[The replacement processes of growth plate cartilage to bone in endochondral ossification of mandibular condyle]

Using 3-week-old male beagle dogs, we examined the cartilage-bone replacement processes in the mandibular condyle by means of light and electron microscopy. Calcification of the cartilage matrix occurred in the central area of the longitudinal septa, but not in the transverse septa. Perivascular mononuclear cells absorbed the transverse septa which initiated the opening of the chondrocytic lacunae. These cells phagocytosed septal cartilage fragments. Shortly thereafter, a thin bone layer was deposited on the remaining longitudinal septa by invading osteoblasts. Osteoclasts in lacunae developed neither ruffled borders nor clear zones in the cartilage matrix, but once the bone layer has been deposited in the remaining cartilage, these structures formed. Our results suggest that the cartilage-bone replacement in mandibular condyle involves three sequential processes: 1) degradation and phagocytosis of cartilage fragments in the transverse septa by mononuclear cells, 2) bone deposition over the remaining longitudinal septa, and 3) degradation of both calcified cartilage and bone by osteoclasts.