Heterogeneity of fiber characteristics in the rat masseter and digastric muscles

Abstract

The functional requirements in muscle use are related to the fiber type composition of the muscles and the cross-sectional area of the individual fibers. We investigated the heterogeneity in the fiber type composition and fiber cross-sectional area in two muscles with an opposing function, namely the digastric and masseter muscles (n = 5 for each muscle) of adult male rats, by means of immunohistochemical staining according to their myosin heavy chain (MyHC) content. The digastric and masseter muscles were taken from Wistar strain male rats 10 weeks old. In the masseter six predefined sample locations were examined; in the digastric four. Most regions showed dominant proportions of type IIA and IIX fibers. However, both muscles also 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 masseter, the type I fibers were absent. In the superficial masseter the distribution of IIA and IIB 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. The cross-sectional areas of type IIB fibers were always the largest, followed by the type IIX and IIA fibers. Only a few differences in cross-sectional area of corresponding fiber types were detected between the various sites. In conclusion, the masseter and digastric muscles showed an obvious heterogeneity of fiber type composition and fiber cross-sectional area. Their heterogeneity reflects the complex role of the both muscles during function. This detailed description of the fiber type composition can serve as a reference for future studies examining the muscular adaptations after the onset of various diseases in the masticatory system.

Fig. 1

The jaw muscles in an adult rat. (A) Lateral view. The two lines indicate the level of sectioning for deep and superficial masseter. (B) Ventral view. The two lines indicate the level of sectioning for the anterior and posterior bellies of the digastric muscle.

Fig. 2

Overview of the anterior (A) and posterior (B) belly of the digastric incubated with a monoclonal antibody against MyHC-I. Higher magnifications of the central (C) and peripheral (D) area in the anterior digastric. Bar = 0.25 mm.

Fig. 3

Proportion of fiber types in the anterior and posterior digastric. Error bars indicate a standard deviation. *Significance of difference between the values (P < 0.05); **significance of difference between the values (P < 0.01).

Fig. 4

Proportion of fiber types in the deep and superficial masseter. Error bars indicate a standard deviation. *Significance of difference between the values (P < 0.05); **significance of difference between the values (P < 0.01). (a) P < 0.05 compared with the proportions of type IIB fiber at the anteromedial, anterolateral and posterolateral regions.