Dissecting stages of mesenchymal-to-epithelial conversion during kidney development

During embryonic development, the structures of the nephron from the glomerulus to distal tubule derive from the metanephric mesenchyme. The mesenchymal cells change their cell type and produce highly organized epithelia under the influence of signals from the ureteric bud. The morphological sequence of this conversion includes the formation of a corona of mesenchymal cells surrounding the tips of the ureteric bud, followed by the development of a pre-tubular aggregate, which evolves into preliminary forms of the segmented nephron. Currently, these stages are largely based on histomorphologic criteria and expression of marker molecules. However, to dissect the effects of inductive signals from the ureteric bud in more detail, a sophisticated readout of stages in the conversion process is required, based on the onset of epithelial polarity and the occurrence of vectorial transport. In this review, we discuss some of the new approaches in establishing the staging of the conversion process.