G3 Terc^−/− P1 neonates do not increase CM proliferation in response to cardiac cryoinjury. (A) Representative Masson’s trichrome staining in WT and G3 Terc^−/− hearts after cryoinjury at P1. Bars, 1 mm. (B) Quantification of the fibrotic area. n indicates the number of animals analyzed. (C) Detail of Masson’s trichrome staining at 28 dpi. Bars, 200 µm. (D) Quantification of the area covered by CMs within the remaining fibrotic tissue at 28 dpi. n indicates the number of animals analyzed. (E) pH3 and TnT immunofluorescence detail (7 dpi). Arrowheads indicate CMs in mitosis. Controls are age-matched uninjured animals. Cryo, cryoinjury. Bars, 100 µm. (F) Quantification of CM mitosis in WT and G3 Terc^−/− mice. n indicates the number of animals analyzed. (G) Detail of Aurora B kinase and TnT immunofluorescence at 7 dpi. The arrowhead indicates the Aurora B signal in the cleavage furrow. Bar, 10 µm. (H) Quantification of CM cytokinesis (7 dpi). n indicates the number of animals analyzed. (I) Detail of WGA staining at 28 dpi. Bars, 50 µm. (J) Quantification of CM area in transverse sections at 28 dpi; 100 CMs from the injury vicinity were analyzed per animal. n indicates the number of animals analyzed. (K) Quantification of CMs stained with the γH2AX DNA damage protein at telomeres in control (P8) and cryoinjured (7 dpi at P1) animals. n indicates the number of animals analyzed. (L) Detail of telomere (Tel) Q-FISH and 53BP1/TnT immunofluorescence at 7 dpi. Arrowheads indicate colocalization of telomere and 53BP1 signals. Bars, 10 µm. (M) Quantification of CMs with 53BP1 at telomeres in control (P8) and cryoinjured (7 dpi at P1) animals. n indicates the number of animals analyzed. Data are mean ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, nonsignificant.