From the Harvard Gazette: "It has long been a given that adult humans - and mammals in general - lack the capacity to grow new nephrons, the kidney's delicate blood filtering tubules, which has meant that dialysis, and ultimately kidney transplantation, is the only option for the more than 450,000 Americans who have kidney failure. But a new study [may] turn that paradigm on its head, and someday lead to a new treatment for those suffering from kidney disease. ... [researchers have] identified adult kidney stem cells in the zebrafish that can generate new nephrons. These cells can be transplanted from one fish to another fish, whereby they grow into functional nephrons in the transplanted recipient. [This] provides a proof of principle that adult kidney stem cells exist in nature and that treating renal disease with a stem cell-type therapy is theoretically possible. ... Investigators have been searching for the adult renal stem cell in mice and humans for some time, but so far the results have been controversial. ... There is a general belief that while there are kidney stem cells in mammals, including humans, they generate nephrons and then disappear during fetal development or soon after birth. ... The mystery is why these particular cells aren't retained in the adult kidney, where they could be called upon to make new nephrons."