Changes in muscle plasma membranes in young mice with X chromosome-linked muscular dystrophy: a freeze-fracture study.

The structure of the muscle plasma membrane of tibialis anterior muscles of X chromosome-linked muscular dystrophy (mdx) mice was studied by the freeze-fracture technique at 3, 7 and 14 days after birth. The ultrastructural features of the freeze-fracture replicas of the muscle plasma membrane alterations in young mdx mice showed a decrease of orthogonal array, orthogonal array subunit particle and intramembranous particle densities on the protoplasmic face. The results are consistent with the previous studies which have shown that the orthogonal arrays are significantly decreased in number in muscle plasma membranes of adult mdx mice and in those of Duchenne dystrophy. However, the immature mdx mouse membranes at 3 days after birth contained as many orthogonal arrays as controls and did not show a statistically significant decrease (P greater than 0.1 by the Wilcoxon rank-sum test). Moreover, the orthogonal arrays were also numerous in young mdx mouse muscle plasma membranes at 7 and 14 days after birth, although the density was less than that of the control mice (P less than 0.01 by the Wilcoxon rank-sum test). These changes in young mdx mouse plasma membranes may precede the later muscle fibre degeneration in this mouse dystrophy and may provide us with an additional clue to the mechanism why mdx mice scarcely show any disability despite the absence of dystrophyn.