RESUMEN
There is an evidence that mitotic activity of human cardiomyocytes in late fetal and early postnatal ontogenesis is very low. But little is known of the division of human cardiomyocytes at earlier stages of development. In this study mitotic activity of ventricular and atrial human cardiomyocytes of 4-8-week-old embryos and 17-32-week-old fetuses has been studied. On these stages the mitotic index is relatively low to reduce moderately within the 1st to the 3rd trimester of pregnancy from 1.4 to 0.7%. These findings are consistent with the data on cell ploidy demonstrating the presence of relatively small share of myocytes with 3c and 4c DNA in ventricles of 6-8-week-old embryos and 12-22-week-old fetuses. The share of such cells in the 1st and 2nd trimesters of pregnancy varies from 19 to 24% and from 8 to 18%, respectively. Cells with 3c and 4c DNA are most likely to be in mitotic cycle. This assumption is supported by electron microscope pictures showing all phases of typical mitosis. Cyclic changes of myofibrillar ultrastructure during mitosis of prenatal human cardiomyocytes are the same as during mitosis of low differentiated myocytes in mouse and rat hearts. These results suggest that in prenatal human cardiomyogenesis the level of myocyte differentiation and the cell number increase at slow rate.