Kidney development: The inductive interactions

The kidney derives from a set of inductive interactions between two tissues, a blastema of metanephric mesenchyme (MM) that arises from within the intermediate mesoderm, and the ureteric bud, a diverticulum off the nephric duct. The first of these interactions (probably long range) derives from the MM and the signal, almost certainly chemotactic and under the indirect control of WT1, causes the bud to sprout off the duct. The second (again probably long range) is from the bud to the MM and causes it to divide and enter a stem-cell phase rather than undergo apoptosis. The third is also from the bud and causes groups of stem cells to aggregate, epithelialize and form nephrons; this is likely to be a short-range interaction. The fourth is from the stem cells; it supplements the initial interaction underpinning bud growth and leads to bud bifurcation; together, these two interactions regulate the formation of the collecting duct system. There are almost certainly other inductive interactions involved in the later stages of nephrogenesis, but, in this review, we focus on the molecular basis of the four prime inductive steps and their downstream effects. (C) 1996 Academic Press Ltd