Introduction.

adult kidney has been debated for some time. In recent years, conclusive evidence for renal progenitors has been reported in lower vertebrates such as fish and insects, as well as in mammals, like mice, rats, and humans. A renal progenitor system consisting of bipotent progenitors, tubular progenitors, and podocyte progenitors was characterized in the human. The identification of renal progenitors is increasing the knowledge about the mechanisms of kidney regeneration. Indeed, growing evidence suggests that some renal disorders can be related to renal progenitor dysfunction. For example, recent evidence suggests that impaired podocyte progenitor differentiation driven by high proteinuria may cause focal segmental glomerulosclerosis and that human tubular progenitors may represent the cell of origin of papillary renal cell carcinoma. These results open important perspectives to modulate renal progenitor function for therapeutic purposes. Other possible strategies are envisaged to promote kidney regeneration and replace kidney function. For example, several groups have reported the use of lineagespecifying factors to differentiate human embryonic stem cells into intermediate mesoderm, from which most nephron-specific cell types are derived. The study of embryonic stem cells and the pursuit of reprogramming methods aim to manipulate differentiated cells and obtain induced pluripotent stem cells that have a broad lineage potential similar to embryonic stem cells. The advent Renal diseases represent one of the major global health burdens of the 21st century. Acute kidney injury affects 1 in 5 people admitted to hospital via emergency departments and it is estimated to be fatal in around 25–30% of cases. In addition, more than 10% of people have chronic kidney disease and the overall prevalence exceeds that of diabetes. For this reason, the replacement of lost renal tissue is a primary target of regenerative medicine research. Kidney regeneration is a challenge. Indeed, the kidney has a complexity that is comparable only to that of the brain, being constituted by more than 26 different cell types that interact to build nephrons – the basic functional units of the kidney – and the surrounding interstitium. Pluripotent stem cells capable of giving rise to any cell lineage of the kidney can be isolated from early-stage mammalian embryos. As development progresses, lineage-restricted stem cells produce the tissues and organs of the body. Development does not necessarily exhaust stem cell pools and usually leads to the formation of tissue-specific adult stem cells that typically show a more restricted potency (e.g. they are multi-, bi-, or unipotent) and are thus also more often defined as progenitors. Adult stem cells and progenitor cells can respond dynamically to injury and fuel substantial regeneration of damaged tissues. For these reasons, they are thought to have important roles in the etiology of disease, malignancy, and aging. The existence of renal stem or progenitor cells in the Published online: May 19, 2014