Histological Features of Bone Formation in Mandibular Condylar Head of Beagle Dog

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  • ビーグル犬の下顎骨下顎頭における骨形成の組織学的特性

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Abstract

Using 3-week-old male beagle dogs, the bone formation processes in the mandibular condylar head were examined by means of light and electron microscopy. There were two bone formative patterns in the mandibular condylar head : intramembranous and endochondral bone formation. In the peripheral area of the condylar head, connective tissue derived from the cartilaginous membrane invaded deeply into hyaline cartilaginous tissues and directly formed membranous bone. At the initial stage of membranous bone formation, TRAP-positive osteoclasts were not observed. Cartilaginous tissue was absorbed by mononuclear phagocytes (macrophages). In the central area of the condylar head, irregular endochondral bone formation was observed. In the endochondral bone formation, chondrocytic lacunae were opened by the removal of septal cartilage by macrophages. Shortly thereafter, a thin bone layer was deposited on the remaining calcified septal cartilage by invading osteoblasts. Multinucleated preosteoclasts in lacunar canals did not develop either a ruffled border or a clear zone in the cartilage matrix, but once the bone layer had been deposited on the remaining cartilage, these structures formed. Our results suggest that cartilage-bone replacement in the mandibular condylar head involves three sequential processes : (1) phagocytosis of calcified cartilage fragments by macrophages, (2) bone deposition on the remaining calcified septal cartilage, and (3) degradation of both bone and calcified cartilage by differentiated osteoclasts.

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