Fig. 4.

Ki67 lineage tracing demonstrates de novo generation of cardiomyocytes in the neonatal murine heart. (A) Genetic cross between Mki67^IRES-CreERT2 and LSL-tdTomato reporter mice to lineage trace Mki67-expressing cells. TAM, tamoxifen. (B) Neonatal hearts were collected 1 and 7 d post tamoxifen (dpt) as well as 8 wk post tamoxifen injection. Tamoxifen was injected at the age of 1 wk. IP, i.p. injection. (C–E) tdTomato labeling 1 d (C), 7 d (D), and 2 mo (E) after tamoxifen injection at postnatal day 7 (P7). Nuclei were stained with DAPI (blue). Phalloidin (green or gray) was used to stain polymerized F-actin. (Scale bars: 500 μm.) (F and G) Number of tdTomato⁺ cardiomyocytes per visible field (F) and percentage of tdTomato⁺ cardiomyocytes of all tdTomato⁺ cells (G) at 1 dpt (P8; n = 3 mice), 7 dpt (P14; n = 2 mice), and 2 mo post tamoxifen (mpt) (n = 3 mice).