The purpose of the present study is to investigate postnatal developmental changes in muscle fiber type composition depending on the anatomical region in the masseter muscle of the rabbit. Muscle fiber type composition was examined at 7 postnatal developmental stages, at 4, 6, 9, 12, 18, 33 and 50 weeks. In sections of the masseter muscle, the ten sample sites were selected in the transverse plane following anatomical compartments. The fiber types were identified histologically as 6 different types, type I, IC, IIC, IIA, IIAB or IIB fibers, by myosin-ATPase staining and immunohistochemistry using the anti-myosin heavy chain monoclonal antibodies MHC-slow and MHC-fast.
The ten sample sites of the masseter muscle were classified into one of 3 areas (anterior superficial, posterior superficial and deep layer) depending on the number of type I and IIB fibers. The percentage of type IIB fibers was higher in the posterior superficial area at 50 weeks: posterior superficial (36.8%), anterior superficial (11.6%) and deep layers (0.05%). In the anterior superficial and deep layers, the percentage of type I fibers increased and type IIA fibers decreased until 12 weeks (fast-to-slow transition). However, the opposite change occurred from 12 to 50 weeks (slow-to-fast transition). In the posterior superficial area, a transformation toward fast phenotypes occurred from early development stage.
The results presented in this study suggest that the regional and compartmental specialisation within the masseter permits the muscle to perform many different functional roles in the developmental stages and control of jaw movements and jaw position.