Existing cardiomyocytes generate cardiomyocytes at a low rate after birth in mice

Significance The nature of postnatal cardiomyogenesis in mammals remains in dispute. Here, we provide cell-level evidence for the birth of cardiomyocytes in newborn and adult mice. Our clonal analysis, based on the mosaic analysis with double markers mouse model, shows that cardiomyocytes are the parent cell of origin of cardiomyocytes that are generated postnatally. Our findings confirm that limited, symmetric division of cardiomyocytes is a rare phenomenon in the mouse heart after birth.