Regional differences in fiber characteristics in the rat temporalis muscle

Abstract

The behavioral differences in muscle use are related to the fiber type composition of the muscles among other variables. The aim of this study was to examine the degree of heterogeneity in the fiber type composition in the rat temporalis muscle. The temporalis muscle was taken from 10-week-old Wistar strain male rats (n = 5). Fiber types were classified by immunohistochemical staining according to their myosin heavy chain content. The anterior temporalis revealed an obvious regional difference of the fiber type distribution, whereas the posterior temporalis was homogeneous. The deep anterior temporalis showed a predominant proportion of type IIA fibers and was the only muscle portion displaying slow type fibers (< 10%). The other two muscle portions, the superficial anterior and posterior temporalis, did not differ significantly from each other and contained mainly type IIB fibers. Moreover, the deep anterior temporalis was the only muscle portion showing slow type fibers (< 10%). In the deep portion, type IIX fibers revealed the largest cross-sectional area (1943.1 +/- 613.7 microm(2)), which was significantly (P < 0.01) larger than those of type IIA and I + IIA fibers. The cross-sectional area of type IIB fibers was the largest in the remaining two muscle portions and was significantly (P < 0.01) larger than that of type IIX fibers. In conclusion, temporalis muscle in rats showed an obvious heterogeneity of fiber type composition and fiber cross-sectional area, which suggests multiple functions of this muscle.

Fig. 1

(A) Lateral aspect of the temporalis muscle in an adult rat. The line indicates the level of muscle sectioning. (B) Panoramic view of the temporalis muscle incubated with monoclonal antibody against MyHC-IIA. A clear heterogeneity in fiber type composition can be seen between the anterior deep and superficial portions. Bar = 2 mm.

Fig. 3

Total proportions of fiber types in the superficial and deep anterior temporalis and the posterior temporalis muscle. Error bars indicate S.D. (n = 5). **Significance of difference between the values (P < 0.01).

Fig. 2

Light micrographs of five consecutive sections of the deep portion of the anterior temporalis muscle incubated with monoclonal antibodies against MyHC-I (A), MyHC-IIA (B), MyHC-IIX (C), MyHC-IIA + IIX + IIB (D) and MyHC-cardiac α (E). Bar = 200 µm.