In the past few years, a number of studies have shown that stem cells can be found in virtually every organ of the adult organism. The kidney is not an exception, and resident stem cells have been identified both in the papilla and along the tubules. Of interest, kidney-bound stem cells have been identified also in the bone marrow. When injected, both resident and bone marrow-derived stem cells are able to reach the injured renal tissue and, once there, to differentiate into renal cells. The

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... dence that, in humans, some of the acute and most of the chronic renal damages lead to ESRD suggests that in normal conditions, the reservoir of stem cells (considering both resident and bone marrow-derived stem cells) is insufficient to allow a major renal regeneration. Probably the number of stem cells that are ready to intervene in an adult kidney are sufficient to compensate for the normal cell turnover but largely inadequate to counteract a major injury. This is confirmed further by the finding that, even by transplanting a syngenic bone marrow in rats with ablation of 5/6 of the renal function, it is not possible to increase the life expectancy of the animals. Altogether, this evidence suggests that, to clarify the potentiality of a stem cell therapy for renal diseases, experiments that aim to clarify the ideal concentration of stem cells to be injected and to identify the best way of administration are needed.