To quantify epicardial cell proliferation, images of the wound area and the uninjured right ventricular wall were captured at 7 dpi using a Zeiss AXIO imager M1 microscope with a 10× objective (967 × 267 pixels). The numbers of tcf21:DsRed2+DAPI+ nuclei and tcf21:DsRed2+EdU+ nuclei were manually determined using ImageJ software. The epicardial cell proliferation index was defined as the percentage of tcf21:DsRed2+EdU+ nuclei in three selected sections from each heart.