Heterogeneity of fiber characteristics in rat masseter and digastric muscles

Objectives: The aim of this study was to examine the heterogeneity in the fiber type composition and cross-sectional area in two jaw muscles of the rat with an opposing function, namely the digastric and the masseter. Methods: The digastric and masseter muscles were taken from 5 Wistar strain male rats of 10-weeks old. The heterogeneity in the fiber type composition and fiber cross-sectional area in ten locations of the two muscleswas investigated by means of immunohistochemical staining according to their myosin heavy chain (MyHC) content. Results: Both muscles revealed a regional heterogeneity in their fiber type distribution. In the digastric, type I fibers were detected only at the central and deep areas of the anterior and posterior belly, respectively. Meanwhile, the peripheral area of the anterior belly contained a higher proportion of type IIB fibers. In the superficial masseter, the distribution of IIA (superior, 16.8% ; inferior, 26.1%) and IIB (superior, 28.3% ; inferior, 11.9%) fibers was significantly different between the superior and inferior regions. In the deep masseter, regional differences were observed among all four examined areas, of which the posterolateral region contained the highest proportion of type IIB fibers. In the masseter, the type I fibers were absent. The cross-sectional areas of type IIB fibers were always the largest, followed by the type IIX and IIA fibers. Conclusion: The masseter and digastric muscles showed an obvious heterogeneity of fiber type composition and fiber cross-sectional area. These findings would serve as a reference for future studies to examine the nature of muscular adaptations after the onset of various disorders.