Phenotypic conversions in renal development.

The transporting epithelia of the kidney are derived from an embryonic rudiment containing two distinct cell populations: ureteric bud epithelia and mesenchymal cells of the metanephric blastema. The ureteric bud is a caudal outgrowth of the Wolffian Duct and gives rise to the renal collecting system by branching morphogenesis. The metanephric blastema gives rise to diverse cells of the nephron after receiving an inductive stimulus. It has been proposed that mesenchymal progenitors of the metanephric blastema derive directly from intermediate mesoderm, although this hypothesis has never been tested directly. Utilizing direct lineage analysis techniques we demonstrate, in an organ culture system, that mesenchymal nephron progenitors are immediate descendants of ureteric bud epithelia. Ureteric bud epithelia can give rise to mesenchymal nephron progenitors that populate the metanephric blastema by undergoing an epithelial-to-mesenchymal transition followed by delamination. If this process occurs in vivo, renal morphogenesis can be characterized by two phenotypic conversions: an epithelial-to-mesenchymal transition leading to the generation of mesenchymal-nephron progenitors, followed by a mesenchymal-to-epithelial transition leading to the generation of diverse nephron epithelial cell types. We have immortalized an embryonic renal mesenchymal cell line and demonstrate that the clonal cell line, RSTEM-1, undergoes phenotypic conversions in vitro, providing a suitable model to study the regulation of the epithelial phenotype.