Sexual dimorphism in the rabbit masseter muscle: Myosin heavy chain composition of neuromuscular compartments

The myosin heavy chain (MyHC) isoform composition of six adult (>7 months o ld) male and female rabbit masseter muscles was studied using seven monoclo nal antibodies. In matched serial tissue sections, muscle fibers in 10 diff erent neuromuscular compartments were analyzed. Nearly all fibers were foun d to express one of five phenotypes. They either contained one of four diff erent slow/beta MyHC phenotypes (I-1-I-4), nearly all of which co-express c ardiac alpha MyHC, or they contained type IIa MyHC. Very few fibers contain ed 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. F or 7 of the 10 compartments studied, significant sex differences in the pro portion of II and IIa fibers were found. Males contained more IIa fibers an d fewer II fibers than females. Fibers of the IIa phenotype were significan tly larger than fibers of all of the other phenotypes and larger in males t han females.