DNA synthesis, mitosis and fusion of myocardial cells

Myocardial cells obtained from embryonic chick ventricles have been used to investigate (1) whether differentiated cells can undergo DNA synthesis and mitosis and, (2) whether heart cells when grown in culture can fuse with each other and with chick skeletal myoblasts to form heterokaryon myotubes. Electron microscopic observations have shown that myocardial cells of day 3 and day 20 chick embryos did contain myofibrils with defined sarcomeres; these cells have been observed in mitosis. Cells obtained by tryptic digestion of day 12 chick ventricles when grown in culture continued to replicate their DNA as shown by thymidine-³H radioautography with DNase controls and were observed in all stages of mitosis. Electron microscopy showed that myofibrils were present in some of the cultured cells. Bi-, tri- and tetranucleate cells were observed in the cultures. Thymidine-³H radioautography showed that these cells were formed by karyokinesis without cytokinesis and by the fusion of uninucleate cells. Since the heart cells could fuse with each other, we tested the possibility that they could fuse with skeletal myoblasts to form heterokaryon myotubes. This was accomplished by co-culturing thymidine-³H labeled ventricular cells and unlabeled skeletal myoblasts. Radioautography with DNase controls showed that some of the myotubes consisted of unlabeled skeletal muscle nuclei and labeled heart nuclei in varied proportions. The factors initiating the formation of these heterokaryons have not been elucidated.