Cardiac stem cells and myocardial regeneration.

Until recently, the accepted paradigm considered the adult mammalian heart a post-mitotic organ without intrinsic regenerative capacity where neither myocyte death nor new myocyte formation played any role in its homeostasis and could be safely ignored. We have recently identified in the adult mammalian myocardium a small cell population expressing surface antigens commonly associated with a variety of stem cells. These cells have the behaviour and potential of bonafide cardiac stem cells (CSCs): they are clonogenic, self-renewing and multipotent. Their presence has identified myocyte death and myocyte renewal as the two sides of the proverbial coin of cardiac homeostasis. Myocyte renewal depends on the differentiation of the CSCs into immature myocytes that divide two to four times before becoming terminally differentiated. Both in vivo and in vitro the progeny of a single CSC can generate the three major cell types of the myocardium: myocytes, smooth muscle and endothelial vascular cells. More interestingly, when directly injected or activated with growth factors in the post-ischaemic myocardium, these cells are able to reconstitute a functional ventricular wall. Thus, although in the adult heart most cardiac myocytes are permanently withdrawn from the cell cycle, the heart has an intrinsic regenerative potential and it is not a terminally differentiated organ.