ANAT 12. Histology of Bone.docx

Learning Objectives: List the functions of the skeleton. Describe the types of cells, fibers and ground substance present in bone. Based on function of the various types of bone cells, predict the morphology and intracellular components of each cell type. Describe the process of ECM synthesi...

Learning Objectives: List the functions of the skeleton. Describe the types of cells, fibers and ground substance present in bone. Based on function of the various types of bone cells, predict the morphology and intracellular components of each cell type. Describe the process of ECM synthesis and mineralization. Location of new bone production within a bone. Describe location of the different cell types in the bone. Know how bone cells communicate and are regulated by different substances. Know how bone synthesis can be measured clinically. Identify and describe the [two types] of bone. Explain the different patterns of organization within compact bone. Describe the basic organization of the tendon-to-bone attachment. Relate [basic] knowledge of histology of bone to histopathology of this tissue. Specialized form of CT, differs by its [mineralization] of ECM, forming Calcium hydroxyapatite. Bone vs. Cartilage: hard, vascularized, innervated, great healing ability, mineralized. General functions of Skeleton: Support, protection(cranial/thoracic cavity), movement, hematopoiesis(blood prod), mineral homeostasis(Ca, phosphorus), Fat storage, Mastication. Periosteum: dense irregular CT, covers external surface(NOT articular surface) - - Perforating(Sharpey's) Fibers: CT matrix(type 1 collagen fibers), connect Periosteum-Bone. Compact: 80% of bone mass, stronger/denser to withstand stress. Deep to periosteum. Cancellous(Spongy): inner regions, needle-like projections(Trabeculae) line marrow cavities & Dipole of calvarium(skullcap). ([covered by endosteum]) Bone Tissue Components: -CT: periosteum(location of osteoblasts), endosteum(lines internal surface, has osteoblasts). -Cells: 3 types. - - - -ECM: type 1 collagen(90%), non-collagenous proteins(osteonectin etc.), hydroxyapatite & water. Central(Haversian) canal connected horizontally by perforating(Volkmann) canal. Development of Osteon:  Osteoclasts tunnel into existing bone(as cutting cone). Follows osteoBlasts, lines walls of tunnel, secrete osteoid in cyclic manner, producing layers of new matrix(lamellae) & trapping some osteocytes in lacunae. Tunnel constriction, due to multiple lamellae. Lumen finally exists as only a narrow central canal with small blood vessels. Osteoblasts: Release vesicles filled w/ [alkaline phosphatase] ↑PO4- → Ca + PO4- = crystal precipitation. (Reside [in perio- & endo-steum]) - - - Osteocytes: inactive, mature osteoblasts in lacunae. Maintenance/regulation of bone remodelin via [sclerostin], sensitive to mechanical stress. Osteoclasts: giant [multinucleated] cell([fused monocytes]), secrete acid to breakdown bone. Anchored to bone at the ruffled border by podosomes. (In Howship's lacunae) - \*\*[Stain cytoplasm Acidophilic]. Mesenchymal stem cells→ osteoprogenitor cells→ Osteoblasts→ Osteocytes & bone-lining cell. ~~MSCs induced via expression of Runx2, Dlx5, Osx. Then pre-osteoblast(**columnar**) mature into osteoblast by ↑ expression of Osx & secretion of bone-matrix proteins(type 1 collagen, osteocalcin, bone sialoprotein).~~ Bone Resorption by Osteoclasts: At [ruffled border], H+ & [TRAP] & [MMP-9] are released into Howship's lacuna→ bone degradation → ↑ Ca & PO4- → endocytosis → transcytosis(in vesicles)→ exocytosis(into bloodstream). Bone Matrix Synthesis & Minerlizations(by Osteoblasts): 1\. Deposition of organic matrix: Osteoblasts secrete type 1 collagen & [non-collagen proteins]; (osteocalcin, BSP, osteopontin, proteoglycan). 2\. Subsequent calcification(mineralization) of organic matrix. - - Structure of bone Type 1 collagen: tropocollagen(triple-helix): Glycine-Proline-hydroxyproline. Polymerizes to form collagen fibrils, that aggregate to form long fibers(Type 1). Bone mineral content: inorganic material \~50% of bone dry-weight, e.g. Hydroxyapatite, CaP. Enthesis: site of attachment of tendon into bone. (insertion site) - - - Enthesis develops Post-Natally, consists of 4 zones: - - - - - Clinical Application: Ricket: Ca deficiency during growth, ↓dietary vitamin D, no sun, kidney/GIT issues, ↓ vitamin D during pregnancy. (malnutrition) Osteoporosis: ↓ bone density, due to menopause, ↓ dietary calcium, smoking, ↓exercise etc... - Osteosarcoma(primary): arise in osteoprogenitor cells, uncommon \~ 0.5% of all cancer deaths. Secondary tumors: metastasis into bones via blood/lymphatics, most common from breast, lung, prostate, kidney, thyroid. \*\*Bone histology- [acidophilic] due to higher concentration of collagen fibers.