[DNA synthesis and mitotic division of myocytes of the ventricles, atria and conduction system of the heart during the myocardial development in mammals].

The degree of differentiation of various types of muscle and non-muscle cells that synthesize DNA in myocardia of mouse embryos and suckling rats was estimated by means of electron microscopic autoradiography with 3H-thymidine. No morphologically undifferentiated myoblasts were observed. DNA synthesizing capacity and mitotic activity are typical of numerous moderately differentiated myocytes in all myocardial compartments studied. As judged from proliferation kinetics studies, ventricular myocytes proliferate at embryonal stages and during the 1st postnatal week more actively than do atrial ones, specialized muscle cells from the conductive system replicating much less intensely as compared with both these kinds of myocytes. However, at the 2nd postnatal week, the withdrawal of myocytes from the mitotic cycle proceeds more rapidly in ventricles than in other heart compartments which results in a relatively more active proliferation of myocytes in atria and conductive system beginning from the end of the 2nd postnatal week. By the 17--18th days of the postnatal life, practically all the myocytes, irrespective of their topology, cease to proliferate. Nevertheless, even after this term, up to 0.1--0.5% of atrial and/or conductive system myocytes still go on entering periodically the mitotic cycle. The duration of S and G2 + 1/2 M periods is similar in both the ventricular and atrial myocytes of suckling rats. Probable causes and significance of the observed asynchrony of the myocyte proliferation rates in different heart compartments is discussed.