Methods: Female and male rats were obtained at the ages of 2, 3, 4, 5, 6 and 7 months. For each sex, the temporomandibular joints tissue blocks from four rats were subjected to histological assessment of cartilage thickness and subchondral bone architecture; for the remaining three rats, the mandibular condyles were delivered for gross measurement and evaluation of the mineralization and architecture of subchondral bone by micro-CT.
Results: The results showed that cartilage thickness decreased sharply from 2 to 3 months old in female and 2 to 4 months old in male (P < 0.05) and then remained stable. The subchondral bone density increased sharply from 3 to 4 months old in female and 3 to 5 months old in male, whereas bone volume fraction and trabecular thickness increased but bone surface-to-volume ratio and trabecular number decreased rapidly from 3 to 4 months old in both sexes (P < 0.05). In addition, the significant enlargement of the mandibular condyle in length and width occur sharply at 4 or 5 months old in female but at 5 or 6 months old in male (P < 0.05).
Conclusion: We concluded that the rapid changes of the rat condylar cartilage and subchondral bone primarily occur before 4 months old. These developmental activities result in thinner cartilage but larger and thicker subchondral bone, and they were succeeded by the occurrence of rapid growth in the size of mandibular condyle. Sex differences were identified that the endochondral ossification of fibrocartilage and formation of subchondral bone were faster in female than in male rats, which in turn lead to the earlier enlargement of condyle in female than in male rats.