Measuring cardiomyocyte cell-cycle activity and proliferation in the age of heart regeneration

During the past two decades, the field of mammalian myocardial regeneration has grown dramatically, and with this expanded interest comes increasing claims of experimental manipulations that mediate bona fide proliferation of cardiomyocytes. Too often, however, insufficient evidence or improper controls are provided to support claims that cardiomyocytes have definitively proliferated, a process that should be strictly defined as the generation of two de novo functional cardiomyocytes from one original cardiomyocyte. Throughout the literature, one finds inconsistent levels of experimental rigor applied, and frequently the specific data supplied as evidence of cardiomyocyte proliferation simply indicate cell-cycle activation or DNA synthesis, which do not necessarily lead to the generation of new cardiomyocytes. In this review, we highlight potential problems and limitations faced when characterizing cardiomyocyte proliferation in the mammalian heart, and summarize tools and experimental standards, which should be used to support claims of proliferation-based remuscularization. In the end, definitive establishment of de novo cardiomyogenesis can be difficult to prove; therefore, rigorous experimental strategies should be used for such claims.