Figure 4. Representative maps of regions within transverse sections of the female (A and C) and male (B and D) masseter that contained muscle fibers expressing type IIa (A and B) or IIb (C and D) MyHC were generated by the segmentation of immunostained fibers. Sections are displayed from ventral to dorsal. The histogram illustrates the percent area of each section occupied by fibers expressing the MyHC isoform, as well as the percent of the total area occupied. Note that, in both the male and female, fibers expressing type IIa MyHC are localized to the more anterior and ventral regions of the muscle and are absent from the superficial layer. Expression of the type IIa isoform is more pronounced in the female than in the male. Fibers expressing type IIb MyHC are localized to the posterior and dorsal regions of the muscle, and the expression is more pronounced in the male than in the female.