The identification of cardiac progenitor cells in mammals raises the possibility that the human heart contains a population of stem cells capable of generating cardiomyocytes and coronary vessels. The characterization of human cardiac stem cells (hCSCs) would have important clinical implications for the management of the failing heart. We have established the conditions for the isolation and expansion of c-kit-positive hCSCs from small samples of myocardium. Additionally, we have tested whether these cells have the ability to form functionally competent human myocardium after infarction in immunocompromised animals. Here, we report the identification in vitro of a class of human c-kit-positive cardiac cells that possess the fundamental properties of stem cells: they are self-renewing, clonogenic, and multipotent. hCSCs differentiate predominantly into cardiomyocytes and, to a lesser extent, into smooth muscle cells and endothelial cells. When locally injected in the infarcted myocardium of immunodeficient mice and immunosuppressed rats, hCSCs generate a chimeric heart, which contains human myocardium composed of myocytes, coronary resistance arterioles, and capillaries. The human myocardium is structurally and functionally integrated with the rodent myocardium and contributes to the performance of the infarcted heart. Differentiated human cardiac cells possess only one set of human sex chromosomes excluding cell fusion. The lack of cell fusion was confirmed by the Cre-lox strategy. Thus, hCSCs can be isolated and expanded in vitro for subsequent autologous regeneration of dead myocardium in patients affected by heart failure of ischemic and nonischemic origin.generation of human myocardium ͉ progenitor cells ͉ stem cell niches

In this study, we tested whether the human heart possesses a cardiac stem cell (CSC) pool that promotes regeneration after infarction. For this purpose, CSC growth and senescence were measured in 20 hearts with acute infarcts, 20 hearts with end-stage postinfarction cardiomyopathy, and 12 control hearts. CSC number increased markedly in acute and, to a lesser extent, in chronic infarcts. CSC growth correlated with the increase in telomerasecompetent dividing CSCs from 1.5% in controls to 28% in acute infarcts and 14% in chronic infarcts. The CSC mitotic index increased 29-fold in acute and 14-fold in chronic infarcts. CSCs committed to the myocyte, smooth muscle, and endothelial cell lineages increased Ϸ85-fold in acute infarcts and Ϸ25-fold in chronic infarcts. However, p16 INK4a -p53-positive senescent CSCs also increased and were 10%, 18%, and 40% in controls, acute infarcts, and chronic infarcts, respectively. Old CSCs had short telomeres and apoptosis involved 0.3%, 3.8%, and 9.6% of CSCs in controls, acute infarcts, and chronic infarcts, respectively. These variables reduced the number of functionally competent CSCs from Ϸ26,000͞cm 3 of viable myocardium in acute to Ϸ7,000͞cm 3 in chronic infarcts, respectively. In seven acute infarcts, foci of spontaneous myocardial regeneration that did not involve cell fusion were identified. In conclusion, the human heart possesses a CSC compartment, and CSC activation occurs in response to ischemic injury. The loss of functionally competent CSCs in chronic ischemic cardiomyopathy may underlie the progressive functional deterioration and the onset of terminal failure.cardiac progenitor cells ͉ human heart ͉ myocardial infarction M yocardial regeneration occurs in humans after ischemic injury (1, 2), and myocyte proliferation appears to be restricted to the viable myocardium adjacent to and remote from the infarct (2). The identification of cardiac stem cells (CSCs) in the adult heart (3-7) suggests that replicating myocytes may constitute a subpopulation of rapidly growing amplifying cells originated from more primitive cells. CSCs are distributed throughout the heart, raising the possibility that those located within the infarct or in its proximity could divide and differentiate reconstituting dead myocardium. If this hypothesis were the case, strategies may be developed to enhance myocardial growth promoting partial restoration of the infarct. This response would reduce infarct size, improve function, and decrease mortality. Myocardial regeneration within the infarct could have escaped earlier observations because the heart was not viewed as a self-renewing organ, and myocyte replacement was considered to be regulated by a subset of cells capable of a few rounds of doubling, located by necessity in the spared portion of the ventricle (2). Alternatively, the lack of myocardial regeneration might reflect CSC death within the infarct and͞or the inability of CSCs to migrate and reach the necrotic area. Thus far, no evidence has been presented that CSCs can reconstitute i...