Systematic distribution of muscle fiber types in the medial gastrocnemius of the laboratory mouse: a morphometric analysis.

Midbelly cross sections of the medial gastrocnemius muscle of young adult male laboratory mice were subjected to ATPase histochemistry with preincubation at pH 4.6. Through the use of a sampling grid and computer-assisted morphometric analysis, 26 to 35% of the total muscle fibers were sampled and classified as type I, IIa, or IIb. Photomicrographs (16 X 20 in.) of five muscles were divided into octants according to a standardized procedure. Total fiber counts and percent of fibers sampled were determined. Variability of sample size per octant was noted, but when averaged across entire muscles, it was in all instances greater than 33%. Fiber type frequency per octant was tested for goodness of fit to a random model by means of a chi-square statistic for equal expected frequencies. Deviation from random fiber type frequency was significant at the P = 0.001 level for every muscle. More importantly, when these data were pooled and again tested using the same method, the probability estimate was less than P = 0.001. This established that the variations in the fiber type proportions found in each mouse followed a common pattern. The systematic fiber type distribution confirmed by these morphometric and statistical methods supports the impression expressed by many muscle biologists that this muscle displays a consistent and complex intramuscular organization.