BIO 201: Bone
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This 13 page Class Notes was uploaded by ASUNursing19 on Saturday March 26, 2016. The Class Notes belongs to BIO 201 at Arizona State University taught by Dr. Penkrot in Winter 2016. Since its upload, it has received 56 views. For similar materials see Human Anatomy/Physiology I in Biology at Arizona State University.
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Date Created: 03/26/16
Osseous Tissue Bone Tissues and organs of the skeletal system Histology of osseous tissue Bone development Physiology of osseous tissue Bone disorders Bone as a Tissue Osteology : the study of bone o Also, skeletal specimen collections Skeletal system: composed of bones, cartilages, and ligaments o Form strong flexible framework of the body o Cartilage: forerunner of most bones Covers many joint surfaces of mature bone Ligaments: hold bones together at the joints Tendons : attach muscle to bone Classification of Bones 206 named bones in human skeleton Divided into two groups based on location o Axial skeleton Long axis of body Skull, vertebral column, rib cage o Appendicular skeleton Bones of upper and lower limbs Girdles attaching limbs to axial skeleton 6.3 Classification of Bones Bones are also classified according to one of four shapes: o Long bones Longer than they are wide Limb bones o Short bones Cubeshaped bones (in wrist and ankle) Sesamoid bones form within tendons (example: patella) Vary in size and number in different individuals o Flat bones Thin, flat, slightly curved Sternum, scapulae, ribs, most skull bones o Irregular bones Complicated shapes Vertebrae and hip bones Functions of the Skeleton Support : hold the body up, supports muscles, mandible and maxilla support teeth Protection: brain, spinal cord, heart, lungs Movement : limb movements, breathing, action of muscle on bone (system of levers) Electrolyte balance: calcium and phosphate ions Acidbase balance : buffers blood against excessive pH changes Blood formation : red bone marrow is the chief producer of blood cells Bones and Osseous Tissue Bone (osseous tissue) : connective tissue with the matrix hardened by calcium phosphate and other minerals Mineralization or calcification: the hardening process of bone Individual bones consist of tissue, marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue Continually remodels itself and interacts physiologically with all other organ systems General Features of Bones Compact (dense) bone : outer shell of long bone Diaphysis (shaft) : cylinder of compact bone to provide leverage Medullary cavity (marrow cavity) : space in the diaphysis Epiphyses: enlarged ends of a long bone o Enlarged to strengthen joint and attach ligaments and tendons Spongy (cancellous) bone: covered by more durable compact bone o Skeleton about threefourths compact and onefourth spongy bone by weight o Spongy bone found in ends of long bones, and the middle of nearly all others Articular cartilage : a layer of hyaline cartilage that covers the joint surface where one bone meets another Nutrient foramina : minute holes in the bone surface that allows blood vessels and nerves to penetrate periosteum Periosteum : external sheath that covers bone except where there is articular cartilage o Outer fibrous layer of collagen Some outer fibers continuous with the tendons that attach muscle to bone Perforating (Sharpey's) fibers : other outer fibers that penetrate into the bone matrix and attach tendon into bone tissue. Very strong! Strong attachments and continuity from muscle to tendon to bone o Inner osteogenic layer of bone forming cells Important to growth of bone and healing of fractures Endosteum: thin layer of reticular connective tissue lining marrow cavity o Has cells that dissolve osseous tissue and others that deposit it Epiphyseal plate (growth plate): area of hyaline cartilage that separates the marrow spaces of the epiphysis and diaphysis o Enables growth in length o Epiphyseal line: in adults, a bony scar that marks where growth plate used to be Bone Markings Sites of muscle, ligament, and tendon attachment on external surfaces Areas involved in joint formation or conduits for blood vessels and nerves Three types of markings: o Projections: outward bulge of bone May be due to increased stress from muscle pull or is a modification for joints o Depression : bowl or groovelike cutout that can serve as passageways for vessels and nerves, or plays a role in joints o Opening : hole or canal in bone that serves as passageways for blood vessels and nerves Histology of Osseous Tissue Bone is connective tissue that consists of cells, fibers and ground substance Four principal types of bone cells o Osteogenic (osteoprogenator) cells: stem cell of the bone Osteogenic (osteoprogentor) cells : stem cells found in endosteum, periosteum Multiply continuously to produce new osteoblasts o Osteoblasts: build bone o Osteocytes : mature bone cells o Osteoclasts: recycle bone Osteoblasts: bone forming cells o Line up as single layer of cells under endosteum and periosteum o Are nonmitotic o Synthesize soft organic matter of matrix which then hardens by mineral deposition o Stress and fractures stimulate osteogenic cells to multiply more rapidly and increase number of osteocytes to reinforce or rebuild bone o Secrete osteocalcin: thought to be the structural protein of bone (?) Stimulates insulin secretion of pancreas Increases insulin sensitivity in adipocytes which limit the growth of adipose tissue May have a role in male fertility by boosting testosterone synthesis (?) Osteocytes : former osteoblasts that have become trapped in the matrix they have deposited o Lacunae : tiny cavities where osteocytes reside o Canaliculi: little channels that connect lacunae o Cytoplasmic process reach into canaliculi o Some osteocytes reabsorb bone matrix while others deposit it o Contribute to homeostatic mechanism of bone density and calcium and phosphate ions o When stressed, produce biochemical signals (piezoelectric) that regular bone remodeling Osteoclasts: bonedissolving cells found just deep to periosteum o Osteoclasts develop from same bone marrow stem cells that give rise to blood cells o Different developmental origin from rest of bone cells o Unusually large cells formed from the fusion of several stem cells Typically have 3 to 4 nuclei, may have up to 50 o Ruffled border : side facing bone surface Several deep infoldings of the plasma membrane which increases surface area and resorption efficiency o Resorption bays (Howship lacunae): pits on surface of bone where osteoclasts reside o Remodeling: results from combined action of the bonedissolving osteoclasts and the bonedepositing osteoblasts) The Matrix Matrix of osseous tissue is, by dry weight, about 1/3 organic 2/3 inorganic Organic matter : synthesized by osteoblasts o Collagen, carbohydrate: protein complexes, such as glycosaminoglycans, proteoglycans, and glycoproteins Inorganic matter o 85% hydroxyapatite (crystallized calcium phosphate salt) o 10% calcium carbonate, 5% other minerals (fluoride, sodium, potassium, magnesium) Bone is a composite combination of a ceramic and a polymer o Combines optimal mechanical properties of each component o Bone combines the polymer, collagen, with the ceramic, hydroxyapatite and other minerals o Ceramic portion allows the bone to support body weight, and protein portion (collagen) gives bone some degree of flexibility Compact Bone Osteon (haversian system): the basic structural unit of compact bone o Formed by a central canal and its concentric lamella connected to each other by canaliculi o Perforating (Volkmann) canals are transverse or diagonal passages along the length of the osteon o Collaged fibers "corkscrew" down the matrix of the lamella giving it a helical arrangement o Blood flow : skeleton receives about half a liter of blood per minute o Nutrient foramina: on the surface of bone tissue that allow blood vessels and nerves to enter the bone Open into the perforating canals that cross the matrix and feed into the central canals Innermost osteocytes near central canal receive nutrients and pass them along through gap junction to neighboring osteocytes They also receive wastes from their neighbors and transform them to the central canal maintaining a twoway flow a nutrients and waste o Not all of the matrix is organized into osteons Circumferential lamellae : inner and outer boundaries of dense bone Run parallel to bone surface Interstitial lamellae: remains of old osteons that broke down as bone grew and remodeled itself Blood Vessels of Bone Nutrient foramina: on bone surface Perforating (Volkmann's) canals: transverse or diagonal canals Central canals: vertical canals Circumferential lamellae Interstitial lamellae Spongy Bone Spongelike appearance Spongy bone consists of: o Slivers of bone called picules o Thin plates of bone called rabeculae o Spaces filled with red bone marrow o In skull: iploe Few osteons and no central canals o All osteocytes close to bone marrow Provides strength with minimal weight o Trabeculae develop along bone's lines of stress Osteoporosis Decrease in the density and mass of osseous tissue o Reduced BMD (bone mineral density) o Affects both gender, but females more Small stature; hereditary factors; Asian or European ancestry especially prone o Risk decreased with diet and exercise o Significantly increased risk of fractures "Dowager's hump" Additional risk factors for osteoporosis: o Petite body form o Insufficient exercise to stress bones o Diet poor in calcium and protein o Smoking o Hormonerelated conditions Hyperthyroidism Low blood levels of thyroidstimulating hormone Diabetes mellitus o Immobility o Males with prostate cancer taking androgensuppressing drugs Bone Marrow Bone marrow : general term for soft tissue that occupies the marrow cavity of a long bone and small spaces amid the trabeculae of sponge bone Red marrow ( myeloid tissue ) o In nearly every bone in a child o Hemopoietic (bloodforming) tissue : produces blood cells and is composed of multiple tissues in a delicate, but intricate arrangement that is an organ to itself o In adults, found in skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur Yellow marrow found in adults o Most red marrow turns into fatty yellow marrow o No longer produces blood Bone Development Ossification or steogenesis : the formation of bone In the human fetus and infant, bone develops by two methods: o Intramembranous ossification o Endochondral ossification Ossification (teogenesis ) is the process of bone tissue formation o Formation of bony skeleton begins in month 2 of development o Postnatal bone growth occurs until early childhood o Bone remodeling and repair are lifelong Endochondral Ossification Endochondral ossification: process in which bone develops from preexisting cartilage model o Beginning the 6th fetal week and ending in early 20s o Most bones develop by this process Mesenchyme develops into a body of hyaline cartilage in location of future bone o Covered with fibrous perichondrium o Osteoblasts form a bony collar around middle of cartilage model former perichondrium is now considered to be periosteum o Primary ossification center: chondrocytes in the middle of the model enlarge Blood vessels penetrate the bony collar and invade primary ossification center o Primary marrow cavity: forms from blood and stem cells filling hollow cavity Formation of the Bony Skeleton Up to about week 8, fibrous membranes and hyaline cartilage of fetal skeleton are replaced with bone tissue Endochondral ossification o Bone forms by replacing hyaline cartilage o Bones are called artilage (ndochondral bo) s o Form most of skeleton o Forms essentially all bones inferior to base of skull, except clavicles o Begins late in month 2 of development o Uses previously formed hyaline cartilage models o Requires breakdown of hyaline cartilage prior to ossification o Beings at primary ossification center in center of shaft Blood vessels infiltrate perichondrium, converting it to periosteum Mesenchymal cells specialize into osteoblasts Five main steps in the process of ossification: 1. Bone collar forms around diaphysis of cartilage model 2. Central cartilage in diaphysis calcifies, then develops cavities 3. Periosteal bud invades cavities, leading to formation of spongy bone o Bud is made up of blood vessels, nerves, red marrow, osteogenic cells, and osteoclasts 4. Diaphysis elongates, and medullary cavity forms o Secondary ossification centers appear in epiphyses 5. Epiphyses ossify o Hyaline cartilage remains only in epiphyseal plates and articular cartilages Intramembranous ossification o Bone develops form fibrous membrane o Bones are called membrane bones o Begins within fibrous connective tissue membranes formed by mesenchymal cells o Forms frontal, parietal, occipital, temporal, and clavicle bones Four major steps are involved: 1. Ossification centers are formed when mesenchymal cells cluster and become osteoblasts 2. Osteoid is secreted, then calcified 3. Woven bone is formed when osteoid is laid down around blood vessels, resulting in trabeculae o Outer layer of woven bone forms periosteum 4. Lamellar bone replaces woven bone, and red marrow appears Postnatal Bone Growth Long bones grow lengthwise by interstitial (longitudinal) growth of epiphyseal plate Bones increase thickness through appositional growth Bones stop growing during adolescence o Some facial bones continue to grow slowly through life Growth in Length of Long Bones Interstitial growth requires presence of epiphyseal cartilage in the epiphyseal plate Epiphyseal plate maintains constant thickness o Rate of cartilage growth on one side balanced by bone replacement on other Epiphyseal plate consists of five zones: o Resting (quiescent) zone o Proliferation (growth) zone o Hypertrophic zone o Calcification zone o Ossification (osteogenic) zone 1 Resting (quiescent) zone o Area of cartilage on epiphyseal side of epiphyseal plate that is relatively inactive 2 Proliferation (growth) zone o Area of cartilage on diaphysis side of epiphyseal plate that is rapidly dividing o New cells formed move upward, pushing epiphysis away from diaphysis, causing lengthening 3 Hypertrophic zone o Area with older chondrocytes closer to diaphysis o Cartilage lacunae enlarge and erode, forming interconnective spaces 4 Calcification zone o Surrounding cartilage matrix calcifies; chondrocytes die and deteriorate 5 Ossification zone o Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysisdiaphysis junction o Spicules are then eroded by osetoclasts and are covered with new bone by osteoblasts o Ultimately replaced with spongy bone o Medullary cavity enlarged as spicules are eroded Bone Growth and Remodeling Interstitial growth: bones increase in length o Bone elongation is really a result of cartilage growth within epiphyseal plate o Epiphyses close when cartilage is gone piphyseal line o Lengthwise growth is finished (occurs at differed ages in different bones) Appositional growth: bones increase in width throughout life o The deposition of new bone at the surface o Osteoblasts on deep side of periosteum deposit osteoid tissue o Forms circumferential lamellae over surface Bone remodeling occurs throughout life 10% per year o Repairs microfractures, released minerals into blood, reshapes bones in response to use and disuse Wolff's law of bone : architecture of bone determined by mechanical stressed placed on it and bones adapt to withstand those stresses o Remodeling is a collaborative and precise action of osteoblasts and osteoclasts o Bony processes grow larger in response to mechanical stress Control of Remodeling Wolff's law also explains: o Hardedness (right or lefthanded) results in thicker and stronger bone of the corresponding upper limb o Curved bone of the corresponding upper limb o Curved bones are thickest where most likely to buckle o Trabecular form trusses along lines of stress o Large, bony projections occur where heavy, active muscles attach Weight lifters have enormous thickenings at muscle attachment sites of most and used muscles o Bones of fetus and bedridden people are featureless because of lack of stress on bones Growth in Length of Long Bones Near end of adolescence, chondroblasts divide less often Epiphyseal plate thins, then is replaced by bone Epiphyseal plate closure occurs when epiphysis and diaphysis fuse Bone lengthening ceases o Females: occurs around 18 years of age o Males: occurs around 21 years of age Growth in Width (Thickness) Growing bones widen as they lengthen through appositional growth o Can occur throughout life Bones thicken in response to increased stress from muscle activity or added weight Osteoblasts beneath periosteum secret bone matrix on external bone Osteoclasts remove bone on endosteal surface Usually more building up than breaking down which leads to thicker, stronger bone that is not too heavy Hormonal Regulation of Bone Growth Growth hormone: most important hormone in stimulating epiphyseal plate activity in infancy and childhood Thyroid hormone: modulates activity of growth hormone, ensuring proper proportions Testosterone (males) and estrogens (females) at puberty: promote adolescent growth spurt o End growth by including epiphyseal plate closure Excesses or deficits of any hormones cause abnormal skeletal growth 6.6 Bone Remodeling About 57% of bone mass is recycle each week o Spongy bone replaced ~ every 34 years o Compact bone replaced ~ every 10 years Bone remodeling consists of both bone deposit and bone resorption o Occurs at surfaces of both periosteum and endosteum o Remodeling units: packets of adjacent osteoblasts and osteoclasts coordinate remodeling process Mineral Deposition Mineral deposition (mineralization): a crystallization process in which calcium phosphate, and other ions, are taken from the blood plasma and deposited in bone tissue o Osteoblasts produce collagen fibers that spiral the length of the osteon o Fibers become encrusted with minerals that harder the matrix Calcium and phosphate (hydroxyapatite) from blood plasma are deposited along the fibers The calcium and phosphate ion concentration must reach a critical value called the solubility product for crystal formation to occur Osteoblasts neutralize normal tissue inhibitors, present throughout the body and allow salts to precipitate in the bone matrix Ectopic Ossification Abnormal calcification (ectopic ossification) o May occur in lungs, brain, eyes, muscles, tendons, or arteries (arteriosclerosis) o Calculus: calcified mass in an otherwise soft organ such as the lung Lithopedion: "stone baby" Mineral Resorption Mineral resorption: the process of dissolving bone and releasing minerals into the blood o Performed by osteoclasts at the "ruffled border" just deep to the periosteum o Hydrogen pumps in membrane secrete hydrogen into space between the osteoclast and bone surface o Chloride ions follow by electrical attraction o Hydrochloric acid (pH 4) dissolves bone minerals o Acid phosphate enzyme digests the collagen Bone Resorption Resorption is function of osteoclasts o Dig depressions or grooves as they break down o Secrete lysosomal enzymes and protons (H+) that digest matrix o Acidity converts calcium salts to soluble forms Osteoclasts also phagocytize demineralized matrix and dead osteocytes o Digested products are transcytosed across cell and released into interstitial fluid and then into blood o Once resorption is complete, osteoclasts undergo apoptosis Osteoclast activation involved PTH (parathyroid hormone) and immune T cell proteins Control of Remodeling Remodeling occurs continuously but is regulated by genetic factors and two control loops 1. Hormonal Controls o Negative feedback loop that controls blood Ca2+ levels o Calcium functions in many processes, such as nerve transmission, muscle contraction, blood coagulation, gland and nerve secretions, as well as cell division o 99% of 12001400 gms of calcium are found in bone o Intestinal absorption of Ca2+ requires vitamin D 2. Response to Mechanical Stress 1 Hormonal Controls o Parathyroid hormone (PTH): produced by parathyroid glands in response to low blood calcium levels o Stimulates osteoclasts to resorb bone o Calcium is released into blood, raising levels o PTH secretion stops when homeostatic calcium levels are reached o Calcitonin: produced by parafollicular cells of thyroid gland in response to high levels of blood calcium levels o Effects are negligible, but at high pharmacological doses it can lower blood calcium levels temporarily Calcium Homeostasis Phosphate is a component of DNA, RNA, ATP, phospholipids, and pH buffers Calcium needed in neuron communication, muscle contraction, blood clotting, and exocytosis Minerals are deposited in the skeleton and withdrawn when they are needed, about 1100g of calcium in adult body o 99% in the skeleton, 18% of adult skeleton exchanged with blood each year Normal calcium concentration in blood plasma is normally 9.2 to 10.4 mg/dl o Hypocalcemia: blood calcium deficiency o Causes excess excitability of muscle, tremors, spasms, or tetany (inability to relax) Na+ enters cells too easily and excited nerves and msucles o Hypercalcemia: blood calcium excess o Sodium channels less responsive and nerve and muscle less excitable than normal (sluggish reflexes, depression) Ion Imbalances Hypercalcemia is rare (calcium will precipitate) Hypocalcemia has a wide variety of causes o Vitamin D deficiency o Diarrhea o Thyroid tumors o Underactive parathyroids o Pregnancy and lactation o Accidental removal of parathyroid glands during thyroid surgery Calcium homeostasis depends on a balance between dietary intake, urinary and fecal loses, and exchanges between osseous tissue Calcium homeostasis is regulated by three hormones: o Calcitriol, calcitonin, and parathyroid hormone Hormonal Control of Calcium Calcitonin > bone deposition (Ca2+) Calcitriol & PTH > Ca2+ form bone into blood Calcitriol (activated Vitamin D3) Calcitriol: a form of vitamin D produced by the sequential action of the skin, liver, and kidneys Produced by the following process: o Epidermal keratinocytes use UV radiation to convert a steroid, 7 dehydrocholesterol to previtamin D3