BONES

BONE is a connective tissue characterized by a mineralized extracellular matrix.The mineral is Calcium phosphate, in the form of Hydroxyapatite crystalsBones also served as a storage tissue for calcium and phosphate

4 main groups of noncollagenous proteins found in bone matrix:Proteoglycan macromolecules: contain a core protein w/ various numbers of covalently attached side chains of glycosaminoglycansMultiadhesive glycoproteins: responsible for attachment of bone cells & collagen fibers to the mineralized ground subsanceOsteonectin:seves as glue bet collagen & hydroxyapatite crystalsOsteopontin: mediates attachment of cells to bone matrixSialoprotein I & II: mediate cell attachment & initiate calcium phosphate formation during the mineralization process

4 main groups of noncollagenous proteins found in bone matrix:Bone specific Vit.K dependent proteins: includes osteocalcin, protein S & matrix Gla-protein(MGP). Osteocalcin captures calcium from the ciculation & attracts & stimulates osteoclasts in bone remodelingGrowth factors & cytokines: small regulatory proteins including insulin-like gowth factors, TNF-, TGF-, platelet derived growth factors, bone morphogenic proteins & interleukins ( I & II)

Bone matrix contains lacunae connected by a network of canaliculiw/in the bone matrix are spaces called lacunae, where each of which contains osteocyte. 4 other cell types are associated w/ bone:Osteoprogenitor cells: derived from mesenchymal cells; they give rise to osteoblastsOsteoblasts: secrets the extracellular matrix of boneBone-lining cells: cells that remain on bone surface when there is no active growthOsteoclasts: bone resorbing cells present on bone surfaces where bone is being removed or remodeled, or where bone has been damaged

Bones and Bone TissueBones are organs of the skeletal system; bone tissue is the structural components of bonesBone is classified as compact (dense) or spongy (cancellous)Bones are also classified into shape; the location of spongy and compact bone varies w/ bone shape

Short bones

Flat bones

Irregular bones

General Structure of BonesBones are covered by periosteum, a sheath of dense fibrous connective tissue containing osteoprogenitor cells

Bones that articulate w/ neighboring bones possess movable joints

General Structure of BonesBone cavitiesAre lined by endosteum, a layer of connective tissue cells that contain osteoprogenitor cellsThe marrow cavity and the spaces in spongy bone contain marrowRed bone marrow: consists of developing blood cells in different stages of developmentTellow bone marrow: consists mainly of fat cells

General Structure of BonesMature bonesConsists of structural units called osteons or haversian systems

General Structure of BonesMature bonesMature spongy bone is structurally similar to mature compact boneThe blod supply to he shaft of a long bone is chiefly by arteries that enter the marrow cavity through nutrient foraminaThe blood supply to bone tissue is essentially centrifugal

General Structure of BonesImmature boneBone tissue formed in the skeleton of a developing fetusDoes not display organized lamellated appearanceContains relatively more cells per unit area than a mature boneCells tend to be randomly arrangedThe matrix has more ground substance than does the matrix of a mature bone

Bone FormationThe develoment of bone is traditionally classified as ENDOCHONDRAL or INTRAMEMBRANOUSEndochondral ossification is wherein a cartilage model serves as the precursor of the boneIntramembranous ossification is a simper method, w/out the intervention of a cartilage precursor

Intramembranous OssificationBone is formed by differentiation of mesenchymal cells into osteoblasts1st evidence is observed in 8th week of gestation in humansNewly formed bone matrix appears as small, irregularly shaped spicules and trabeculae

Endochondral OssificationBegins w/ proliferation and aggregation of mesenchymal cellsat the site of the future boneInitially, a hyaline cartilage model w/ the general shape of the bone is formedThe first sign of ossification is the appearance of a cuff of bone around the cartilage modelConnective tissue surrounding the porion of the cartilage in de eloping bone no longer functions as perichondrium, it is now called periosteum

Endochondral OssificationWith the establishment of periosteal bony collar, the chondrocytes in the mid-region of the cartilage model become hypertrophicThe calcified cartilage matrix inhibits diffusion of nutrients,causing death of chondrocytes in the cartilage modelPeriosteal cells migrate into the cavity along w/ growing blood vessels

Growth of Endochondral BoneEndochondral bone growth begins in the second trimester of fetal life and continues in early adulthoodGrowth in length of bones depends on the presence of epiphyseal cartilageZone of reserve cartilage: exhibits no cellular proliferation or active matrix productionZone of proliferation: cartilage cells undergo division and organize into distinct columns. Cells are larger than those in the reserve zone

Growth of Endochondral BoneZone of hypertrophy: contains greatly enlarged cartilage cellsZone of calcified cartilage: hypertrophied cells begin to degenerate and the cartilage matrix becomes calcifiedZone of resorption: zone nearest the diaphysis.The calcified cartilage here is in direct contact w/ the connective tissue of the marrow cavity

Growth of Endochondral BoneBone deposition occurs on the cartilage spicules in the same manner as described for the formation of the initial ossification centerCartilage of the epiphyseal growth plate is responsible for maintaining the growth processTickness of epiphyseal plate remains relatively constant during growthThe amount of new cartilage produced equals the amount resorbedResorbed cartilage is replaced by spongy bone

Growth of Endochondral BoneAs bones elongate, remodeling is requiredWhen an individual achieves maximal growth, proliferation of new cartilage w/in the epiphyseal plate terminates

Development of Haversian SystemOsteons typically develop in preexisting compact boneIn the development of new osteons, a tunnel is bored through compact bone by osteoclastsCompact adult bone contains Haversian systems of varying age and size

Physiologic Aspects of BoneBone serves as a reservoir for body calciumPTH acts on the bone to raise low blood calcium levels to normalCalcitonin acts to lower elevated blood calcium levels to normal

Physiologic Aspects of BoneBone can repair itself after injuryThe initial response to a fracture is similar to the response to any injury that produces tissue destruction and hemorrhageNeutrophils are the first to arrive on site, followed by macrophages that begin to clean up the site of injuryIn healthy individuals, this process usually takes 6-12 weeks, depending on the severity of fracture and particular bone