[Ultrastructure of the cells and DNA synthesis in skeletal muscle regeneration. A study of the regeneration of the frog sartorius muscle by an electron microscopic autoradiographic method].

The ultrastructure of cells of the regenerating frog's sartorius muscle and their capacity to synthesize DNA was studied by means of 3H-thymidine (3HT) electron microscope autoradiography. On the 8-17th post injury (p.i.) days, 2 hours following 3HT administration, only mononuclear cells were seen labeled, the myotube nuclei incorporating no 3HT. Along with the endothelial cells, fibroblasts, phagocytes and cells identified conventionally as myoblasts, satellite cells examined from both necrotic and viable parts of injured myofibers were labeled. No myoblast sequestration from the injured myofibers occurred. By the 13-15th p.i. days, numerous myoblast-like cells are accumulated beneath the glycocalix layer covering the free ends of myotubes which are rich in ribosomes and display an active sarcomerogenesis. Some of these myoblast-like cells become labeled after 3HT pulse. The 13 day p.i. regenerates examined 72 hours following 3HT injection display labeling in numerous myotube nuclei. This is indicative of the myoblast fusion, which is believed to play a principal role in the regenerative somatic myogenesis. Within the myonuclei adjacent to the areas of the regeneration, membranous and/or fibrillar structures of an unknown origin were frequently observed.