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Mitotic-cyclic and cycle-independent growth of cardiomyocytes
Abstract:
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.
Insights
Heart myocyte number increases shortly after birth. Cell polyploidization occurs, establishing a stable ploidy ratio by 3 weeks, with binucleate cells being most common.
Area of Science:
- Cardiovascular Biology
- Cell Biology
- Developmental Biology
Background:
- Cardiac myocyte number increases significantly in the early postnatal period.
- Myocyte ploidy undergoes changes after birth, transitioning from diploid to polyploid states.
Purpose of the Study:
- To investigate the changes in myocyte number and ploidy during mouse heart development.
- To analyze the relationship between myocyte ploidy and cell mass.
- To understand the growth dynamics of myocytes outside the mitotic cycle.
Main Methods:
- Quantification of myocyte number in mouse heart ventricles across different age groups.
- Analysis of myocyte ploidy distribution (diploid, binucleate, tetraploid, octaploid, hexadecaploid).
- Measurement of myocyte mass and comparison with genome content.
Main Results:
- Myocyte number increased by 25-30% in the first 3 days after birth.
- A stable ploidy ratio, dominated by binucleate 2c x 2 myocytes (~80%), was established by 3 weeks and maintained up to 1 year.
- Myocyte mass did not proportionally correlate with genome ploidy (2c:4c:8c:16c ratio was 2:3.3:5.0:6.3), indicating distinct growth patterns.
Conclusions:
- Postnatal heart development involves significant myocyte proliferation followed by polyploidization.
- The established myocyte ploidy distribution is stable throughout adulthood.
- Myocyte growth is not solely dependent on genome replication, suggesting complex regulatory mechanisms.