Similar to limbs, the regeneration of an internal organ like the kidney is variable in adult animals. Lower animals such as cartilaginous fish can form new nephrons following injury, while higher animals merely demonstrate compensatory growth of the remaining nephrons, namely, “regenerative hypertrophy.”8 The ability to form new nephrons following completion of nephrogenesis is called “neonephrogenesis.” Elasmobranchs, such as skate and dogfish, demonstrate neonephrogenesis as adults, following partial resection of kidney.9 Recently, the work of Diep et al. revealed neonephrogenesis in adult zebrafish.10 In contrast to lower vertebrates, mammals have a very limited regenerative ability in the kidney. The postnatal mammalian kidney responds to partial resection by regenerative hypertrophy without neonephrogenesis.8 However, new nephron formation can be induced in mammals during prenatal period by partial nephrectomy. For example, there is a 45% increase in the nephron number in the contralateral kidney following unilateral nephrectomy at day 100 (term 150 d) of gestation in an unborn sheep.11
The role of stem cells is better defined in organs with high cell turnover, such as bone marrow, intestine and skin. However, their role in organs with low cell turnover, such as kidney, is ill defined. Recent literature has demonstrated the the capacity of the kidney to regenerate. Given the regenerative ability of the adult kidney, the presence of stem cells in the embryonic kidney, neonephrogenesis during prenatal life in vertebrates and pre- and postnatal life in invertebrates, it is likely that the adult mammalian kidney has stem cells that are cellular sources of kidney regeneration.