The notion of the adult heart as terminally differentiated organ without self-renewal potential has been undermined by the existence of a subpopulation of replicating myocytes in normal and pathological states. The origin and significance of these cells has remained obscure for lack of a proper biological context. We report the existence of Lin(-) c-kit(POS) cells with the properties of cardiac stem cells. They are self-renewing, clonogenic, and multipotent, giving rise to myocytes, smooth muscle, and endothelial cells. When injected into an ischemic heart, these cells or their clonal progeny reconstitute well-differentiated myocardium, formed by blood-carrying new vessels and myocytes with the characteristics of young cells, encompassing approximately 70% of the ventricle. Thus, the adult heart, like the brain, is mainly composed of terminally differentiated cells, but is not a terminally differentiated organ because it contains stem cells supporting its regeneration. The existence of these cells opens new opportunities for myocardial repair.

Abstract-To determine whether cellular aging leads to a cardiomyopathy and heart failure, markers of cellular senescence, cell death, telomerase activity, telomere integrity, and cell regeneration were measured in myocytes of aging wild-type mice (WT). These parameters were similarly studied in insulin-like growth factor-1 (IGF-1) transgenic mice (TG) because IGF-1 promotes cell growth and survival and may delay cellular aging. Importantly, the consequences of aging on cardiac stem cell (CSC) growth and senescence were evaluated. Gene products implicated in growth arrest and senescence, such as p27 Kip1 , p53, p16 INK4a , and p19 ARF , were detected in myocytes of young WT mice, and their expression increased with age. IGF-1 attenuated the levels of these proteins at all ages. Telomerase activity decreased in aging WT myocytes but increased in TG, paralleling the changes in Akt phosphorylation. Reduction in nuclear phospho-Akt and telomerase resulted in telomere shortening and uncapping in WT myocytes. Senescence and death of CSCs increased with age in WT impairing the growth and turnover of cells in the heart. DNA damage and myocyte death exceeded cell formation in old WT, leading to a decreased number of myocytes and heart failure. This did not occur in TG in which CSC-mediated myocyte regeneration compensated for the extent of cell death preventing ventricular dysfunction. IGF-1 enhanced nuclear phospho-Akt and telomerase delaying cellular aging and death. The differential response of TG mice to chronological age may result from preservation of functional CSCs undergoing myocyte commitment. In conclusion, senescence of CSCs and myocytes conditions the development of an aging myopathy. Key Words: telomerase Ⅲ telomere dysfunction Ⅲ cellular senescence T he accepted but never proven paradigm is that the heart is a postmitotic organ characterized by a predetermined number of myocytes, which is defined shortly after birth and is preserved throughout life till death of the organism. 1 According to this view, age of cardiomyocytes corresponds to the age of the organ and organism, ie, without exception, cellular, organ, and organism age coincide. Myocytes must age at the same pace and, at any given time, the heart should be composed of a homogeneous population of myocytes of identical age. Therefore, myocardial aging has been interpreted as a time-dependent biological process that interacts with ischemic heart disease, hypertension, diabetes, and other pathological conditions, which together define the clinical phenotype. 2 The possibility that cardiac aging is an independent determinant of morbidity and mortality has faced opposition and emphasis has been placed on age-associated changes, which increase the chances of cardiovascular events in the elderly. Treatment of cardiac diseases in old patients has resulted in a prolongation of average lifespan. However, maximum lifespan has not increased in the last 70 years, 3 suggesting that cellular aging may play a more important role than generally expected.Several...