(A-C), at P1; D-F, at P21. (A,D) Histology showing normal heart morphology. Scale bars: 1 mm. (B,E) Calculation of heart weight to body weight ratio. Differences are not statistically significant. (C,F) Ventricular cardiomyocyte area shown as a box and whiskers plot (median, middle quartiles, and full range of the data indicated; no outliers excluded), based on n = 110 (Nkx2.5-Cre P1), n = 110 (Nkx2.5-Cre/Igf2 P1); n = 115 (Nkx2.5-Cre P21), n = 116 (Nkx2.5-Cre/Igf2 P21) cells. Differences are not statistically significant. (G,H) Calculation of ventricular mononuclear cardiomyocyte frequency (G) and the nuclear ploidy of the mononuclear cardiomyocyte subpopulation (H) in cellular preparations from neonates at P3 and P7 as indicated. All pups were on an identical B6AF1 genetic background. The measured difference in mononuclear cardiomyocyte percentage at P7 is statistically significant although of uncertain biological relevance (see text). Differences in nuclear ploidy are not statistically significant (at P3, p=0.19; at P7, p=0.38). (I) Images of cell preparations from the same analysis shown in panels G-H, stained for the cardiomyocyte marker TNNC (green) and DAPI to visualize nuclei; scale bar for all panels = 100µ.