Sexually dimorphic expression of myosin heavy chains in the adult mouse masseter

Little is known regarding the role of androgenic hormones in the maintenanc e of myosin heavy chain (MHC) composition of rodent masticatory muscles. Be cause the masseter is the principal jaw closer in rodents, we felt it was i mportant to characterize the influence of androgenic hormones on the MHC co mposition 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 s tained tissue sections with antibodies specific to type IIa and IIb MHC iso forms. 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 mous e masseter. The rostral portions of the masseter are composed mostly of typ e 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 ha s no effect on the fiber type composition in females. We conclude that test osterone plays a role in the maintenance of MHC expression in the adult mal e mouse masseter.