[Muscle tissue regeneration of the lymph hearts in adult Rana temporaria frogs. A study by light and electron microscopy autoradiography methods].

The regeneration response of adult frog lymph heart muscle tissue was studied from 2 to 3 weeks after mechanical injury. High resolution autoradiographic studies showed that regenerative necrotic zones have many actively proliferating mononuclear cells deprived of cytoplasmic myofilaments. Some of them have numerous free ribosomes, so they might be identified as myoblasts. On the 13th day after injury newly-formed myotubes with chains of myonuclei and pictures of active sarcomerogenesis were observed. On the other hand, the surviving muscle fibers of the perinecrotic zone were rich in myonuclei at their growing ends. In the vicinity of nuclei, accumulation of a mass of non-differentiated cytoplasm rich in free ribosomes and polysomes, rough endoplasmic reticulum, Golgi apparatus, and centrioles are seen. Tritiated thymidine pulse-labeling showed that only rare myonuclei of the perinecrotic zone muscle fibers were labeled, whereas numerous non-differentiated cells of granulation tissue and myosatellites incorporated thymidine. The number of labeled myonuclei markedly increased 96 hours after 3HTdr administration. These data evidence that the myoblastic mechanism is predominant in the regeneration of adult frog lymph heart muscle tissue. It is necessary to emphasize that during the lymph heart muscle tissue reparative myogenesis some of the perinecrotic myonuclei are able to synthesize DNA and to divide mitotically, which distinguishes this type of muscle from skeletal muscle tissue of vertebrates.