Figure 4.

In the LV (A) and RV (B), the percent of cardiomyocytes with 1 nucleus declines rapidly before birth (n = 79 animals, at least 300 myocytes per ventricle of each animal were counted). Most of these myocytes become binucleated. A small but growing proportion of neonatal cardiomyocytes have 4 or more nuclei. In the LV (C) and RV (D), the proportion of myocytes in the cell cycle is low at birth (n = 79 animals, at least 500 myocytes per ventricle of each animal were counted). Insets: Cell cycle activity normalized to the proportion of myocytes that are mononucleated. Fetal data from Jonker et al. (3). Regression lines (solid) and 95% CI (dashed; best fit to each data set) are shown; bars show quadrinucleate 95% CI of the breakpoints. Regression data are included Tables 1, 2, and 3. mono, mononucleate.