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Spatial Distribution of Myosin Heavy-chain Isoforms in Mouse Masseter

Dept. of Orthodontics, Box 100444, JHMHSC, University of Florida, Gainesville, FL 32610-0444;

View larger version (38K): [in a new window]   Figure 1. Mouse masseter muscle anatomy depicted for each layer of the muscle: superficial (A), intermediate (B), and deep (C). Tendons of origin (tendon a and tendon b) and insertion (tendon c) are shown, along with the motor endplate distribution. Note that each layer has a continuous row of endplates that does not subdivide the muscle into compartments. The locations of the horizontal sections are also shown (1-4).

View larger version (36K): [in a new window]   Figure 2. (A) Representative immunoblots of the mouse masseter MyHC. Immunoblots are shown for types II, IIa, and IIb MyHC isoforms with the use of MY32, SC-71, and BF-F3, respectively. (B) The normalized integrated optical densities (IOD) of mouse masseter muscle fiber MyHC contents of six different regions were calculated and are shown for the two major MyHC isoforms, IIa and IIb. Note the general distribution of IIa MyHC in the muscle fibers of the anterior regions of the masseter, while IIb predominates in the posterior region.

View larger version (53K): [in a new window]   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.

View larger version (38K): [in a new window]   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.