Age-related histological changes in rat mandibular condyle

Abstract

The effect of aging on rat mandibular condyle was histologically assessed using 1-, 4-, 9-, and 16-month-old male Fischer rats (Eight rats of each group). The medial and anterior portions of the condyle protruded with age. The hypertrophic cell layer of the condylar cartilage occurring up to 4 months was no longer observable at 9 months, after which calcification of the cartilage layer with a tidemark was featured. The chondrocytes in the calcified cartilage layer were reduced in size and intensely stained by toluidine blue. A mineralizing front was formed parallel to, and migrated toward, the condyle surface. At 9 months, osteon-like osteogenesis occurred around blood vessels on the border between the calcified cartilage and the subchondral bone. Condyles of 16-month-old rats exhibited osteosclerotic changes. The calcified cartilage of the middle portion of the condyle formed a remarkably thick layer. The increased volume of the trabecular bone appears to lead to enhanced osteosclerotic changes of the condyle. The aging-related protrusion of the anterior and medial portions of the condyle could be the result of a combination of mechanical forces, e.g., occlusion and mastication. Further, our study revealed that the turnover from cartilage to bone occurring in rat condyle through the process of aging involved an osteon-like bone formation around the blood vessels on the border between the calcified cartilage layer and the bone. This process differs from the endochondral ossification process observed in younger rats.