Polyploidization of myocytes in the cardiac ventricle of mice occurs predominantly during the first week of the postnatal life. Using isolated cells it was shown that about 70% of the cardiomyocytes become binuclear at this time (2c x 2), while 10% are mononuclear but contain 4c of DNA, where c was the haploid level of DNA. About 2% of the cell population were reprsented by 4c x 2 or 8c cells. Cytophotometry of Feulgen-stained DNA in 14C-thymidine-labeled nuclei has shown that the cells that enter the mitotic cycle are mainly diploid. After mitosis (30 h or more after thymidine application) the label was found predominantly in 2c x 2 and 4c cell types. Comparison of the curves presenting dynamics of labeled mitoses and the accumulation of labeled binuclear cells reveals that binuclear 2c x 2 cells are formed by acytokinetic mitosis. The formation of 4c mononuclear cells is accomplished via other types of mitotic arrest; this may be due, for example, to a block in the pro- or metaphase. Only very rare cases of cytotomy were detected and the number of newly formed 2c cells was very low. It is concluded that cell multiplication is practically arrested at this period of life, and growth of the ventricular mass is due to polyploidization of virtually all cycling cells, while their number remains unchanged. Mechanisms and functional significance cardiomyocyte polyploidization are discussed.

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