Near the middle of the model periosteal capillaries grow into the cartilage

Near the middle of the model periosteal capillaries

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Near the middle of the model, periosteal capillaries grow into the cartilage model, inducing the formation of a primary ossification center Osteoblasts will deposit bone extracellular matrix over remnants of calcified cartilage, forming spongy bone trabeculae Primary ossification spreads from the central location towards both ends of the cartilage model 34
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Endochondral ossification The fourth step is formation of the medullary cavity The primary ossification center works towards the ends of the bones Osteoclasts break down some of the newly formed spongy bone This activity leaves a cavity in the diaphysis known as the medullary cavity Eventually most of the wall of the diaphysis is replace by compact bone 35
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Endochondral ossification The fifth step is the development of secondary ossification centers When branches of the epiphyseal arteries enter the epiphyses, secondary ossification centers develop This usually happens around the time of birth Bone formation is similar to primary ossification except: Spongy bone remains in the interior (no cavity formation ) Ossification proceeds outward, not inward 36
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Endochondral ossification The sixth and final step is formation of articular cartilage and epiphyseal plate The hyaline cartilage that covers the epiphyses becomes the articular cartilage Prior to adulthood, hyaline cartilage remains between the diaphysis and the epiphysis as the epiphyseal plate This region is responsible for lengthwise growth of bone, discussed next 37
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Endochondral ossification 38
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Endochondral ossification 39
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Bone growth During infancy, childhood and adolescence bones grow in both length and thickness Growth in length requires two major events: Interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate and replacement of cartilage on the diaphysis side of the growth plate To better understand the process, one needs to understand the layers or zones of the epiphyseal plate Zone of resting cartilage is nearest the epiphysis consists of small scattered chondrocytes The term resting implies that this zone is not participating in bone growth This zone anchors the epiphyseal plate to the epiphysis of the bone 40
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Bone growth – lengthwise The zone of proliferating cartilage contains larger chondrocytes that are stacked like coins The chondrocytes undergo interstitial growth as they divide and secrete extracellular matrix Division replaces cells that die at the diaphyseal side of the epiphyseal plate Zone of hypertrophic cartilage contains large maturing chondrocytes arranged in columns Zone of calcified cartilage is only a few cells thick and contains mostly dead chondrocytes Died from the calcification of the extracellular matrix around them Osteoclasts will dissolve the calcified extracellular matrix Blood vessels and osteoblasts will invade the area laying down bone extracellular matrix replacing the cartilage by endochondral ossification 41
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