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
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
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
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
Endochondral ossification 38
Endochondral ossification 39
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
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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- Summer '19