CM proliferation is diminished in G3 Terc^−/− P1 neonates. (A) Representative Masson’s trichrome staining on WT and G3 Terc^−/− heart sections at P1. n indicates the number of animals analyzed. RV, right ventricle; LV, left ventricle. Bars, 500 µm. (B) WT and G3 Terc^−/− heart weight at P1. n indicates the number of animals analyzed. (C) Heart weight (HW) to body weight (BW) ratio in WT and G3 Terc^−/− P1 hearts. n indicates the number of animals analyzed. (D) WGA staining in WT and G3 Terc^−/− P1 hearts. Bars, 25 µm. (E) CM size quantification in P1 heart transverse sections; 100 CMs were analyzed per animal. n indicates the number of animals analyzed. (F) Detail of pH3 and TnT immunofluorescence at P1. Arrowheads indicate CMs in mitosis. Bars, 100 µm. (G) Quantification of mitotic CMs at P1. n indicates the number of animals analyzed. (H) Detail of Aurora B kinase and TnT immunofluorescence. The arrowhead indicates the Aurora B signal in the cleavage furrow. Bar, 10 µm. (I) Quantification of CM cytokinesis at P1. n indicates the number of animals analyzed. (J) Quantification of P1 WT and G3 Terc^−/− CMs stained with the DDR protein γH2AX at telomeres. n indicates the number of animals analyzed. (K) Representative image of DNA bridges in P1 G3 Terc^−/− CMs. Arrowheads indicate DNA bridges in dividing CMs. Bars, 10 µm. (L) Percentage of anaphases-telophases with DNA bridges in P1 WT and G3 Terc^−/− CMs. n indicates the number of animals analyzed per group. (M) Detail of mono- and binucleated CMs. Bars, 25 µm. (N) Percentage of mono- and binucleated CMs at P1. n indicates the number of animals analyzed. Data are mean ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, nonsignificant.