Chronic kidney disease (CKD) is increasing at the rate of 6-8% per annum in the US alone. At present, dialysis and transplantation remain the only treatment options. However, there is hope that stem cells and regenerative medicine may provide additional regenerative options for kidney disease. Such new treatments might involve induction of repair using endogenous or exogenous stem cells or the reprogramming of the organ to reinitiate development. This review addresses the current state of understanding with respect to the ability of non-renal stem cell sources to influence renal repair, the existence of endogenous renal stem cells and the biology of normal renal repair in response to damage.
Understanding repair options via an understanding of renal developmentThe prevalence and incidence of chronic kidney disease (CKD) is increasing at 6-8% per annum in the USA alone, largely as a result of increased prevalence of diabetes and obesity [1]. To understand what might be possible with respect to cellular therapies or regenerative medicine for the kidney, one first needs to consider what is understood about normal renal development and response to injury. The kidney has been classically regarded as an organ with minimal cellular turnover and no capacity for regeneration. The subsequent identification of stem cells in a number of such organs, including the brain, has challenged this view. However, the dogma remains that the kidney reaches a maximal complement of nephrons and then loses these over time [2]. This dogma is drawn from our understanding of the development of this organ.
The progenitor population during developmentThe kidney is mesodermal in origin and develops from two populations derived from intermediate mesoderm, the ureteric bud (UB) and the metanephric mesenchyme (MM). While an even broader potential had been proposed [3], genetic and lineage analyses have confirmed that the MM gives rise to all portions of the nephron other than the collecting ducts and also gives rise to elements of the renal interstitium [4][5][6]. The UB gives rise to the ureter and collecting ducts only. The MM is apparently not homogeneous but forms condensed mesenchyme around the tips of the branching UB. This cap mesenchyme (CM) contains self-renewing progenitors capable of generating all cells of the nephron other than the collecting ducts via an initial mesenchyme-epithelial transition (MET) event throughout the prenatal developmental period [6]. The continued expression of the transcription factor Six2 in CM is required for maintenance of this stem cell population during kidney development [5]. As the UB branches and extends through the MM, individual MET events occur at the underside of each tip to form a new nephron [2]. This nephron endowment therefore proceeds from the centre of the kidney out to the periphery, with the CM stem cell field remaining on the outer edge of the expanding organ. Endowment of new nephrons is restricted to prenatal development in humans, while in rodents it persists only until the imm...