Matrix vesicle calcification in bones of adult rats.

To clarify the calcification mechanism that functions in bone formation in adult rats, the ultrastructure of tibial trabeculae and calvarial endostea obtained from 8- to 18-month-old rats was investigated morphologically, and compared with that of 19.5-day post-coitum fetal rats. In both samples, osteoid was observed between the activated osteoblasts and the calcified matrix, which contained matrix vesicles enclosed by a biological membrane. Some of these vesicles contained needle-like crystals thought to be hydroxyapatite, suggesting probable matrix vesicle calcification. These results indicate that matrix vesicle function not only in the initial calcification that occurs during embryonic ossification but also contribute to bone formation in adults.