POSTNATAL CHANGES IN THE EXTRAFUSAL MUSCLE-FIBER AND MUSCLE-SPINDLE IN THE MASSETER MUSCLE OF THE DYSTROPHIC MOUSE

In dystrophic (C57BL/6J-dy/dy) mice, postnatal changes in the masseter muscle were analyzed morphometrically and histochemically. From the 6 th to 14th postnatal week, the superficial portion of the masseter mus cle, which was composed of white and intermediate muscle fibers, was a trophied in dystrophic mice. The atrophy was severest in the most late ral component of the muscle. On the contrary, distinct hypertrophy was evident in red muscle fibers in the deep portion. These changes incre ased with time in the diseased animals. The distribution of formazan p articles produced by succinate dehydrogenase (SDH) staining was irregu lar in the muscle of dystrophic mice. In muscle spindles, mean diamete rs of intrafusal muscle fibers and outer capsule were significantly de creased in the diseased mice. The difference in the SDH activity betwe en intrafusal muscle fibers was not often detectable because all fiber s had very low SDH activity. The results strongly suggest that the dis tinct decrease in muscle volume that accompanies with progression of m uscular dystrophy may result from severe atrophy of white and intermed iate muscle fibers. Red muscle fibers in the deep portion may cause ad aptive hypertrophy to compensate the function of the muscle, which is gradually being lost in the superficial portion.