Histogenesis of Myotubes in Regenerating Striated Muscle

¹ Department of Oral Pathology, Loyola University School of Dentistry, Chicago, Illinois

Two groups of 80- to 120-Gm. Sprague-Dawley strain rats were injured by cut, crush and punch wounds of the external abdominal oblique muscle. Series 1 rats were given injections of tritiated thymidine 6 hours after injury and killed at 10, 13, 16, 24, 36, or 44 hours and 3, 4, 6, or 8 days. Series 2 rats were given injections 1 hour before killing at the same time intervals. Autoradiography and special staining were done on all specimens. Labeled perivascular undifferentiated connective tissue cells of the endomysium began to undergo mitotic division 16 hours after injury and rapidly rose to peak numbers at 3 to 4 days, then leveled off. The number of cells in Series 2 rats also rose to a peak at 3 days, at a lower order of magnitude, but fell after 6 days.

Aggregates of mononuclear cells formed on the surface and injured ends of surviving muscle. In Series 1 rats, some cells were labeled, indicating that the source of the cells was from the perivascular mononuclear cells; furthermore, it shows that such cells fused to form the myotube. Only the mononuclear cells were labeled in Series 2 rats, indicating that only mononuclear cells synthesize DNA in preparation for mitosis in injured muscle. This totally excluded cells either of myotubes or of mature muscle. As there is an increase in cellularity in regenerating muscle, however, only the perivascular undifferentiated connective tissue can contribute to such cell source.

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