Figure 3. Representative sections of female and male masseter muscles immunostained for the MyHC isoform type IIa (A) or IIb (C). Images have been inverted for easier visualization of immunofluorescently stained fibers. Note that immunostained fibers for IIa are localized mainly to the anterior and deep regions of the masseter, and that different size populations of fibers can be identified. Type IIb immunostained fibers are localized mainly to the posterior regions of the masseter. Fibers expressing the type IIb isoform appear larger than those expressing IIa and are fairly uniform in size. Histograms of the minimum cross-sectional fiber diameter are shown for the anterior and posterior regions for IIa MyHC (B) and the posterior region for IIb (D). Note the anterior and posterior region differences in fiber diameter distributions for the female IIa MyHC, while no difference was found for the male.