Enhanced cardiomyocyte proliferation during heart regeneration. (A-D) Confocal images of the heart from transgenic fish expressing nuclear DsRed2Nuc under the control of cmlc-2 cardiomyocyte-specific promoter. Immunostaining for Tropomyosin (blue) labels the myocardium, MCM5 (green) detects mitotic cells and DAPI (yellow in the left panels) marks all the nuclei. Proliferating cardiomyocytes are identified by the co-expression of DsRed2-Nuc and MCM5. Dashed line denotes the post-infarct. (A', B', C' and D') Higher magnifications of the framed area in the left panels. At 4 dpci (A-A'), 7 dpci (B-B'), 14 dpci (C-C') and 30 dpci (D-D'), many proliferating cardiomyocytes are located in the vicinity of the post-infarct. At 4, 7 and 14 dpci, a layer of proliferating non-myocytes surrounds the periphery of the scar (arrows in A, B and C). Bars in (A and A'), 300 μm. (E) Ratios of MCM5/DsRed2-Nuc-positive nuclei relative to DsRed2-Nuc nuclei. (n = 10, 2 representative sections of 5 hearts, *P < 0.01 compared to uncut).