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Skeletal muscles attach to bones in three basic patterns3,4), and the attachment of muscles greatly influences ossification5). Furthermore, several types of muscle bundles with different functions are found in one muscle1,2). Thus, in order to determine the function of a particular muscle, the role of each muscle bundle that makes up that muscle must be clarified.

The results of the present study show that the attachment site of the deep layer of the masseter muscle in rats can be divided into three areas: the tendinous attachment, periosteal attachment and transition phase between the tendinous attachment and periosteal attachment. We surgically damaged these areas and observed its healing process to investigate the role of muscle bundles with different attachment morphology. Since the rats appeared to regain their masticating function 14 days after surgery, chronological changes in the deep layer of the masseter muscle were analyzed over this period of time. We found that, 14 days after surgery, the tendinous fibers on the outer most layer of the deep layer of the masseter muscle had been repaired and had regained their alignment. However, the morphology of the peritoneal attachment of muscular fiber bundles had not been repaired. These findings suggest that in order for the masseter muscle to recover after injury, the tendinous fibers must be repaired so that the entire muscle can be protected and recover functionality. Future research will examine recovery of the entire masseter muscle in order to analyze the repair of muscle fibers, attachment of muscle to bone and bone tissue.

This study was supported by Oral Health Science Center grant 961D01 from Tokyo Dental College.


1)Abe, S., Ouchi, Y., Ide, Y. and Yonezu, H.(1997). Perspectives on the role of the lateral pterygoid muscle and the sphenomandibular ligament in temporomandibular joint function, Cranio 15: 203-207.

2)Abe, S., Iida, T., Ide, Y. and Saitoh, C.(1997). An anatomical study of a muscle bundle separated from the medial pterygoid muscle, Cranio 15: 341-344.

3)Chong, D. A. and Evans, C. A.(1982). Histologic study of the attachment of muscles to the rat mandible, Archs oral Biol 27: 519-527.

4)Moss, M. L. and Moss-Salentijin, L.(1978). The muscle-bone interface : An analysis of a morphological boundary. In : Muscle Adaptation in the Craniofacial Region (Edited by Carlson D. S. and McNamara J. A., Jr), Monogr. No.8, Center for Human Growth and Development, Univ of Michigan, Ann Arbor.

5)Yamada,K.(1985). A histological study on the migration mechanism of the attachment of the deep layer of the masseter muscle to the rat mandible during growth, Jpn J Oral Biol 27:254-271.(in Japanese)

6)Yamada, K., Hanada, K. and Ozawa, E.(1988). Ultrastructural study on the migration mechanism of the attachment of the deep layer of masseter muscle to the rat mandible during growth, Jpn J Oral Biol 30: 664-682.(in Japanese)

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