Heart muscle cell nuclearity varies among species: While humans have mainly uninucleate myocytes, rats have mainly binucleate myocytes which appear between 5-13 days of age. The dog is intermediate in this regard (Figure 2.10).
Polyploidy, that is, multiple copies of the nuclear diploid DNA, is common in humans and increases with age. Ploidies of 4, 8, 16, and 32 are not uncommon. In contrast, polyploidy occurs little in rats. It is unknown whether the presence of multiple DNA copies confers an advantage to the myocardium.
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