythroblast nucleus still rather large taking up most of the cell nucleus not condensed cytoplasm still very blue or basophilic Basophilic erythroblast very dif cult to distinguish from the proerythroblast Polychromatophilic erythroblast nucleus is more condensed than that of the proerythroblast cytoplasm less blue more grayish Orthochromatophilic erythroblast nucleus more condensed smaller than that of previous cells and looks pyknotic by comparison cytoplasm beginning to take on a more pinkish cast Reticulocyte no nucleus cytoplasm still stains somewhat bluish due to presence of remnants of polyribosomes Erythrocyte mature erythrocyte has no nucleus in mammals cytoplasm stains very pink due to lack of ribosomes and presence of high amounts of protein ie hemoglobin GRANULOCYTE DEVELOPMENT Unipotent stem cell cannot be distinguished from other unipotent stem cells by histology Myeloblast large cell with bluestaining cytoplasm large nucleus often has a clear area near the nucleus this is where the rather large Golgi is located Promyelocyte still a rather large cell with azurophilic not speci cally stained granules Myelocyte overall cell still rather large nucleus still round without indentation granules staining appropriately for the series ie pink for eosinophilic blue for basophilic neutral for neutrophilic Metamyelocyte cell is about the size of a mature granulocyte nucleus with slight indentation granules present that stain appropriately for the series ie pink for eosinophilic blue for basophilic neutral for neutrophilic Band cell cell is about the size of a mature granulocyte nucleus with de nite indentation looks like a horseshoe prominent granules that stain appropriately for the series Mature segmented granulocyte cell is mature and looks like normal mature granulocytes in the blood with lobed nucleus and prominent granules that stain appropriately for the series 21 MONOCYTES Not responsible for knowing the sequence of development of monocytes PLATELETS Platelets also called thrombocytes play an important role in hemostasis by plugging holes in blood vessels to prevent bleeding promoting formation of clots to further prevent bleeding helping to repair damaged blood vessels Platelet granules contain 1 the secretory material that platelets produce to help repair damaged blood vessels 2 growth factor and 3many other proteins Some of these are platelet factor 4 regulates vascular permeability calcium mobilization from bone chemotaXis of monocytes and neutrophils beta thromboglobulin function unknown used to monitor activation of platelets in some diseases coagulation factors fibrinogen factor V factor VIII fibronectin thrombospondin plateletderived growth factor all may be involved in repair of damaged blood vessels serotonin taken up from plasma and stored in granules lysosomal enzymes such as hydrolases Platelets appear as round oval or biconcave disks and have a diameter of about 15 to 35 pm They are somewhat difficult to see in blood smears because of their small size and because they are often clumped together Despite their small size they contain all of the normal organelles and are rich in granules that are difficult to resolve with the light microscope but can be easily seen with the electron microscope Platelets are formed in the bone marrow from megakaryocytes 30100 um diameter very large cells with a polyploid multilobed nucleus Platelets are released from fragmenting megakaryocytes in at least two ways extension of pseudopodia through the wall of the sinuses pseudopodia contain quotstringsquot of platelets that are pinched off and released into the circulation passage of mature megakaryocyte into circulation and fragmenation in the pulmonary vascular bed 22 Immune System This system consists of cells and tissues that have as their main function the protection of the body from the invasion by microorganisms and diseaseproducing entities foreign to the animal To achieve this goal this system has components spread widely throughout the body with concentrations in speci c places Components of the system may be single lymphocytes located strategically in the epithelium of mucous membranes aggregations of lymphocytes associated with the mucosa of strategically placed organs or entire organs highly organized and strategically located in reference to lymph and blood ow patterns The components are Lymphocytes T cells B cells Plasma cells Bone marrow Thymus Lymph Node Mucosa associated lymphoid tissue MALT Spleen NOTE The bone marrow and thymus are considered as primary immunelymphoid components because they contain the stem cells that will develop into T cells B cells and natural killer cells of the functioning immune and lymphatic systems lLymphocytes and Plasma Cells There are basically two different types of lymphocytes T lymphocytes T cells that are involved in cellmediated immunity and B lymphocytes B cells that are involved in humoral immunity Both types of cells originate from stem cells in bone ma1row In addition there are many types of T lymphocytes depending on their specific role in the immune response In circulating blood lymphocytes may be either small lymphocytes 69 pm or large lymphocytes 915 pm with the latter representing only about 3 of circulating lymphocytes Although it is not possible by routine histological methods to differentiate the various types of small lymphocytes found in blood they are of several different types that are in the process of migrating through the circulation to take up residence in an organ or they are quotsearchingquot for foreign antigen Large lymphocytes are mostly activated B lymphocytes Immature T lymphocytes move from the bone marrow into the thymus take up residence and become thymusdependent or mature T lymphocytes These mature T cells then pass through the circulation to find homes in lymph nodes mucosaassociated lymphoid tissue or the spleen There are several types of T lymphocytes ie T helper cells cytotoxic T cells and suppressor T cells 23 B lymphocytes originate and mature in the bone marrow then move through the circulation to various sites throughout the body Upon interaction with foreign antigen and usually with the assistance of T helper cells B lymphocytes become mature antibody secreting cells called plasma cells Clones of plasma cells making specific immunoglobulins are produced thus providing the large numbers of plasma cells needed to mount a good antibody humoral immune response Plasma cells are rarely found in the circulation but reside mostly in connective tissue lamina propria beneath epithelia in the medullary cords of lymph nodes and in the white pulp of the spleen These immune cells are strategically located in areas that come in close contact with foreign substances They represent one of the first lines of defense against invading microorganisms viruses and parasites A good example is the small intestine In these types of locations they are perfectly positioned to interact with invading foreign substances and they recognize these substances as nonself or foreign Upon such quotrecognitionquot lymphocytes are activated and function to neutralize or destroy the invading foreign substance Plasma cells are derived from activated B lymphocytes that have left the blood stream and taken up residence in connective tissue They are easily identified in histological sections due to their unique morphology which re ects their high protein synthetic activity Usually the round to oval nucleus is eccentrically located in the cell due to the presence of a large Golgi apparatus where immunoblobulin synthesis is completed and the molecules packaged for secretion The predominant staining pattern of the cytoplasm is bluish to purple basophilic due to the large amount of rough endoplasmic reticulum and associated ribosomes Usually the cytoplasm is packed with rough ER In a very well stained relatively thin seciton the nucleus has the appearance ofbeing quotspokedquot or having a quotclock facequot lThymus Located posterior to the sternum in the anterior part of the mediastinum the thymus is a bilobed nodular organ that is very large in the rst year or two of life reaching maximum size at puberty then becoming smaller in a process called involution During this degenerative process connective tissue fibers and fat cells replace the previously functional tissue parenchyma of the organ and even though only a few pieces of functional tissue remain it is enough to continue to supply the organism with sufficient mature lymphocytes Immature T lymphocytes move from the bone marrow into the thymus where they become immunocompetent T cells These T cells then leave the thymus go into the circulation and eventually find their way to lymph nodes mucosaassociated lymphoid tissue or the spleen Functions production of immunocompetent T lymphocytes production of mature but naive T cells for peripheral tissues and circulation immunological selftolerance regulation of T cell maturation proliferation and function via secretion of hormones 24 Histology of the Thymus Each lobule has an outer darker staining cortex and an inner paler staining medulla High concentrations of T lymphocytes in the cortex are the basis for the intense basophilia of this region and this is the site of precursor cell proliferation and maturation Mature immunocompetent T cells then move from the cortex toward the medulla where they enter the bloodstream to be taken out of the thymus The thymus has two tissue components parenchyma and stroma The parenchyma is composed mostly of T lymphocytes in various stages of development into mature T cells whereas the stroma is composed of special thymic epithelial cells The parenchyma and stroma have different appearances depending on whether you are looking at the cortex or the medulla Cortex In the cortex the parenchyma consists mostly of the developing T lymphocytes It is here that the T cell receptor TCR genes are rearranged so that the mature T cells obtain their specific surface markers The stroma consists of sparse delicate epithelial cells obscured by all of the lymphocytes These epithelial cells form the support structure for the developing T cells but also play an important role in isolating the T cells from foreign anitgens during their development Medulla In the medulla the stroma consists of prominent epithelial cells that have large palestaining nuclei and substantial amounts of eosinophilic pinkstaining cytoplasm There are fewer T cells because most of them have entered the blood stream via vessels at the corticomedullary junction Antigen presenting cells APC are also found in the medulla where they are called thymic interdigitating cells These cells are thought to present selfantigens to the matured T cells T cells that recognize these selfantigens are removed by a process called apotosis This process helps to prevent autoimmune diseases One prominent and identifying feature of the medulla is the presence of Hassall s corpuscles thought to represent degenerating epithelial cells These impressive structures begin to form in the fetus and increase in number and size as the animal ages 25 Structural basis for function of the thymus In performing its major functions of producing immunocompetent but naive T cells and in achieving immunological selftolerance the thymus has some special structural arrangements unlike those found in other organs First to keep the developing T lymphocytes quotprotectedquot so that they can develop their surface receptors in a quotclimatequot that is not in uenced by antigens the thymic epithelial cells form a continuous layer along the inner surface of the capsule extending into the thymus along the septa and along blood vessels These cells actually provide a cellular framework for a space that is kept separate from other spaces such as the bloodstream This separation is maintained by desmosomes between adjacent epithelial cells and their close contact with endothelial cells of capillaries The quotbarrierquot that results is called the bloodthymus barrier it is similar in structure to the bloodbrain barrier It is within this confined and protected space that the T lymphocytes develop into immunocompetent yet naive T cells The integrity of the space within the epithelial cell framework is extremely important because it prevents the premature stimulation of T cells by antigens Blood thymus barrier components tight junctions between endothelial cells basal lamina of endothelium small connective tissue space basal lamina of epithelial cell continuous sheet of epithelial cell Second to provide a mechanism by which the newly developed immunocompetent and naive T cells can be added back to the circulation the blood supply of the thymus also has some peculiarities Most arteries enter the thymus through the capsule course via connective septae through the cortex down to the level of the corticomedullary junction where they then actually enter the parenchyma of the organ Capillaries from these arterial branches return to the region of the cortex within the parenchyma These capillaries are special in that they are not permeable to macromolecules thus preventing any antigenic contact with developing T cells in the cortex Postcapillary venules that derive from these same capillaries are permeable to macromolecules and lymphocytes The new immunocompetent T cells move into these postcapillary venules to eventually join the general circulation and move to the other tissues and organs that are part of the immune system Some capillaries from the arterial branches entering the thymus from the capsule extend down directly into the medulla to supply the tissue with oxygen and nutrients then reconvene as postcapillary venules that join the postcapillary venules coming from capillaries in the cortex Thus blood draining the cortex and the medulla combine in the postcapillary venules and exit the thymus through typical venous pathways Third to ensure that selftolerance is acheived the medulla of the thymus has antigen presenting cells APC that are thought to present selfantigens to the matured T cells 26 Any T cells that recognize these selfantigens are removed thus preventing development of autoimmune diseases I Lymph Nodes After maturing in the thymus T cells move through the circulation to other organs including lymph nodes Lymph nodes are small limabean shaped organs that are spread throughout the body but occur in groups in areas where lymphatic vessels come together to form larger vessels such as in the groins neck and axilla Lymph nodes are also part of the lymphatic system that includes the lymphatic vessels lymphoid tissue and lymphoid organs Lymphatic vessels drain uid lymph from peripheral tissues and bring it to the venous system Lymph consists of interstitial uid that is similar to blood plasma but with a lower protein concentration lymphocytes and macrophages Lymph nodes lter and purify the lymph before it flows into the venous system Functions lter debris and microorganisms via phagocytosis by fixed macrophages facilitate the interaction between antigen presenting cells and circulating lymphocytes to initiate an immune response B lymphocytes activation and proliferation plasma cell formation and antibody production in response to antigens T lymphocytes activation to become T helper and T cytotoxic cells The location and structural organization of lymph nodes makes them perfect for the above functions They are positioned so that all lymphatic vessels draining back to the venous circulation from the tissues pass through a lymph node The afferent lymphatic vessels branch outside the organ penetrate the capsule and empty into the subcapsular sinus From here the lymph ows into and through cortical sinuses enabling the lymph to come in close contact with cells in the cortex of the node In the medulla there are also sinuses medullary sinuses that enable the lymph to ow toward the hilum and enter efferent lymphatic vessels Eventually the filtered lymph enters the bloodstream through the thoracic duct or right lymphatic duct Lymph nodes are surrounded by a fibrous connective tissue capsule that enters the organ as trabeculae which define a cortex and medulla The capsule and trabeculae are the source of reticulin fibers that are found throughout the node and form the main supporting network of the organ These fibers serve to keep the sinuses open and to support the massive number of lymphocytes and macrophages Beneath the capsule is a subcapsular sinus into which lymph ows from the afferent lymphatic vessels 27 IHistological organization of Lymph Nodes lThe Cortex The cortex is composed of the cortical sinuses surrounded by dense accumulations of lymphocytes In the more superficial cortex the lymphocytes are arranged into spherical follicles lymphoid follicles It is here that B lymphocytes are activated and undergo proliferation Germinal Center GC The open palestaining nature of the nuclei of these cells indicate that they are B lymphocytes undergoing active proliferation Other cells include follicular dendritic cells that present antigen to the B cells tingible body macrophages that engulfed dead B cells that have died by apotosis Resting B cells enter the lymph node parenchyma though the high endothelial venules and if they encounter an antigen with which they can react they then enter the cycle of blast transformation to produce clones of plasma cells and B memory cells This production of clones occurs in the germinal centers of lymphoid follicles Paracortical zone Deeper regions of the cortex contain primarily T lymphocytes that do not form into follicles T lymphocytes helper and cytotoxicsuppresor arrive through the circulation enter the lymph node parenchyma through the high endothelial venules and take up residence in the paracortical zone If activated the T lymphocytes undergo active proliferation to produce expanded clones of activated T lymphocytes T lymphocytes that arrive at the lymph node via the arterial blood stream gain access to the parenchyma of the lymph node through the wall of the high endothelial venules located in the paracortical zone These blood vessels contain endothelial cells that are expressing specific lymphocyte binding molecules called addressins These surface molecules are available to bind to lymphocytes that recognize them the lymphoctyes bind to the surface of the endothelium then cross the vessel wall and enter the lymph node parenchyma Mantle zone corona The germinal center is surrounded by a ring of darkerstaining cells The condensed nature of their nuclei indicates that these are resting B cells Also present in the mantle zone are T helper cells macrophages and dendritic cells 28 In a T celldominated response the paracortical zone of the lymph nodes may be greatly enlarged lnterdigitating dendritic cells are the main antigen presenting cell in the paracortical zone IThe Medulla The medulla of a lymph node is composed of medullary cords interspersed between medullary sinuses The medullary cords are composed of plasma cells producing antibodies their precursors macrophages and T helper cells The most prominent cell in the cord is the precursor to plasma cells or immunoblasts that came from the germinal centers of the lymphoid follicles in the cortex of the node In the medullary cords the plasma cells undergo nal maturation and secrete antibodies into the lymph that is collected by efferent lymphatic vessels in the node and eventually carried to the general circulation Plasma cells may also get into the general circulation in this manner The medullary sinuses are composed primarily of reticular bers RF providing the support framework reticular cells broblastlike cells that secret the reticulin RC and macrophages Overview of the Blood Flow Pattern in a Lymph Node Arteries enter at the hilus I Arterioles distributed through trabeculae I Capillaries of Medulla Capillaries of and Cortex Paracortical Region Vern 13985 the VElnS High endothelial venules and out of node lymphocytes enter node 29 High Endothelial Venules B cells to T cells to cortex paracortical zone Efferent Lymphatic Vessels Thoracic Duct V 39 39 lymphoid tissue MALT MALT is really connective tissue located beneath mucous membranes in which the lymphocyte is the predominant cell type Examples occur in the respiratory gastroin stinal 39 and reproductive tracts The exact extent of these aggregations of lymphocytes is not easily discernible because they have no distinct capsule like that of lymph nodes However they are like lymph nodes in that they o en have a palestaining germinal center containing active y dividing lymphocytes like the germinal centers in lymph nodes The larger aggregations contain B and T cell zones and antigen processing cells the smaller more scattered MALT components such as those in the intestines and respimtory tmct are mostly T lymphocytes Some B cells and plasma cells are also present lgistribution of MALT In the digestive system 0 in the Wall of the pharynx tonsils palatine lingual pharyngeal o in the Wall of the small intestine aggregate lymphoid nodules o in the Wall of the colonaggregate lymphoid nodules in the Walls of the appendix In the reproductive system 0 in the Wall of the vagina 30 Types of MALT and Functional aspects larger aggregates function much like lymph nodes smaller scattered MALT are mostly T lymphocytes but also have B cells and plasma cells mostly IgA in the intestines and respiratory tract to protect against pathogens that may gain access to underlying tissues IgG and IgM secreted into lamina propria to counteract pathogens that have gained access to connective tissue IgE secreted into lamina propria mediates the release of histamine from mast cells single lymphocytes found within the lamina epithelialis are mostly T lymphocytes MALT is drained by efferent lymphatics but there are no afferent lymphatics lymphocytes exposed in MALT regions go through regional lymph nodes then return to the MALT region after activation lSpleen Located between the stomach left kidney and diaphragm the spleen is the largest lymphoid organ in the body performing functions for the blood similar to those performed by the lymph nodes for the lymph It is a soft organ conforming to the contours of the organs and structures surrounding it At the hilus on the visceral surface the splenic artery brings blood into the spleen the splenic vein takes blood from the spleen to the hepatic portal system and lymphatics drain lymph from the spleen In some domestic species such as the horse and dog the spleen functions as a reservoir from which blood can be mobilized when needed and in these species smooth muscle is a prominent feature of the capsule and trabeculae of the spleen Functions removal of abnormal blood cells and particulate matter via phagocytosis storage of iron from recycled red blood cells initiation of the immune responses by B cells and T cells in response to antigens circulating in the blood hematopoiesis in fetus and sometimes in adult Histology The exterior surface of the spleen consists of a capsule containing collagen and elastic bers the interior components are collectively called the quotpulpquot Upon gross examinagtion of a slice of the spleen the pulp has two very different appearances red and white The organ appears as a large expanse of red pulp dotted with white pulp Histologicallyred pulp is quotredquot due to the presence of large numbers of erythrocytes in blood vessels called sinuses and white pulp is quotwhitequot due to lack of these sinuses and consequently fewer erythrocytes The red pulp surrounds the white pulp while the latter looks like lymphatic nodules Closer inspection of the white pulp indicates 31 that there is a quotcentral arteriolequot sometimes called a central artery close to the center of each area of white pulp IRed Pulp The red pulp of the spleen is characterized by a parenchyma PN that consists of macrophages of the sheathed capillaries as well as other macrophages and blood cells that have not yet entered the venous sinuses The rest of the red pulp is occupied by numerous venous sinuses VS The walls ofthe sinuses are very open and can be easily traversed by blood cells Their lining consists of long endothelial cells arrows oriented along the longitudinal aXis of the vessel Large spaces occur between adjacent endothelial cells and the underlying basement membrane is discontinuous thus blood cells can easily pass between the endothelial cells and gain access to the bloodstream on the venous side A continuous reticulin network forms the framework that supports the macrophages and a few fibroblasts responsible for producing the reticulin bers special stains are required to visualize the reticular network IWhite Pulp The white pulp of the spleen is characterized by a parenchyma that consists of two types of lymphocytes ie B cells and T cells located in two different areas ofthe spleen B cells are located in the lymphoid follicles scattered throughout the organ In younger animals a germinal center can be seen as seen in lymph nodes In fact this type of white pulp functions much in the manner that lymphoid follicles of lymph nodes function ie initiation of immune responses by B cells to foreign antigens in the blood T cells are located around the central arteries and form a kind of sheath This site is called the periarteriolar lymphoid sheath Blood flow in the spleen To properly understand the histology of the spleen it is necessary to understand its blood supply The splenic artery enters the spleen at the hilus then branches into numerous arterioles that run through the parenchyma or pulp of the spleen When these arterioles acquire a coating of T cells the arterioles are called central arteries and the surrounding lymphoid tissue is called the PALS ie the periarteriolar lymphoid sheath 32 Arteries sheamed api ary Veins PC Penlcillary arteries Sinus PALS y L l are speclal caplllarles whlch actually have no endothellal eells and end blmdly These unlque After ofslnuses Eonsldered part of any true blood vessel than the blood eells reeenlee the bloodstream through the walls of the sinuses This particular arrangement of quotblood flowquot in the spleen is considered to be an open circulation pattern Integument The Integument the skin and all of its derivatives Components skin epidermis dermis hypodermis derivatives sweat glands sebaceous glands mammary glands hair nails claws hooves horns antlers combs wattles and feathers Functions protection from drying out from invasion by microorganisms from UV light and from insults mechanical chemical or thermal sensation for touch pressure pain and temperature thermoregulation decreases heat loss in cold temperatures increases heat loss in hot temperatures metabolic functions energy stored in fat deposits synthesis of vitamin D Structure of the Skin Three distinct layers can be seen in the skin Epidermis consists of keratinizing stratified squamous epithelium Dermis consists of fibroelastic connective tissue Hypodermis consists mostly of white adipose tissue sometimes referred to as the subcutis The thickness of these layers varies depending on the specific location inon the body and in a given location on the amount of exposure to wear and tear For example in the buccal cavity the epidermis consists of a moist stratified squamous epithelium which is relatively think and not highly keratinized whereas the epidermis of skin on the ball of the foot is thick and highly keratinized The skin covering the dorsum of the hand has a rather thin hypodermis whereas the skin over the buttocks has a very thick hypodermis containing numerous fat cells 34 Epidermis Layers of the Epidermis in order from outermost surface to innermost deepest Stratum comeum consists of the remains of keratinocytes mostly composed of the protein keratin Stratum lucidum present only in very thick skin palestaining layer of cells between the stratum comeum and stratum granulosum in which the dying keratinocytes contain a lot of keratin but are not completely replaced by it Stratum granulosum consists of keratinocytes containing large numbers of granules that contribute to the process of keratinization Stratum spinosum consists of large polyhedral keratinocytes that are actively synthesizing keratin which is inserted as tono brils into the area of the plasma membrane beneath desmosomes that connect adjacent cells together These quotconnectionsquot or desmosomes between cells in this layer help hold them together and result in the quotspinyquot appearance of the cells that gives this layer its name Stratum basale consists of keratinocytes undergoing mitosis to produce the constant supply of keratinocytes needed for replacement of the dead and dying cells in the more super cial layers of the epidermis Dermis Two zones of the dermis papillary zone consists of loose areolar connective tissue containing collagen and ne elastic bers connects the epidermis to the thicker and denser reticular zone of the dermis reticular zone contains dense irregular and coarse collagen bers and thick elastic bers interspersed with broblasts and blood vessels and nerves Glands in the skin Several different types of glands are located in the dermis of the skin serving a variety of functions Sebaceous glands Apocrine sweat glands 35 Merocrine eccrine sweat glands Sebaceous Glands The epithelium of this gland is an outgrowth of the external root sheath of the hair follicle and the gland empties its oily product directly into the follicle itself The glands are of a branched acinar type and produce a lipid product called sebum that serves to reduce the entry of microorganisms into the body through the skin to lubricate the hair and preventing it from drying out The secretory cells die and become part of the product a holocrine mode of secretion These glands are not found in hooves foot pads claws or horns Apocrine Sweat Glands These glands are coiled tubular glands with a large lumen and a duct connecting it to an adjacent hair follicle These glands secrete a viscid milky product and are analogous to odiferous glands of many mammals Once thought to use the apocrine mode of secretion it is now known that their mode of secretion is more like that of the merocrine sweat glands These glands are the primary sweat gland of domestic animals and are especially prominent in the horse Merocrine Sweat Glands These glands are unbranched tubular in form and appear as a mass of tubules in cross section They are plentiful in the upper regions of the fatty hypodermis They secrete a watery product that is hypotonic to the plasma It is the evaporation of this secretion on the surface of the skin that aids in thermoregulation These are sometimes called eccrine sweat glands Hair General structure of hair and associated structures Hair shaft the pa1t of the hair above the surface of the skin Hair root the part of the hair below the surface Bulb an enlarged hollow portion at the base of the root Hair papilla projection of dermis into center of the bulb Follicle the indentation in the skin within which the root lies Hair has three layers Cuticle the outermost layer Single layer of attened keratinized cells Overlap like shingles with free edge distally Cortex the thickest intermediate layer Consists of several layers of keratinized cells containing hard keratin If hair is colored these cells contain pigment Cells held together by desmosomes Medulla central core loosely packed cuboidal cells The structure and organization of the cuticle and medulla cells are speciesspecific 36 lHair follicles The hair follicle is the structure that anchors the hair in the dermis and produces the hair itself It is composed of ve layers of epithelial cells arranged concentrically The inner three layers form the hair shaft through a process of keratinzation while the outer two layers form the hair sheath cells in the innermost layer form the medulla of the hair or core of the hair shaft cells in the next layer form the cortex that makes up most of the hair cells in the third layer form the cuticle on the surface of the hair cells in the fourth layer make up the internal root sheath cells in the fth or outermost layer form a layer called the external root sheath that does not take pa1t in hair formation Mw The external root sheath is separated from the surrounding connective tissue by a thick basement membrane known as the glassy membrane Types of follicles Hair follicles can be classi ed in two ways based on their size ie diameter and based on their organization Based on size diameter Primary hair follicle large having sweat gland sebaceous gland and arrector pili muscle eX overcoat or guard hairs in dogs Secondary hair follicle smaller lacking sweat glands and a1rector pili muscle eX underhair Based on Organization Simple follicle a single hair from one follicle Compound follicle cluster of several follicles with several hairs emerging from one opening onto surface of skin The Hoof The equine foot includes the hoof dermis first second and third phalanges and associated structures The hoof itself is the comifled layer of the epidermis lacking the stratum granulosum and stratum lucidum It is important to understand the histology of the hoof because a disease involving the epithelium of the foot called lamim39tz39s is the most devastating clinical disease of the foot 37 The peculiar histology of the hoof is formed from special relationships between the dermis or corium and the overlying epidermis In some places the dermal papillae and epidermal pegs are con uent forming apparent layers ie they are laminar or consist of lamellae in other places they are more typical It is this lamellar interaction between the epidermis and dermis that gives the hoof its strength The wall of the hoof is that part of the hoof which is visible when the foot is on the ground and it can be divided into three layers From outside to inside they are the stratum externum tectorium the stratum medium and the stratum internum lamellatum lThe Wall of the Hoof The stratum externum or tectorium is an extension of the perioplic epidermis and is composed of comified eipithelial cells which appear as a soft white shiny material This tissue attaches the hoof to the epidermis of the skin of the foot The stratum medium or coronary epidermis is composed of prominent tubular and intertubular horn and this layer comprises the bulk of the wall of the hoof The stratum internum or stratum lamellatum is the epidermis in the laminar region Stratum Lamellum This layer is made of nontubular horn which fuses with the stratum medium and helps hold the wall to the foot In this region the dermal papillae and epidermal pegs form elongated ridges oriented perpendicular to the ground These ridges are formed from primary and secondary laminae the secondary laminae being oriented at close to a right angle to the primary laminae There are about 600 primary laminae and about 100200 secondary laminae for each primary lamina This system of interdigitating primary and secondary laminae provides the tight bond between the wall of the hoof and the underlying dermis Thus damage to the laminae leads to disruption of this interdigitating system which results in separation of the hoof wall from the dermis and phalanx beneath it Laminitis acute laminar degeneration is an in ammation of the laminae within the hoof Many pathophysiologic mechanisms are thought to cause laminitis among them vasoconstriction within the digit perivascular edema arteriovenous shunting of blood at the level of the coronary band ictiou and ruler quot 39 39 These lead to less than normal perfusion of blood to the digit resulting in ischemia edema and eventually necrosis of the laminae 38 IMale Reproductive System The primary function of the male reproductive system is reproduction which includes the production of spermatozoa the transportation of spermatozoa from the testes out of the male body the secretion by glands and the placement of spermatozoa in the female reproductive tract Spermatozoa are produced in the testes then transported from the testes by a series of ducts which become gradually larger and connect with the urethra of the penis Various accessory glands in the male system secrete materials which together with the spermatozoa constitutes the semen A secondary function of the male reproductive system is the production of the male hormones which are responsible for the secondary sex characteristics of the male animal The male reproductive system is composed of several distinct organs These include the testes epididymis deferent ducts accessory glands and the penis The testis plural testes is both an exocrine organ compound coiled tubular gland producing cells ie spermatozoa and an endocrine organ secreting hormones ie testosterone Accessory glands not all are present in all species include the ampullary glands vesicular glands prostate gland bulbourethral gland and urethral glands The testes are paired organs and each one is enclosed in a fibrous white capsule of dense connective tissue tunica albuginea containing blood vessels the stratum vasculare A layer of peritoneum is tightly adhered to the tunica albuginea of each testis The stallion has obvious smooth muscle fibers in the capsule The connective tissue of the capsule continues into the testis on the posterior aspect as the mediastinum testis The dense connective tissue of the tunica albuginea is continuous with the loose areolar connective tissue of the septuli testis septa which extend through the parenchyma of the testis and divide it into lobules Each lobule is composed of several seminiferous tubules tubuli contorti and the surrounding connective tissue Spermatogenesis formation of spermatozoa occurs in the epithelial lining of the seminiferous tubules The interstitium is composed of loose connective tissue containing fibroblasts and Leydig cells interstitial cells Spermatozoa produced in the seminiferous epithelium move through the lumen of the tubules to the tubuli Lcti straight tubes which extend to a network of spaces in the mediastinum the rete testis except in the stallion Efferent ductules ductuli efferentes carry the spermatozoa from the rete testis then converge to form the ductus epididymis a convoluted duct The ductus epididymis straightens and becomes the ductus deferens In domestic mammals testes are not in a major body cavity but are enclosed in the scrotum Each testis is suspended at the end of a tissue called the spermatic cord which contains the ductus deferens the blood vessels and the nerves supplying the testis Testis Each testis is composed of an exocrine part seminiferous tubules and an endocrine part interstitial or Leydig cells The testis is divided into lobules by septa consisting of loose areolar connective tissue Several seminiferous tubules are found in each lobule and interstitial cells are found in the connective tissue septa surrounding the seminiferous tubules The seminiferous tubules are the exocrine portion of the testis producing and quotexcretingquot 39 spermatozoa These tubules are lined by a stratified epithelium that consists of the developing spermatozoa and supporting cells Sertoli cells Seminiferous tubules The stratified epithelium of the seminiferous tubules is composed of different stages of developing sperm cells Spermatogonia are stem cells located near the basement membrane of the tubule which proliferate by mitosis Some of the progeny cells differentiate into sperm and move away from the basement membrane toward the lumen of the tubule These differentiating cells first undergo meiosis then undergo a morphological change to become spermatozoa Some of the progeny cells undergo mitosis again to produce more progeny cells providing a continuous source of stem cells for the production of spermatozoa Interstitium The interstitial tissue of the testis consists of loose areolar connective tissue containing numerous reticular fibers which serves to support the seminiferous tubules The interstitial cells Leydig cells located in this connective tissue are responsible for the synthesis and secretion of the steroid hormone testosterone Spermatogenesis the process by which stem cells develop into mature spermatozoa There are three phases 1 Spermatocytogenesis Mitosis 2 Meiosis and 3 Spermiogenesis l Spermatocytogenesis also called Mitosis Stem cells Type A spermatogonia singular spermatogonium divide mitotically to replace themselves and to produce cells that begin differentiation Type B spermatogonia Spermatogonia have spherical or oval nuclei and rest on the basement membrane You are not responsible for distinguishing between Type A and Type B spermatogonia in lab 2 Meiosis Cells in prophase of the first meiotic division are primary spermatocytes They are characterized by highly condensed chromosomes giving the nucleus a coarse chromatin pattern and an intermediate position in the seminiferous epithelium This is a long stage so many primary spermatocytes can be seen Primary spermatocytes go through the first meiotic division and become secondary spermatocytes The cells quickly proceed through this stage and complete the second meiotic division Because this stage is short there are few secondary spermatocytes to be seen in sections You are not responsible for identifying secondary spermatocytes in lab Meiosis is the process by which the diploid number of chromosomes present in spermatogonia the stem cells is reduced to the haploid number present in mature spermatozoa The products of the second meiotic division are called spermatids They are spherical cells with interphase nuclei positioned high in the epithelium Since spermatids go through a metamorphosis into spermatozoa they occur in early through late stages You are not responsible for distinguishing the different stages of spermatids but you are required to identify a spermatid All of these progeny cells remain attached to each other by cytoplasmic bridges The bridges remain until sperm are fully differentiated 3 Spermiogenesis This is the metamorphosis of spherical spermatids into elongated spermatozoa No further mitosis or meiosis occurs During spermiogenesis the acrosome forms the agellar apparatus 40 forms and most excess cytoplasm the residual body is separated and left in the Sertoli cell Spermatozoa are released into the lumen of the seminiferous tubule A small amount of excess cytoplasm the cytoplasmic droplet is shed later in the epididymis Sertoli Cell amp Developing Sperm Cells an interaction The Interaction At all stages of differentiation the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule although its cytoplasm is difficult to distinguish at the light microscopic level They are characterized by the presence of a vesicular oval basally positioned nucleus which contains a prominent nucleolus The nuclear envelope often contains a definite fold The significance of the very close association of the two types of cells is unknown Sertoli cells are endocrine cells they secrete the polypeptide hormone inhibin Inhibin acts at the level of the pituitary to reduce the secretion of follicle stimulating hormone Blood testis barrier Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells The spermatogonia are in the basal compartment deep to the level of the tight junctions and the more mature forms such as primary and secondary spermatocytes and spermatids are in the adluminal compartment The function of the bloodtestis ba1rier red highlight in diagram above may be to prevent an autoimmune reaction Mature sperm and their antigens arise long after immune tolerance is established therefore a male animal is capable of making antibodies against his own sperm Injection of sperm antigens causes in ammation of the testis autoimmune orchitis and reduced fertility Thus the bloodtestis ba1rier may reduce the likelihood that sperm proteins will induce an immune response The Duct System After production in the testes spermatozoa pass through a series of ducts in their journey out of the male system Tubuli recti Rete Testis Efferent ductules The genital ducts are tubular organs in which the lamina epithelialis varies from the stratified epithelium in the testes to a transitional epithelium in the urethra In the terminal part of the seminiferous tubules the epithelium contains only Sertoli cells which gradually blends with the squamous cuboidal or columnar epithelium species variation of the tubuli recti and the rete testis These epithelial cells may actually represent a continuation of the Sertoli cells which line the seminiferous tubules Epididymis The ductus epididymis is lined with a pseudostratified stereociliated columnar epithelium Stereocilia are actually nonmotile long microvilli which serve to increase the absorptive andor secretory surface of this epithelium With its associated connective tissue and muscle the ductus epididymis coils to form the head body and tail of the epididymis which then continues into the 41 ductus deferens Spermatozoa are stored within the epididymis while they undergo maturation to become mature sperm Ductus Deferens The ductus epididymis continues as the ductus deferens which is also lined by a pseudostratified columnar epithelium However the lamina propria submucosa of the ductus deferens is areolar loose connective tissue and the tunica muscularis is very thick and contains two layers of smooth muscle The tunica serosa is present and typical Urethra The male urethra consists of two portions the pelvic urethra and the penile urethra Both portions are lined with transitional epithelium both contain erectile tissue and both contain species variable branched tubular mucous glands the urethral glands In the pelvic urethra the three layers of smooth muscle in the tunica muscularis near the bladder are replaced or joined in some species by the striated urethral muscle The tunica adventitia is present and typical The tunica muscularis is smooth muscle and cavernous corpus cavernosum urethra tissue is present in the connective tissue beneath the epithelium In the penile urethra the corpus cavernosum penis is also present Accessory Glands Ampullary vesicular prostate bulbourethral and urethral glands The products of these glands serve to nourish and activate the spermatozoa to clear the urethral tract prior to ejaculation serve as the vehicle of transport of the spermatozoa in the female tract and to plug the female tract after placement of spermatozoa to help ensure fertilization Although the glands are ususally described as being branched tubular or branched tubuloalveolar they vary in their organization and in their distribution in different species Ampullary Glands This branched tubular gland lined by simple columnar epithelium is an enlargement of the ductus deferens in its terminal portion This is a typical tubular gland in ruminants horses and dogs absent in the cat and poorly developed in boars The function of the white serous secretion is not known Vesicular Glands The secretory endpieces of this gland are lined with simple columnar epithelium the main ducts are lined with stratified columnar epithelium These glands do not occur in carnivores but are present in some form in horses ruminants and swine Seminal fuid the product of this gland serves as a vehicle for the transport of spermatozoa Prostate Gland 42 Grossly the prostate gland can be divided into two parts the body and the disseminate part Low cuboidal to low columnar epithelium provides the lining for this compound tubuloalveolar gland which consists primarily of serous secretory end pieces The secretion of this gland is more serous in dogs and more mucous in bulls It serves to promote the movement of spermatozoa and to form a vaginal plug Additionally in bulls the secretion contains high amounts of fructose and citric acid Concretions may be present in the secretory end pieces as well as parts of the duct system Bulbourethral Glands The lining of these paired compound tubuloalveloar glands is simple columnar epithelium A capsule of dense connective tissue contains some smooth muscle as well as skeletal muscle of the bulbocavemous and urethral muscles All domestic species have these glands except the dog and their mucous secretion serves to clear the urethra of urine and to lubricate it and the vagina The product may also serve as an energy source for the spermatozoa Urethral Glands In some species branched tubular mucous glands are found along the length of the urethra especially dorsal to the lumen of the urethra The exact function of their product is not clear Penis the copulatory organ The penis provides an outlet for both urine and the copulatory ejaculate spermatozoa and semen The histology and gross anatomy of the penis varies dramatically from species to species and from region to region within the same species In general the body of the penis consists of the urethra erectile tissue corpora cavemosa penis and corpora cavemosum urethra smooth and skeletal muscle touch and pressure receptors Pacinian corpuscles and a dense connective tissue capsule tunica albuginea Erectile tissue and the erectile mechanism The erectile tissue is composed of dense irregular connective tissue which contains numerous elastic fibers and sinuses Under stimulation the primary blood supply of the penis is directed through helicine arteries which open into the venous sinuses During erection these vessels and the sinuses become engorged with blood and the thinwalled veins beneath the tunica albuginea are effectively closed further increasing the rigidity of the organ Because the capsule around the erectile tissue of the corpus cavemosum urethra is not as thick as that around the corpus cavemosum penis the urethra is not occluded during erection After ejaculation the helicine arteries contract and regain their normal tone resulting in a relaxing of the pressure around the veins which leads to the restoration of normal bloo d ow to the region 43 IFemale Reproductive System The primary function of the female reproductive system is reproduction which includes the production of ova the transportation of ova from the ovary to the site of fertilization transportation of spermatozoa from the point of deposition in the female tract to the site of fertilization nourishment of the developing embryo and fetus parturition and nourishment of the infant Included in the reproductive function of this system is the production of the female hormones which are responsible for the secondary sex characteristics of the female animal as well as the development of follicles in the ovary ovulation preparation of the uterus for implantation of an embryo maintenance of pregnancy parturition and preparation of the mammary glands for milk production Structure The female reproductive system is composed of several distinct organs These include the paired ovaries paired uterine tubes uterus uterine horns cervix vagina and the mammary glands The ovaries are both an exocrine organ producing cells ie ova and an endocrine organ secreting hormones ie estrogen and progesterone Note in domestic animals the oviducts are usually called uterine tubes and the uterus is called uterine horns due to the structure of these organs Ova are produced in the ovaries then transported from the ovaries to the site of fertilization in the upper part of the uterine tube Sperm are transported from the site of deposition near the vagina and uterus to the site of fertilization If fertilization occurs the uterus serves to nourish the developing embryo and fetus until the time of parturition The vagina receives the male copulatory organ the penis during copulation and is the birth canal for the infant during parturition Mammary glands serve to nourish the infant Although all mammals have the same basic organs their individual structure and association with each other varies according to species The structure of the uterine tubes and uterus are especially variable Function The ovary or female gonad is 1 an exocrine gland producing ova 2 an endocrine gland secreting 44 a the female hormones estrogen and progesterone androgens typically considered male hormones The surface of the ovary is covered with surface epithelium a simple epithelium which changes from squamous to cuboidal with age Immediately beneath this surface epithelium there is a dense connective tissue sheath the tunica albuginea ovarii In most species the ovaries are composed of an outer cortex and inner medulla except in the mare where the cortical region is interior to the medulla The cortex is composed of ovarian follicles developing oocytes with their associated follicular cells interstitial gland cells and stromal elements Ovarian follicles are in different stages of development least mature to most mature primordial primary secondary secondaryvesicular and mature The cortex usually also contains the remains of degenerated follicles called atretic follicles which may arise at any stage of follicular development Interstitial gland cells are also present in the cortex Although the function of these cells is not known for sure they are thought to secrete estrogen since they have the structure of steroid secreting cells The atretic follicles and the interstitial gland cells though not shown on this diagram will be discussed later The medulla is composed of loose areolar connective tissue containing numerous elastic and reticular fibers large blood vessels nerves and lymphatics The hilus is the region through which blood vessels lymphatics and nerves enter and leave the ovary It is contiguous with and histologically similar to the medulla Oogenesis the production of female gametes the ova 1 Early in embryogenesis primordial germ cells migrate from the yolk sac endoderm to the genital ridge developing ovary where they take up residence and are called oogonia 2 These diploid oogonia undergo several mitotic divisions prior to or shortly after parturition thus providing the developing ovary with a large supply of future ova eggs 3 When oogonia begin the first meiotic division they are called primary oocytes 4 Primary oocytes are arrested in prophase of Meiosis I until the female reaches sexual maturity They grow in size during this arrested phase but do not divide A human female is born with about 2 million primary oocytes in her ovaries but by the time of puberty only about 400000 are left due to atresia degeneration When the female reaches sexual maturity and under the in uence of follicle stimulating hormone FSH a small number of primary oocytes are stimulated to continue through Meiosis I 45 6 During this process the number of chromosomes is reduced from the diploid number 2N to the haploid number 1N 7 This division is uneven in that although the chromosomes are divided equally most of the cytoplasm stays with the oocyte The smaller polar body contains half the chromosomes but only a small amount of cytoplasm and will eventually degenerate 8 After a primary oocyte completes the first meiotic division it is called a secondary oocyte 1N In most species Meiosis I is completed just before ovulation release of the ovum from the ovary However in horses and dogs Meiosis I is completed after ovulation 9 If a secondary oocyte is not penetrated by a sperm it will degenerate 10 If fertilization and pregnancy do not occur a new cycle will begin in which FSH from the pituitary gland will stimulate a few more primary oocytes to continue through Meiosis I 11 The process is the same as previously described and a secondary oocyte is formed 12 However some of the time a sperm will penetrate the zona pellucida and the secondary oocyte is stimulated to continue through Meiosis II forming a second polar body and a mature ovum 1N Again the polar body contains half of the chromosome material but little cytoplasm and it eventually degenerates 13 After a sperm enters the cytoplasm of the ovum two pronuclei form containing genetic material from the ovum or the sperm 14 Fertilization is complete when the two pronuclei fuse and restore the diploid chromosome number 15 If fertilization is completed the zygote undergoes several mitotic changes to become an embryo otherwise it degenerates There are three stages in the development of follicles l preovulation 2 ovulation 3 postovulation The pars distalis of the pituitary gland hypophysis controls the process of follicular development and maturation by the secretion of l gonadotropic hormones FSH follicle stimulating hormone 2 LH luteinizing hormone 3 prolactin in some species The hormonal 39 quot will be 1 39 39 J in the J39 39 ofthe ovarian cycle Preovulation Development of Follicles in the Ovary 46 The primordial quiescent follicle consists of a primary oocyte and a single layer of attened follicular cells As the follicle develops alterations occur in the primary oocyte and the surrounding follicular cells The primary oocyte produces yolk granules and the follicular cells change from attened to cuboidal or columnar The primary follicle consists of a primary oocyte with a single layer of cuboidalcolumnar follicular cells As development proceeds the number of follicular cells increases by mitosis forming several layers around the primary oocyte As these cells enlarge they release steroid hormones called estrogens of which estradiol is the dominant one prior to ovulation During each cycle a few primary follicles will continue to develop into secondary follicles The secondary follicle consists of several layers of cuboidal columnar follicular cells now collectively called the membrana granulosa which begin to secrete follicular uid A thick amorphous layer the zona pellucida forms between the primary oocyte and the membrana granulosa Previously undifferentiated stromal cells now develop into two distinct layers around the developing follicle the theca interna and the theca externa Cells in the theca interna are large rounded and epitheliallike cells in the theca externa are smaller fibroblasts Both layers of theca cells are separated from the membrana granulosa cells of the follicle by a basement membrane As the follicular uid secreted by the membrana graulosa cells accumulates small pockets of uid between granulosa cells begin to appear Usually in human females only one secondary follicle will continue to develop The secondary vesicular follicle is characterized by the presence of pockets of follicular uid within the membrana granulosa As the follicle continues to develop the separate pockets fuse to form one large pocket of uid called the follicular antrum During this development of the follicular antrum the oocyte is still a primary oocyte arrested in prophase of Meiosis I It is still surrounded by granulosa cells which are contiguous with the membrana granulosa present around the periphery of the growing follicle Two regions of cells can be identi ed in the layer of granulosa cells surrounding the oocyte l the corona radiata contains granulosa cells which remain attached to the oocyte after ovulation and are in close contact with the oocyte through cytoplasmic processes which pass through the zona pellucida and contact microvilli of the oocyte 2 the cumulus oophorus contains granulosa cells which surround the oocyte and are continuous with the displaced cells of the membrana granulosa but remains in the ovary after ovulation The other granulosa cells form a layer around the periphery of the follicle and are separated from the theca interna cells by a distinct basement membrane The mature follicle sometimes called the preovulatory follicle has all of the components of the secondaryvesicular follicle but is much larger and contains one single large antrum of follicular uid These follicles are very large and usually extend from the deepest parts of the cortex and protrude from the surface of the ovary In some species just before ovulation the primary oocyte in the mature follicle completes meiosis I producing a secondary oocyte and a polar body 47 Preovulation Development of Theca Cells in the Ovary As the oocyte and follicular granulosa cells are growing and developing in the ovary the stromal cells differentiate and develop into the theca interna and theca externa cells As a follicle goes from a primary to a secondary follicle the stromal cells immediately surrounding the follicle differentiate into the theca folliculi The cells closest to the follicle become the theca interna cells round foamy cells that secrete androgens including testosterone These two male hormones are converted by the granulosa cells to estrogens The stomal cells farther away from the developing follicle become the theca externa cells broblastlike cells arranged around the follicle outside the theca interna cells Postovulation 1 corpus hemorrhagicum After ovulation hemorrhage into the remains of the follicle usually occurs resulting in a structure called a corpus hemorrhagicum This transitory structure develops into a corpus luteum 2 corpus luteum yellow body In most species LH from the pituitary gland initiates this luteinization and stimulates the granulosa cells to secrete progesterone The granulosa cells undergo hyperplasia proliferation hypertrophy enlargement and are transformed into granulosa lutein cells In several species including the human the accumulation of a yellow lipid pigment lutein and other lipids marks the transition to granulosa lutein cells The cells of the theca interna are also transformed into lipidforming cells called theca lutein cells The resulting structure is highly vascular If fertilization occurs the corpus luteum persists and secretes progesterone 3 orpus albicans If fertilization does not occur the corpus luteum degenerates and is replaced by connective tissue forming a corpus albicans The Ovarian Cycle Primordial follicles in the cortex of the ovary are stimulated by FSH follicle stimulating hormone secreted by the pars distalis of the pituitary gland FSH stimulates the development of one or more primordial follicles in the ovary to begin the development into a mature Graafian follicle ready for ovulation A surge of LH from the pars distalis initiates ovulation and induces luteinization of the granulosa and theca cells of the ruptured follicle The oocyte with its surrounding corona radiata and cumulus cells moves into the uterine tubes while the ruptured follicle left behind becomes a corpus hemorrhagicum and under the in uence of LH develops into a corpus luteum If fertilization and implantation occur the corpus luteum persists as the corpus luteum of pregnancy but if fertilization does not occur the corpus luteum degenerates into a corpus albicans which remains as a scar in the ovary The Uterine Tubes Function The uterine tubes also called Fallopian tubes or oviducts 1 transport the ovum from the ovary to the site of fertilization 48 2 help transport spermatozoa the haploid male gametes from the site of deposition to the site of fertilization 3 provide an appropriate environment for fertilization 4 transport the fertilized ovum embryo to the uterine horns where implantation and further development may occur Structure The uterine tubes can be divided into three major parts 1 the infundibulum 2 the ampulla 3 the isthmus The infundibulum is the region most proximal to the ovary It is funnelshaped and has finger like projections called fimbriae that extend into the pelvic cavity and make close contact with the ovaries The tunica mucosa occupies most of the thickness of the wall of the organ The ampulla is the middle onethird region in which fertilization usually occurs Histologically it is very similar to the infundibulum having a very thick tunica mucosa and relatively thick tunica muscularis The thickwalled isthmus is the lower onethird region most proximal to the uterine horns The smooth muscle in the wall of the isthmus helps propel by peristalsis the fertilized ovum toward the uterine horns and body of the uterus where implantation occurs The tunica muscularis is the thickest part of the wall and the tunica submucosa is very thin as in the infundibulum and ampulla About the time of ovulation the infundibulum closest to the ovary moves to cover the site of rupture of the mature Graafian follicle The ovum moves down the infundibulum of the uterine tube toward the ampulla assisted by peristaltic contractions of the smooth muscle in the wall of the tube as well as uid moved by ciliated epithelial cells in the mucosa of the tube The ampulla is usually the site of fertilization After fertilization the embryo moves down through the isthmus which connects the uterine tube with the uterine horns or uterus The thick muscular wall of the isthmus of the uterine tube helps propel the embryo into the uterus where it can be nourished during further development Regional Variations The uterine tubes are paired tubular organs with the typical organization of a tubular organ ie four tunics consisting of l tunica mucosa 2 tunica submucosa 3 tunica muscularis 4 tunica serosa 49 The thickness and specific characteristics of these tunics varies with the region of the uterine tube The tunica mucosa of the infundibulum helps capture the ovum from the surface of the ovary bathes it in a supportive uid and helps move it toward the uterus The tunica mucosa of the ampulla provides the proper environment for fertilization Consequently in the infundibulum and ampulla the tunica mucosa is thick and highly developed It is the tunica muscularis of the isthmus that provides the strong contractions that at the right time propel the ovum or embryo into the uterine horns As a result in the isthmus the tunica mucosa is reduced in thickness and the tunica muscularis is much thicker Histology The lamina epithelialis of the tunica mucosa of the uterine tubes is an intermittently ciliated columnar epithelium that contains two types of cells a ciliated cell and a nonciliated secretory cell In the cow and sow the lamina epithelialis may be pseudostratified intermittently ciliated columnar The secretory product of the nonciliated secretory cells is moved toward the uterine horns by the movement of the cilia on the ciliated cells This secretion probably also protects and nourishes the ovum The lamina propria consists of a typical loose areolar connective tissue without glands and it blends with the underlying thin tunica submucosa There is no lamina muscularis mucosae in the entire female reproductive tract The tunica muscularis is sparse in the infundibulum and the ampulla but thick in the isthmus consisting of an inner circular and an outer longitudinal layer of smooth muscle The tunica serosa is typical containing many blood vessels in a distinct vascular layer Cyclic Changes in the Epithelium Under the in uence of estrogen the ciliated epithelial cells increase in height and in the number of cilia Under the in uence of progesterone these cells decrease in height and in the number of cilia These cells are at their tallest with the most numerous cilia at the time of ovulation Their main function is to assist in the movement of the ovum toward the site of fertilization and the embryo toward the uterus This action is secondary to the peristaltic movement of the isthmus region Clinical The uterine tubes are the site of tubal ectopic pregnancies They can also be the site of bacterial infection which can lead to Pelvic In ammatory Disease a major cause of infertility in women The Uterus Functions Ni serves to receive the sperm in mares transports sperm from site of deposition to uterine tubes for fertilization provides suitable environment for a implantation of the embryo b nourishment of the embryo amp fetus during pregnancy E 50 4 provides mechanical protection of the fetus 5 eXpels the mature fetus at the end of pregnancy In the fundus and body of the uterus the wall is divided into the l endometrium tunica mucosa and tunica submucosa 2 myometrium tunica muscularis 3 perimetrium tunica serosa These are terms speci c to the uterus that apply to the typical tunics of a tubular organ The endometrium comprises the tunica mucosa and the tunica submucosa of the uterus In the tunica mucosa the lamina epithelialis is usually simple columnar except in the sow and ruminants where it may be pseudostratified columnar The lamina propria consists of loose connective tissue full of neutrophils and lymphocytes It blends with the underlying tunica submucosa since there is no lamina muscularis mucosae in the entire female reproductive tract Uterine glands are simple or branched tubular glands located in the lamina propriatunica submucosa Some regions of the endometrium in ruminants are void of glands and are highly vascular It is in these regions called caruncles that contacts between the uterus and the extraembryonic membranes are made The myometrium is the tunica muscularis of the uterus It is composed of a thick inner circular layer and a thinner outer longitudinal layer of smooth muscle The region in between the two layers of smooth muscle contains large blood vessels The perimetrium is the tunica serosa of the uterus It has the typical composition of loose connective tissue but contains a large number of lymphatic vessels The stratum vasculare is a layer of large blood vessels located between the inner and outer layers of smooth muscle of the myometrium In the sow the stratum vasculare is indistinct and in the cow it may be located in the outer half of the circular muscle layer Glands Uterine glands are simple or branched tubular glands and may be coiled distally Distal portions of the glands are in the lamina propriatunica submucosa The glands secrete mucus glycogen proteins and lipids The remainder of the endometrial tissue is loose connective tissue The Uterus Changes in the Uterus during the Estrous Cycle Domestic animals are either monestrous or polyestrous Monestrous animals such as bitches have one estrous cycle per year each cycle is followed by a long anestrous period Polyestrous animals are either continuously cycling without an anestrous period such as cows and sows or seasonally cycling with an anestrous period such as mares ewes goats and queens The uterine wall especially the endometrium undergoes greater changes in the monestrous animal than in the polyestrous animal but changes can be clearly seen in polyestrous animals 51 During the estrous cycle the uterus undergoes changes controlled by hormones secreted by the ovary The changes are most pronounced in the glands in the tunica submucosa of the endometrium and in the smooth muscle and stratum vasculare located in the tunica muscularis The Chart and Diagrams below allow you to see the differences in the uterine wall between Proestrus early in the estrous cycle and Diestrus later in the estrous cycle First read through the text and view the associated images then review the material by comparing the appearance of the uterus in proestrus versus diestrus by using the colored bars below the text Proestrus l endometrial glands at lowest level of size amp secretion 2 estrogen rises from granulosa cells in the developing follicles of the ovary 3 uterine epithelial cells hypertrophy 4 uterine glands proliferate 5 vascular supply increases Estrus 1 after ovulation progesterone rises from granulosa amp theca lutein cells in the corpus hemorrhagicum granulosa amp theca lutein cells in the corpus luteum 2 uterine epithelial cells continue to hypertrophy 3 uterine glands continue to proliferate Metestrus l progesterone levels remain high 2 uterine glands undergo hyperplasia amp coiling 3 uterine glandular secretory activity high Diestrus Ni endometrial glands at maximum size coiling amp secretion normal state no fertilization secretory activity is arrested lining cells and glands begin to involute vascularity of the entire wall decreases fertilized state secretory activity is maintained in order to nourish embryofetus during pregnancy the wall of the uterus returns to a state that resembles that of the Proestrus uterus E 52 Vagina Histology The epithelium of the vagina is strati ed squamous usually nonglandular It increases in thickness during proestrus and estrus In some species especially rodent and carnivores the epithelium keratinizes during estrus The tunica mucosa and submucosa are highly folded Lymphocytes and lymphatic nodules can be found in the connective tissue The cranial portion of the vagina has a tunica serosa the larger caudal portion has a tunica adventitia Tunica mucosa lamina epithelialis of strati ed squamous epithelium which is nonglandular highly folded lamina propria is loose connective which blends with the denser connective tissue of the tunica submucosa since a lamina muscularis mucosae is not present the tunica mucosa is thin prior to the onset of puberty and in old age thickens under the in uence of estrogens during the reproductive years super cial cells accumulate glycogen which is maximum at the time of ovulation The acid pH app 30 in the vagina is due to the breakdown of this glycogen by commensal lactobacilli which produce lactic acid Thus only acidloving bacteria and fungi can exist in this low pH environment thus detering bacterial pathogens and fungi such as Candida albicans Tunica submucosa highly folded Tunica muscularis composed of twothree layers of smooth muscle Tunica serosa present cranially then turns into a tunica adventitia caudally 53 lEndocrine System Endocrine organs are organs whose cells secrete their products ie hormones into the bloodstream whereas exocrine organs such as sweat glands salivary glands and sebaceous glands secrete their products into a duct system Hormones travel via the blood circulation and when they reach their quottarget organquot they exert their speci c effect Some organs are primarily exocrine while some are primarily endocrine and some contain elements of both The ovary and testes are both exocrine organs quotsecretingquot ova and spermatozoa respectively yet they are endocrine organs as well secreting hormones such as estrogen progesterone and testosterone In the pancreas part of the organ is an exocrine gland the acini secreting digestive enzymes and part is endocrine ie the islets of Langerhans which secrete various hormones such as insulin and glucagon In the digestive system some cells are endocrine cells ie the enteroendocrine cells Other endocrine organs include the thyroid parathyroid adrenal and pituitary gland The Thyroid Gland Gross Anatomy The thyroid gland is located dorsolateral to the trachea close to the larynx It has two lobes that are connected by a narrow isthmus II Histology The thyroid gland is composed of follicles and interfollicular connective tissue The capsule classified as loose areolar connective tissue surrounds the mass of thyroid 54 follicles and sends smaller pieces of connective tissue into the gland to surround the individual thyroid follicles Near the thyroid gland and embedded in the same connective tissue capsule is the parathyroid gland Sometimes patches of lymphocytes can be observed in the thyroidparathyroid glands Thyroid follicles consist of a layer of simple epithelium surrounding a gellike pinkish material called colloid The principal cell is the most numerous cell present in the simple epithelial layer and is responsible for secreting the thyroid hormones as well as thyroglobulin a glycoprotein Thyroid hormones are stored extracellularly as part of the thyroglobulin which is the main component of the colloid The size of follicles and the height of principal cells varies even within one section of the gland Squamous principal cells indicate a relatively inactive gland whereas cuboidal to columnar cells indicate more activity in removing the hormone from the stored form In addition to principal cells there is another type of functional cell in the thyroid gland This is the parafollicular cell which may be found as single cells in the epithelial lining of the follicle or in groups in the connective tissue between follicles They usually appear as large clear cells since they do not stain well with hematoxylin and eosin They are sometimes called parafollicular cells based on their location and clear cells C cells based on their appearance of their cytoplasm Parafollicular cells secrete calcitonin a hormone that lowers the level of calcium in the blood III Function secretes the thyroid hormones triiodothyronine F3 and tetraiodothyronine F4 or thyroxin that help to regulate the metabolic rate also secretes calcitonin that helps control blood calcium concentration IV Mechanism of Secretion of T3 and T4 thyroxin Under the influence of increased TSH from the pituitary gland principal cells concentrate iodine by active transport At the same time they synthesize thyroglobulin and secrete it into the lumen of the thyroid follicle The iodination reaction catalyzed by the enzyme peroxidase is carried out on the large thyroglobulin molecule at the luminal surface of the principal cell Various combinations of iodinated and noniodinated tyrosine are possible If the two molecules of tyrosine are both fully iodinated the hormone resulting upon 55 cleavage is T4 but if one of the tyrosines has only one iodine then the hormone that results is T3 In the circulation T4 is converted to T3 which appears to be the active form of the hormone Under the in uence of rising TSH levels the principal cells take up colloid by pinocytosis the vesicles fuse with lysosomes which hydrolyze throglobulin releasing T3 and T4 thyroxin which diffuse into the blood and lymph V Parafollicular Cells Secrete calcitonin which inhibits osteoclasts from resorbing bone resulting in decrease in calcium in the blood Controlled by the level of calcium in the blood IPARATHYROID GLAND I Gross Anatomy The parathyroid gland is dif cult to see at the gross level It is very close to and usually embedded within the capsule of the thryroid gland II Histology There are three types of cells in the parathyroid gland adipocytes chief cells and oxyphil cells A reticular connective tissue framework surrounds and supports these cells The main secretory cell is the chief cell These cells secrete parathyroid hormone Unfortunately these cells have no distinguishing features Another cell type present is the oxyphil cell in the human ox and horse These are large cells that contain numerous mitochondria Their function is unknown III Function The parathryoid gland secretes parathyroid hormone which is essential for regulating the levels of calcium and phosphate in the blood Parathyroid hormone acts on the following target organs Bone increases blood calcium by inhibiting osteoblast deposition of calcium and stimulating osteoclast removal of calcium Kidney increases blood calcium by increasing calcium ion reabsorption by kidney tubular cells inhibits reabsorption of phosphate ion from the glomerular filtrate Small intestine increases the absorption of calcium from the small intestine IV Summary of hormonal control of blood calcium levels through action on bone 56 Calcitonin As the level of calcium in the blood rises the amount of calcitonin secreted by the C cells of the thyroid increases Calcitonin stimulates osteoblasts to form bone taking calcium out of the circulation At the same time calcitonin inhibits the mobilization of bone and calcium by osteoclasts The end result is a decrease in the level of calcium in the blood thus helping to maintain proper blood calcium levels Parathyroid Hormone A decrease in the normal levels of calcium in the blood causes the chief cells of the parathyroid gland to secrete more parathyroid hormone which stimulates osteoclasts to mobilize bone resulting in an increase in the level of calcium in the blood Parathyroid hormone also increases Ca ion reabsorption in the kidney and decreases the reabsorption of phosphate ions Note Whereas calcitonin is important in regulating the level of calcium in the blood parathyroid hormone is essentiall ADRENAL GLAND I Gross Anatomy The adrenal glands are located at the cranial end of the kidneys They are flat organs embedded in fat Each gland has an outer cortex that appears yellow in fresh tissue and an inner medulla that appears gray in fresh tissue II Histology of the Adrenal Gland and Adrenal Cortex The adrenal gland is surrounded on the surface by a connective tissue capsule This capsule has projections into the cortex and through the cortex down into the medulla in some species In most species 4 cortical zones can be identified From the zone nearest the capsule these are Zona glomerulosa o In ruminants this zone consists of cells in clusters 0 In carnivores horses and pigs this zone consists of columnar cells in arches and is sometimes called a zona arcuata Zona intermedia This zone is relatively thin and contains mostly undifferentiated cells Zona fasciculata This is the largest zone containing large round foamy appearing cells arranged in cords that radiate from the zona glomerulosa down toward the medulla o Sinusoidal capillaries are located between cords of cells that are one cell thick 57 0 Cells in the zona fasiculata have a foamy appearance due to the presence of many lipid droplets prior to processing for microscopy These lipid droplets represent the precursors for steroid hormones Zona reticularis 0 Cells of the zona reticularis are arranged in a network of cords no longer arranged in parallel as in the zona fasiculata III Function of the Adrenal Cortex Zona glomerulosa cells secrete mineralocorticoids principal one is aldosterone These steroid hormones act on the kidney to 0 Increase Na reabsorption 0 Increase K secretion Zona fasciculata and reticularis cells secrete glucocorticoids eg cortisol cortisone corticosterone which control glucose metabolism IV Histology of the Adrenal Medulla Cells in the medulla are arranged in groups or cords clustered around capillaries and venules The cells have secretory granules which contain either epinephrine or norepinephrine When fixed in potassium bichromate the medullary cells become brown Therefore they are called Chromaf n cells The color is the result of a reaction between chromate and epinephrine or norepinephrine Chromaffin cells are derived from neural crest cells They are innervated by preganglionic sympathetic bers They release hormone by exocytosis when stimulated by those fibers lt Function of the Adrenal Medulla Secretes catecholamines epinephrine and norepineprine VI Vasculature of the adrenal gland 58 Blood is supplied to the adrenal gland via the suprarenal arteries These arteries penetrate the capsule and form a plexus just beneath it From this plexus blood is further supplied via two different routes Cortical arteries of the subcapsular plexus branch into sinusoids in the cortex which intimately surround cells in the zona fasciculata and zona reticularis These sinusoids drain into venules that empty into the central vein of the medulla This blood supply provides a mechanism by which cells in the cortex can influence cells in the medulla The second route of blood supply is more direct to the medulla In this case cortical arteries run along trabecular branches of the connective tissue capsule directly into the medulla without forming capillaries or sinusoids in the cortex These arteries then branch into capillaries in the medulla supplying the chromaf n cells with oxygenated blood These capillaries join the same small medullary venules as the sinusoids of the cortex and all the blood flows into the central vein of the medulla Because of this peculiar blood supply to the adrenal gland the cortex has no veins I PITUITARY GLAND HYPOPHYSIS The pituitary gland or hypophysis is located at the base of the brain Its function re ects its development from two different types of ectoderm ie oral ectoderm which forms Rathke39s pouch and neural ectoderm from the base of the diencephalon which forms the infundibulum As developments proceeds the ectoderm of the infundibulum grows downward and wraps around Rathke39s pouch In the adult the infundibulum forms the infundibular stalk and the pars nervosa the ectoderm of Rathke39s pouch forms the pars distalis and the pars intermedia Some of the oral ectoderm remains associated with the infundibular stalk to form in the adult the pars tuberalis A vestige of Rathke39s pouch often remains visible in the adults of many species and forms a cleft that serves to distinguish the anterior lobe of the pituitary from the posterior lobe useful in humans but not in domestic species The gland remains quotconnectedquot to the brain via both vascular and nervous routes see below under Function These relationships with the hypothalamus of the brain provide a basis for understanding the different functions of the pituitary gland Vascular Connection The vascular connection provides the mechanism by which factors secreted by neuroendocrine cells in the hypothalamus reach and affect secretory cells in the pars distalis These secretory cells respond to the quotreleasing factorsquot brought to them through the portal blood vessels Neural Connection The neural connection provides the mechanism by which hormones secreted by neuroendocrine cells located in speci c nuclei in the brain are actually released into the bloodstream at the level of the pars nervosa of the pituitary gland II Histology 59 The pituitary gland can be divided into various regions based on structure and function One way to describe the pituitary gland is by the type of tissue present in different regions If this system is used then regions are as follows Adenohypophysis based on grouping of all regions composed of glandular tissue This includes the pars distalis pars intermedia pars tuberalis Neurohypophysis based on grouping of all regions composed of neural or neurosecretory tissue This includes the median eminence not shown infundibular stalk pars nervosa infundibular process Another way to describe the pituitary gland is using the terms anterior lobe and posterior lobe in domestic species ventral lobe and dorsal lobe as follows Anterior or Ventral Lobe This includes the parsdistalis pars intermedia Posterior or Dorsal Lobe This includes the pars nervosa The Adenohypophysis Pars distalis This region of the pituitary gland is organized as cords or clusters of cells supported by a reticular connective tissue With routine staining two types of cells can be observed 1 chromophiles which stain readily and are either red acidophiles blue or purple basophiles depending on the type of secretory material present and 2 chromophobes which do not take up the stain and thus appear unstained or rather clear Chromophobes may be chromophiles that have lost their secretory granules or chromophiles that have not accumulated large numbers of secretory granules Use of speci c antibodies against the protein secretory products has allowed the identi cation of the different cells The cells of the par distalis are 60 Somatotrophs secrete growth hormone Mammotrophs secrete prolactin Corticotrophs secrete ACTH Thyrotrophs secrete TSH Gonadotrophs secrete FSH and LH Pars intermedia With routine histological staining the cells in the pars intermedia stain bluepurple and thus are basophilic Cells secrete ACTH MSH endorphins and lipotrophins Pars tuberalis This region is an extension of the glandular pituitary gland and its cells resemble those of the pars intermedia and pars distalis The specific function of the cells in the pars tuberalis however is not clear The Neurohypophysis Pars nervosa This region consists of unmyelinated nerve axons cell bodies are in the hypothalamus and supportive cells called pituicytes Secretes ADH antidiuretic hormone which is synthesized by neurons in the supraoptic nucleus of the hypothalamus Secretes vasopressin which is synthesized by neurons in the paraventricular nucleus of the hypothalamus III Function Control of Secretion Adenohypophysis The cells in the adenohypophysis secrete two classes of hormones 1 direct acting and 2 trophic Direct acting hormones include growth hormone GH and prolactin from the pars distalis and melanocyte stimulating hormone MSH from the pars intermedia Trophic hormones include adrenocorticotrophic hormone ACTH thyroid stimulating hormone FSH follicle stimulating hormone FSH and luteinizing hormone LH Secretion of these hormones is controlled by speci c releasing hormones in the hypothalamus Most of the releasing hormones are stimulatory in their action except for the one for prolactin which is inhibitory and the one for growth hormone which has both inhibitory and stimulatory releasing hormones Releasing hormones are produced in the median eminence of the hypothalamus and reach the adenohypophysis via a portal system of veins known as the pituitary portal system Neurohypophysis 61 The cells in the neurohypophysis secrete only direct acting hormones 1 antidiuretic hormone ADH also known as vasopressin secreted by neurons in the supraoptic nucleus in the hypothalamus and 2 oxytocin secreted by neurons in the paraventricular nucleus in the hypothalamus After synthesis in the hypothalamus these hormones move down the axons of the hypothalamohypophyseal tract through the infundibular stalk and terminate near blood vessels in the pars nervosa Accumulations of these hormones bound to specific glycoproteins can be observed along the axons of the hypothalamophypophyseal tract and in the pars nervosa These quotaccumulationsquot often called Herring bodies represent a storage form of the hormone Release of these hormone stores is determined by impulses in the axons of the hypothalamophypophyseal tract originating in the hypothalamus Such a mechanism of secretion controlled by nerve impulses is called quotneurosecretionquot 62