For some stem cells - haematopoietic progenitors, for example - culture systems in which their differentiation can be investigated are well established. Now, on p. 151, Osafune and colleagues describe a colony-forming assay that allows them to isolate renal progenitors from embryonic mouse kidneys for the first time. Their culture system uses a feeder layer of fibroblasts that stably express Wnt4, which is required for the epithelial differentiation of metanephric mesenchyme. Single metanephric mesenchymal cells that strongly express Sal1 (a zinc-finger nuclear factor essential for kidney development) seeded onto this feeder layer form colonies that contain several types of epithelial cells present in glomeruli and renal tubules. In addition, the researchers report that the planar cell polarity pathway acts downstream of Wnt4 to direct renal progenitor differentiation, a novel insight into the mechanisms underlying kidney development.