Comparison of fiber size and phenotypic gene expression in muscles of dystrophic C57BL/6J DY2J/DY2J mice.

Considerable evidence has shown a correlation between fiber size and degree of necrosis in dystrophic muscles of hamster, X-linked muscular dystrophy (MDX) mice and humans. It has been proposed that small-caliber fibers have an immunity to the phenotypic expression of the dystrophic gene(s). The results from the present study show a discordance between fiber size and necrosis in dystrophic muscles of C57BL/6J dy2J/dy2J mice. Extensor carpi radialis longus and brevis muscles (ECRL and ECRB respectively) were compared in normal and dystrophic 2-week, 4-week and 12-month animals by measuring the mean cross-sectional area of type II fibers, determination of relative proportions of types IIA and IIB fibers and calculation of percentage of fibers exhibiting centronucleation in an entire cross-section of muscle (stained for haematoxylin and eosin or ATPase). The ECRL and ECRB muscles were found to have identical sizes of fiber at each of the 3 ages studied and similar proportions of fiber types, yet the former muscle developed and retained significantly more necrosis (manifest as centronucleation) than the latter.