Mitotic-cyclic and cycle-independent growth of cardiomyocytes.

The number of myocytes in mouse heart ventricles increased by 25% to 30%, on average, mainly for 3 days after birth. The mean myocyte number was 1.87 +/- 0.04 X 10(6) in 3-week-old and older mice, while the individual values varied up to 25% in any age group. In newborn mice, up to 90% of the myocytes were diploid cells, then polyploidization occurred. The stable ratio of ploidy classes was observed from 3 weeks to 1 year of age. The main class was always binucleate 2c X 2, comprising approximately 80% of the entire population. Nor were many mononucleate tetraploids (4c) and octaploids of different types (8c, 4c X 2, 2c X 4, 2c + 2c + 4c) observed. An insignificant number of hexadecaploids (16c and 8c X 2) could be found in some animals. The mean protein content in myocytes was not always directly proportional to the weight of the ventricles. These data and a certain discrepancy between calculated wet and measured dry weights allowed us to assume that the water content and the non-muscle component of myocardium varied from heart to heart. In growing myocytes of different ploidy, the ratio of the myocyte masses did not correspond to the genome ratio 2: 4: 8: 16 being equal to 2: 3.3: 5.0: 6.3. Thus the growth of the cells outside mitotic cycle differs from that during polyploidization.