Changes in cardiomyocyte number from Day 1 to Day 5 in each culture condition. (A) Immocytochemical analysis of cell numbers under each cell condition on Day 1 and Day 5; YFP (green), cTnT (red), Hoechst 33258 (blue nuclei), scale bar = 500 μm. (B) Number of cardiomyocytes/culture. Bar graphs depict the fold increase in YFP(+)/cTnT(+) cells. The value for fibroblasts(−) cultures on Day1 is set at 1.0 (N = 3, **P < 0.01). (C–D) Staining for cell proliferation markers Ki67 and Phh3 cardiomyocytes under each culture condition; cTnT (green), Ki67/Phh3 (red), and Hoechst 33258 (blue nuclei), scale bar = 500 μm (N = 4, **P < 0.01).