EDITORIAL To proliferate or not to proliferate

This editorial refers to ‘Cardiomyocyte cell cycle control and growth estimation in vivo—an analysis based on cardiomyo-cyte nuclei ’ by S. Walsh et al., pp. 365–373, this issue. Cardiomyocytes in mammalian hearts undergo significant reduction in their cell proliferation capacity soon after birth, which is not disputed. However, it remains uncertain whether the adult heart contains a limited capacity for cell proliferation. Through the use of immunohis-tochemical staining methods, it has been reported that adult mouse cardiomyocytes can re-enter the cell cycle and duplicate.1,2 However, the origin of the cell-cycle-positive cells was not deter-mined. Others reported permanent inhibition of cell-cycle pro-gression in adult cardiomyocytes in mice.3 With advances in lineage tracing technology, a recent genetic labelling study demonstrated that stem or precursor cells did not contribute significantly to the for-mation of cardiomyocytes during normal ageing up to 1 year in th