Becomes thinner cartilage cells in zone 1 multiply more and more slowly

Becomes thinner cartilage cells in zone 1 multiply

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•Becomes thinner (cartilage cells in zone 1 multiply more and more slowly). •Longitudinal growth ends when bone of the epiphysis and diaphysis fuses: Epiphyseal plate closure (about age 18 in females and 21 in males). •Epiphyseal plate consists of five zones; 1.Resting (quiescent) zone: The cartilage on the side of the epiphyseal plate is inactive. 2.Proliferation (growth) zone: Cells divide quickly, pushing the epiphysis away from the diaphysis. 3.Hypertrophic zone: The older chondrocytes, their lacunae enlarge. 4.Calcification zone: Cartilage matrix calcifies, chondrocytes die, the matrix deteriorate. 5.Ossification (osteogenic) zone: Osteoclasts erode the cartilage, osteoblasts cover them with new bone. 19
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Bone modeling/remodeling •As the long bone lengthens, the shape of the ends must be altered (remodeling). •Growing bones widen as they lengthen through appositional growth. •As the length increases, external surfaces of the ends made slimmer while the internal surface made thicker. •Bone is destroyed by osteoclasts and laid down by osteoblasts on both the inner and outer surfaces of a growing long bone. 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 proportions. •Testosterone (males) and estrogens (females) at puberty promote adolescent growth spurts. -End growth by inducing epiphyseal plate closure. •Excesses or deficits of any hormones cause abnormal skeletal growth. •Bone remodeling consists of both bone deposit and bone resorption. -Occurs at surfaces of both periosteum and endosteum. 20
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-Remodeling units: Packets of adjacent osteoblasts and osteoclasts coordinate remodeling process. •Resorption is function of osteoclasts; -Dig depressions or grooves as they break down matrix. -Secrete lysosomal enzymes and protons (H+) that digest matrix. -Acidity converts calcium salts to soluble forms. •Osteoclast activation involves PTH (parathyroid hormone). •New bone matrix is deposited by osteoblasts. -Osteoid seam: Band of unmineralized bone matrix that marks area of new matrix. -Calcification front: Abrupt transition zone between osteoid seam and older mineralized bone. •Trigger for deposit may include; -Mechanical signals: When you are doing some mechanical work that puts a strain on the bone, bones thicken to the stress. -Increased concentrations of calcium and phosphate ions for hydroxyapatite formation. -Matrix proteins that bind and concentrate calcium. -Appropriate amount of enzyme alkaline phosphatase for mineralization. •Remodeling occurs continuously but is regulated by genetic factors and two control loops; -Hormonal controls: Negative feedback loop that controls blood Ca2+ levels. -Response to mechanical stress.
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