Mesonephric kidney--a stem cell factory?

Mesonephros is a vestige, transient renal organ that functions only during embryonic development. The anatomy, position and even cellular fate of the mesonephric kidney varies drastically among mammalian species. The origin of mesonephros from intermediate mesoderm and the dependence of its differentiation on the nephric or Wolffian duct have been well established. Commonly accepted is also the mesonephric origin of epididymal ducts of the male reproductive tract. Recently, upon the more profound understanding of the molecular mechanisms involved in the development of the permanent mammalian kidney, some light has been shed over the molecular events taking place during the mesonephric development as well. Because of the functional and structural similarities between the mesonephric and metanephric kidneys, it is not surprising that many molecules regulating metanephric development are also activated during mesonephric development. However, the multifunctional nature of mesonephros has been unexpected. First, it serves as an embryonic secretory organ, in some mammalian species more so than in others. It is thereafter removed by programmed cell death. Second, it is a source of multiple stem cells including somatic cells in the male gonad, vascular endothelial cells, and hematopoietic stem cells. Thus, mesonephros is a challenging model for studies on epithelial differentiation and organogenesis, regulation of apoptosis, sex determination and stem cell differentiation. In this review, we focus in the molecular and stem cell aspects in the differentiation of the mammalian mesonephros.