An ultrastructural study of changes in the tendinous attachment of the deep layer of masseter muscle to the rat mandible during bite raising.

To elucidate the ultrastructural changes in the attachment of masticatory muscles during changes in the functions of masticatory muscles, rats ranging in age from 50 to 80 d, were used. 2 d after bite-raising, the fibrocartilage cells in the tendinous attachment of the deep layer of masseter muscle were characterized by ill-developed cell organelles. Undifferentiated cells around blood vessels degenerated. 4 d after bite raising, however, fibrocartilage cells exhibited well-developed cell organelles, cell division and intercellular contacts. After 10 d of bite raising, the morphologic features of the fibrocartilage cells were the same as those observed in the control. These results suggest that the mechanical stress created by the change in muscle function during bite raising caused the degeneration of the fibrocartilage cell. From 4 d after bite raising, cell proliferation and active formation of new extracellular matrix occurred in response to environmental changes. 10 d after bite raising, the remodeling of collagen fiber and the formation of bone tissue had returned to normal by the adaptation of muscle function.