Necrotic extrafusal muscle fibers of the dystrophic mutant mouse: the ultrastructure of the myoneural junction.

At 28 days postpartum, the extensor digitorum longus muscle of the dy2J mutant mouse contains a population of myofibers which exhibit coagulation necrosis for approximately 90% of their length. Using the electron microscope, motor endplates were found on more than half of the necrotic fibers studied, occurring in mildly, moderately, and severely necrotic regions of these fibers. The ultrastructural features of the axonal terminals did not vary with the condition of the fiber segment at which the endplate occurred. No morphological criteria could be established for distinguishing between the axonal terminals of necrotic fibers and those of "healthy" fibers in the dystrophic animal. The principle morphological changes at motor endplates of necrotic fibers involved not the axonal terminal, but the muscle fiber itself. This study demonstrates that the necrotic myofibers, which are present at the onset of the first clinical symptoms of murine dystrophy, are innervated. Therefore, necrosis is not precipitated by structural denervation. Furthermore, observations of motor endplates on mildly, moderately, and severely necrotic regions of the myofibers indicate that regional changes along the necrotic fiber's length are not a function of distance from the motor endplate.