To quantify subepicardial myocardial cell proliferation, images of the injury border were captured using a Zeiss AXIO imager M1 microscope with a 10× objective (967 × 267 pixels). The subepicardial region was defined as the area within approximately 50 μm of the epicardium, and the trabecular area was defined as the area encompassing the injury, excluding the area used to quantify subepicardial myocardial proliferation. The numbers of Mef2+ and Mef2+PCNA+ cells in the subepicardial and trabecular regions were manually quantified using ImageJ software. The cardiomyocyte proliferation index was defined as the percentage of Mef2+PCNA+ cells in three selected sections from each heart.