Rationale: The ability of the human heart to regenerate large quantities of myocytes remains controversial, and the extent of myocyte renewal claimed by different laboratories varies from none to nearly 20% per year. Objective: To address this issue, we examined the percentage of myocytes, endothelial cells, and fibroblasts labeled by iododeoxyuridine in postmortem samples obtained from cancer patients who received the thymidine analog for therapeutic purposes. Additionally, the potential contribution of DNA repair, polyploidy, and cell fusion to the measurement of myocyte regeneration was determined. Methods and Results: The fraction of myocytes labeled by iododeoxyuridine ranged from 2.5% to 46%, and similar values were found in fibroblasts and endothelial cells. An average 22%, 20%, and 13% new myocytes, fibroblasts, and endothelial cells were generated per year, suggesting that the lifespan of these cells was approximately 4.5, 5, and 8 years, respectively. The newly formed cardiac cells showed a fully differentiated adult phenotype and did not express the senescence-associated protein p16 INK4a. Moreover, measurements by confocal microscopy and flow cytometry documented that the human heart is composed predominantly of myocytes with 2n diploid DNA content and that tetraploid and octaploid nuclei constitute only a small fraction of the parenchymal cell pool. Importantly, DNA repair, ploidy formation, and cell fusion were not implicated in the assessment of myocyte regeneration. Conclusions: Our findings indicate that the human heart has a significant growth reserve and replaces its myocyte and nonmyocyte compartment several times during the course of life. (Circ Res. 2010;107:305-315.)Key Words: myocyte regeneration Ⅲ cell lifespan Ⅲ DNA repair Ⅲ ploidy Ⅲ cell fusion F or nearly a century, the adult heart has been considered a postmitotic organ in which the number of parenchymal cells is established at birth and cardiomyocytes lost with age or as a result of cardiac diseases cannot be replaced by newly formed cells. The recent explosion of the field of stem cell biology, with the recognition that the possibility exists for extrinsic and intrinsic regeneration of myocytes and coronary vessels, 1 has imposed a reevaluation of cardiac homeostasis and pathology. Several laboratories have identified resident cardiac stem cells (CSCs) in the developing, postnatal, and adult heart of animals and humans, 2-4 suggesting that myocyte turnover and tissue regeneration may be more profound than previously predicted.The documentation that CSCs reside in the myocardium, are stored in discrete niche structures, and divide symmetrically and asymmetrically in vitro and in vivo 4 makes the heart a selfrenewing organ. Cardiac cells continuously lost by wear and tear are constantly replaced by activation and commitment of CSCs. 5 Based on retrospective 14 C birth dating of cells, the claim has been made that throughout life, myocyte turnover in humans is restricted to a subset of Ϸ50% of cardiomyocytes. 6 Although the process...

Rationale:The turnover of cardiomyocytes in the aging female and male heart is currently unknown, emphasizing the need to define human myocardial biology.Objective: The effects of age and gender on the magnitude of myocyte regeneration and the origin of newly formed cardiomyocytes were determined. Methods and Results:The interaction of myocyte replacement, cellular senescence, growth inhibition, and apoptosis was measured in normal female (n‫)23؍‬ and male (n‫)24؍‬ human hearts collected from patients 19 to 104 years of age who died from causes other than cardiovascular diseases. A progressive loss of telomeric DNA in human cardiac stem cells (hCSCs) occurs with aging and the newly formed cardiomyocytes inherit short telomeres and rapidly reach the senescent phenotype. Our data provide novel information on the superior ability of the female heart to sustain the multiple variables associated with the development of the senescent myopathy. At all ages, the female heart is equipped with a larger pool of functionally competent hCSCs and younger myocytes than the male myocardium. The replicative potential is higher and telomeres are longer in female hCSCs than in male hCSCs. In the female heart, myocyte turnover occurs at a rate of 10%, 14%, and 40% per year at 20, 60, and 100 years of age, respectively. Corresponding values in the male heart are 7%, 12%, and 32% per year, documenting that cardiomyogenesis involves a large and progressively increasing number of parenchymal cells with aging. From 20 to 100 years of age, the myocyte compartment is replaced 15 times in women and 11 times in men. Conclusions:The human heart is a highly dynamic organ regulated by a pool of resident hCSCs that modulate cardiac homeostasis and condition organ aging.

Background Cardiac stem cells (CSCs) delivered to the infarcted heart generate a large number of small fetal-neonatal cardiomyocytes which fail to acquire the differentiated phenotype. However, the interaction of CSCs with post-mitotic myocytes results in the formation of cells with adult characteristics. Methods and Results Based on in vitro and in vivo assays, we report that the commitment of human CSCs (hCSCs) to the myocyte lineage and the generation of mature working cardiomyocytes are influenced by microRNA-499 (miR-499) which is barely detectable in hCSCs, but is highly expressed in post-mitotic human cardiomyocytes. miR-499 traverses gap junction channels and translocates to structurally coupled hCSCs favoring their differentiation into functionally-competent cells. Expression of miR-499 in hCSCs represses the miR-499 target genes Sox6 and Rod1, enhancing cardiomyogenesis in vitro and after infarction in vivo. Although cardiac repair was detected in all cell-treated infarcted hearts, the aggregate volume of the regenerated myocyte mass and myocyte cell volume were greater in animals injected with hCSCs overexpressing miR-499. Treatment with hCSCs resulted in an improvement in ventricular function, consisting of a better preservation of developed pressure, and positive and negative dP/dt after infarction. An additional positive effect on cardiac performance occurred with miR-499, pointing to enhanced myocyte differentiation/hypertrophy as the mechanism by which miR-499 potentiated the restoration of myocardial mass and function in the infarcted heart. Conclusions The recognition that miR-499 promotes the differentiation of hCSCs into mechanically integrated cardiomyocytes has important clinical implications for the treatment of human heart failure.