Bone Tissue PDF

Bone Tissue\ Bone is a dynamic tissue\ -- Always remodeling itself\ Supports & protects the body\ Allows the body to move\ Stem cells for blood formation Flat bones have\ compact bone on\ the outside &\ spongy bone on the\ inside\ No marrow cavity General Bone Cell Process\ Osteogenic...

Bone Tissue\ Bone is a dynamic tissue\ -- Always remodeling itself\ Supports & protects the body\ Allows the body to move\ Stem cells for blood formation Flat bones have\ compact bone on\ the outside &\ spongy bone on the\ inside\ No marrow cavity General Bone Cell Process\ Osteogenic cells in the endosteum & periosteum\ Develop into osteoblasts\ Osteoblast make unmineralized bone\ Minerals infiltrate and harden bone\ Osteoblasts become trapped and are now called osteocytes\ Osteocytes reside in lacunae\ Canaliculi are tube extensions from lacunae Osteoclasts\ Osteoclasts develop in bone marrow (pre-monocytes)\ Their job is to dissolve bone\ They make pits called resorption bays which reabsorb bone material\ Help remodel bone and release minerals to bloodstream, they have microvilli on one side and have a ruffled border like a comb Bone Matrix\ By weight bone is 1/3 organic tissue & 2/3 minerals\ Organic matter (mainly collagen, some carbohydrates)\ -- collagen, GAGs, proteoglycans & glycoproteins\ Minerals\ -- 85% hydroxyapatite (mostly calcium phosphate)\ -- 10% calcium carbonate\ Combination provides for rigidity & toughness\ -- minerals resist compression; collagen resists tension -acids dissolve minerals, leaving collagen behind Two types of Bone tissue: compact and spongey Compact Bone\ Osteons or haversian systems\ -- cylinders of tissue formed from concentric layers of matrix arranged around a central (haversian) canal\ each layer is called a lamella\ -- osteocytes connected to each other\ Perforating canals or Volkmann canals\ -- connect the perpendicular central canals\ -- both canals contact blood vessels\ Classification of lamellae - Interstitial lamellae are irregularly shaped and fill in the spaces between osteons - Circumferential lamellae wrap around the circumference of the entire bone - Concentric lamellae are circular, arranged concentrically around the central canal of each osteon Spongy (Cancellous) Bone\ Spongelike appearance -- trabeculae\ Trabeculae have few osteons\ Spaces filled with red bone marrow\ Provides strength with little weight Bone Marrow\ Soft tissue within the medullary cavity of a long bone or the spaces amid the trabeculae of spongy bone\ Red marrow looks like thick blood\ -- mesh of reticular fibers and immature cells\ -- hematopoeitic tissue\ Yellow marrow\ -- mostly adipose tissue\ -- mesenchymal stem cells Bone Marrow\ Red marrow distribution\ -- Axial skeleton and cranium (pelvis, thoracic, cranial)\ Yellow marrow distribution\ -- long bones (legs, arms)\ Changes with age Bone Development\ Flat bones of skull, scapula & mandible develop by intramembranous ossification\ Other bones develop by endochondral ossification Intramembranous Ossification\ Steps of the process\ -- mesenchyme condenses\ -- osteogenic cells\ -- osteoblast form osteid\ -- minerals deposited\ -- osteocytes\ -- osteoclasts remodel center &\ osteoblasts remodel surface\ -- mesenchyme - periosteum Endochondral ossification is a process that converts cartilage into bone: 1. Cartilage formation: Mesenchymal cells differentiate into cartilage cells, called chondrocytes, through the influence of paracrine factors. 2. Cartilage growth: Chondrocytes proliferate and undergo hypertrophy. 3. Cartilage death: Chondrocytes die due to a lack of nutrients. 4. Blood vessel invasion: Blood vessels invade the cartilage matrix, bringing osteogenic cells. 5. Bone deposition: Osteoblasts deposit bone on the remaining cartilage matrix. Epiphyseal Plate (also called metaphysis)\ A cartilaginous layer separating the diaphysis of a long bone from its epiphysis\ Growth of the bone occurs here Metaphysis\ Transitional zone between diaphysis\ and epiphysis\ Five zones of activity\ -- reserve cartilage\ -- cell proliferation\ -- cell hypertrophy\ -- calcification\ -- bone deposition Appositional growth: Growth that occurs at the surface of a tissue, increasing its diameter. Interstitial growth: Growth that occurs throughout the volume of a tissue, increasing its length. Bone Remodeling (50% apoptosis, 30% bone lining cell, 15% osteocyte)\ About 10% of bone tissue is recycled per year\ Osteoclasts (most are in the bone marrow) breakdown bone and osteoblasts build it back.\ Osteomacs regulate what osteoblasts do, homeostasis of bone surface Bone Growth\ Interstitial growth (length)\ -- Happens at the metaphysis\ Appositional growth (width)\ Bones respond to physical stress\ -- Osteoblasts build up bones being used\ -- Osteoclasts break down unused bones\ These processes must be balanced or bone deformities will result Miscellaneous Growth\ Bone growth is rapid at puberty\ -- More osteogenic cells created\ -- More chondrocytes in epiphyseal plates\ Testosterone/estrogen stimulate osteoblast activity differently\ -- Estrogen leads to early closure of growth plate\ Anabolic steroids can speed up the closing of the epiphyseal plate Abnormal Growth\ Achondroplastic dwarfism\ -- Limb bones stop growing early\ -- Typically arises from a spontaneous genetic mutation in a gamete from someone of normal height\ Pituitary dwarfism\ -- congenital or acquired, not caused by a tumor but by head trauma etc\ Pituitary gigantism\ -- adenoma (benign tumor that grows on pituitary gland and shortens lifespan) Mineralization\ Mineralization - certain ions are removed from blood plasma & deposited in bone tissue\ Steps of mineralization\ -- osteoblasts produce collagen fibers that spiral along the length of the osteon in alternating directions\ -- fibers become encrusted with minerals hardening matrix\ ion concentration must reach the solubility product\ crystal formation, then positive feedback forms more\ Ectopic ossification is abnormal calcification\ -- eyes, brain, arteries Mineral Resorption\ Osteoclasts (OC) dissolve bone so minerals can be released into the blood\ Resorption occurs at the OC ruffled border\ OC release H+ ions, which attract Cl- ions\ HCl dissolves the minerals such as hydroxyapatite\ Enzymes:\ -- acid phosphatase - phos\ -- cathepsin K - collagen Mineral Resorption\ Why release calcium & phosphate into the blood?\ Phosphates are a part of many molecules we\'ve talked about: DNA, RNA, ATP, Phospholipids\ Calcium is important for many functions\ -- Muscle contraction\ -- Neuron communication\ -- Blood clotting\ -- Some exocytosis Mineral Homeostasis\ Changes in phosphate conc. have little effect\ Changes in calcium as little as 8% can be serious\ -- Hypocalcemia (deficiency of blood calcium)\ excessive nerve excitability & muscle contraction\ sodium channels too difficult to open (membrane potential)\ -- Hypercalcemia (excess)\ excessive calcium at cell surface makes sodium channels less\ likely to open, depressing nervous system\ Homeostasis depends on calcitriol (active vitamin D that increases calcium), calcitonin (decreases calcium levels) &\ PTH (parathyroid hormone) stimulate osteoclast and osteoblast activity Rickets are lack of calcium Calcitriol: from vitamin D, increases calcium levels in the blood by helping the gut absorb calcium from food and preventing the kidneys from losing calcium Calcitonin\ Secreted by C cells of the thyroid gland when calcium concentration rises too high\ Functions\ -- reduces osteoclast activity quickly (\~15 min)\ -- increases calcium loss by kidneys\ Important role in children, but little effect in adults\ -- calcitonin deficiency does not cause any adult diseases Parathyroid Hormone\ Secreted by parathyroid glands\ Primary purpose in to raise blood calcium levels\ Functions\ -- stimulates the production of an enzyme in the kidneys that carries out the last step in calcitriol synthesis\ -- other functions Healing of Fractures\ Normally healing takes 8 - 12 weeks\ Stages of healing\ -- fracture hematoma (1)\ broken vessels form a blood clot\ -- granulation tissue (2)\ fibrous tissue formed by fibroblasts & infiltrated by capillaries\ -- callus formation (3)\ soft callus of fibrocartilage replaced by hard callus of bone in 6 weeks\ -- remodeling (4)\ occurs over next 6 months as spongy bone is replaced with compact bone Osteoporosis (shows up in spongey bone)\ Most common bone disease\ Bones lose mass & become brittle\ -- loss of organic matrix & minerals\ Post-menopausal women - most risk\ -- by age 70, average BM loss is 30%\ HRT (hormone replacement therapy) slows progression, but\....\ Best treatment is prevention \--\ exercise & calcium intake (1000\ mg/day) between ages 25 and 40\ Drugs\ -- Fosamax, Boniva, Reclast - osteoclast\ -- Evista -- estrogen agonist Chondrocytes make fibrocartilage Hydroxyapatite (Hap) is a mineral composed mainly of phosphorus and calcium, naturally occurring in human bones and teeth Osteoid is a gelatinous, unmineralized material that makes up bone The endosteum is a thin membrane of connective tissue that lines the inner surface of bones, including the medullary cavity and Haversian canals