Rat myocardial cells in vitro: Mitosis and differentiated properties

Summary

Ventricular heart cultures were prepared from newborn rats and examined in Rose chambers by high resolution, phase-contrast optics. Details of the procedure are given for obtaining a high yield of myocardial or endothelial cells. Photographic records were obtained of living cells by using photomicrography, cinematography, and high speed filming to document the cytological differences between endothelial and myocardial cells, details of mitosis in the two cell types, and patterns of contractility. Myocardial cells can be distinguished from endothelial cells in the living state by the following criteria; dense cytoplasm; giant and pleomorphic mitochondria or sarcosomes: presence of small, round nuclei; binucleation; one or two round, dense nucleoli; a specialized Golgi apparatus around the nucleus; myofobrils; intercalated discs; myopodia; small, cell size; failure of cell migration; spontaneous contractions; formation of synchronized networks; and flattened appearance with adhesion to other cells during mitosis. Contracting myocardial cells are shown to undergo mitosis. Contractions become weaker at metaphase and then cease at anaphase. Daughter cells may resume contractions. Organized myofibrils are generally not detected during the mitotic periods when contractions are minimal or absent. It is proposed that myofibrils undergo a transient disorganization during mid-mitosis and become reorganized in the daughter cell(s). It is suggested that there is a competition for energy at the time of spindle changes and chromosome movements, so that priority is given to the mitotic process rather than to myofibrillar contractions. Myofibrillogenesis is considered to be a relatively rapid process. The average mitotic time is 2.5 hr fore endothelial cells and 6.1 hr for myocardial cells. Failure of cytokinesis frequently occurs in dividing myocardial cells and results in the formation of binucleated cells. A sequence, is presented of a binucleated myocardial cell which undergoes an abormal multipolar mitosis, leading to polynucleation. Myocardial cells incorporate tritiated thymidine into nuclear DNA. The question of mitosis and differentiation of myocardial cells is reviewed. It is concluded that myocardial cells can no longer be cited in support of the dogma, that differentiated cells do no divide. However, these is a temporary competition between the two phenomena at the metaphase-anaphase stages.