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uring 8th to 12th week of development. Chondroblasts secrete cartilage matrix forming bone model. (b) Cartilage calcifies, and a periosteal bonecollar forms around diaphysis. Chondrocytes in the cartilage model produce holes in the matrix and chondrocytes die. Blood vessels grow toward cartilage. Osteoblasts develop and secrete osteoid around calcified cartilage shaft and form a layer of osteoid. Periosteal bone collar formed. (c) Periosteal bud extends into cartilage shaft and the primary ossification center forms in the diaphysis. Most formed by 12thweek of development. Bone development extends in both directions toward epiphyses as bone replaces degenerating cartilage. (d) Secondary ossification centers form in epiphyses. Not all secondary ossification centers are formed at birth. Hyaline cartilage calcifies and degenerates in epiphyses. Blood vessels enter and osteoprogenitor cells enter laying down bone. Osteoclasts resorb some bone matrix creating the hollow medullary cavity. (e) Bone replaces almost all cartilage, except the articular cartilageand epiphyseal cartilage. By late bone development, almost all of the hyaline cartilage is displaced by bone. Hyaline cartilage remains as articular cartilage only on the articular surface of each epiphysis and at the epiphyseal plates. (f) Lengthwise growth continues until the epiphyseal plates ossify and form epiphyseal lines. Lengthwise bone growth continues into puberty until the epiphyseal plate is converted to the epiphyseal line.7.5 Bone Growth & Remodeling13.Describe and compare appositional and interstitial bone growth: