[A histological study on changes in the attachment of deep layer of the masseter muscle after denervation of the masseteric nerve].

To elucidate the histological and histochemical changes on the deep layer of masseter muscle at the site of their attachment to the mandible after masseteric denervation, 35-day old male rats were used in this experiment. The rats were sacrificed at 10 and 20 days after the left masseteric nerve was resected at the mandibular notch. The attachment of deep layer of masseter muscle to the mandible was examined using light microscopy, lead labelling technique (Okada and Mimura 1938), and histochemical analysis for ALPase activity. Electromyographic techniques were used for detecting masseter muscle function. 1. 10 days after denervation, the number, shape and ALPase activities of osteoblasts in the transitional zone and fibrocartilage cells in the tendinous attachment on the operated side were seen to decrease compared with those in control. Bone formation on the operated side was less than that in control. The electrophysical activity of masseter muscle was not detected on the operated side. 2. 20 days after surgery, ALPase activities of osteoblasts and fibrocartilage cells, and bone formation in the transitional zone and tendinous attachment on the operated side increased, compared with those observed in the same area on the operated side in the 10-day group, but still less than those in control. Low levels of electrophysical activity in masseter muscle were detected on the operated side. 3. In the periosteal attachment and inferior surface of the mandible on the operated side, bone formation was also seen to be less than that in control. These results suggested that the function of masseter muscle seems to exert the greatest influence on the proliferation and differentiation of both fibrocartilage cells and osteoblasts, and bone growth in the attachment of masseter muscle.