ARF expressed in ARF:ARF fish suppresses cardiomyocyte proliferation. (a) Myocyte enhancer factor 2 (Mef2), proliferating cell nuclear antigen (PCNA), and 4′,6-diamidino-2-phenylindole (DAPI) staining in WT and ARF:ARF fish at 11 dpi. Blue stains are for all nuclei. Red stains are for Mef2, a cardiomyocyte nuclear marker. Green stains are for PCNA, a cell cycling marker. (b) The cardiomyocyte (CM) proliferation index shows decreased cardiomyocyte cycling in ARF:ARF fish. The CM proliferation index was calculated by counting Mef2+/PCNA+ cells out of total Mef2+ cells in the injury zone. N = 8 hearts. Results are shown as mean ± standard error. The * represents a statistically significant difference.