The myosin heavy chain (MyHC) isoform composition of six adult (>7 months old) male and female rabbit masseter muscles was studied using seven monoclonal antibodies. In matched serial tissue sections, muscle fibers in 10 different neuromuscular compartments were analyzed. Nearly all fibers were found to express one of five phenotypes. They either contained one of four different slow/beta MyHC phenotypes (I₁–I₄), nearly all of which co-express cardiac alpha MyHC, or they contained type IIa MyHC. Very few fibers contained slow/beta or cardiac alpha MyHC only or both the alpha/slow/beta and IIa isoforms. Most, but not all, of the compartments studied contained similar proportions of fibers of the five major phenotypes, at least within sex. For 7 of the 10 compartments studied, significant sex differences in the proportion of I₁ and IIa fibers were found. Males contained more IIa fibers and fewer I₁ fibers than females. Fibers of the IIa phenotype were significantly larger than fibers of all of the other phenotypes and larger in males than females.

Little is known regarding the role of androgenic hormones in the maintenance of myosin heavy chain (MHC) composition of rodent masticatory muscles. Because the masseter is the principal jaw closer in rodents, we felt it was important to characterize the influence of androgenic hormones on the MHC composition of the masseter. To determine the extent of sexual dimorphism in the phenotype of masseter muscle fibers of adult (10-mo-old) C57 mice, we stained tissue sections with antibodies specific to type IIa and IIb MHC isoforms. Females contain twice as many fibers containing the IIa MHC as males, and males contain twice as many fibers containing the IIb MHC as females. There is a modest amount of regionalization of MHC phenotypes in the mouse masseter. The rostral portions of the masseter are composed mostly of type IIa fibers, whereas the midsuperficial and caudal regions contain mostly type IIb fibers. Using immunoblots, we showed that castration results in an increase in the expression of type IIa MHC fibers in males. Ovariectomy has no effect on the fiber type composition in females. We conclude that testosterone plays a role in the maintenance of MHC expression in the adult male mouse masseter.

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