Fig. 1. (A) Stages of renal branching morphogenesis and nephron formation. Ureteric bud (UB) outgrowth from the Wolffian duct is induced by signals from the metanephric mesenchyme (MM) (A). (B,C) Invasion of the MM by the UB is followed by iterative branching of the UB and elongation of UB stalks. (D) At the tips of the branches, the epithelium induces the mesenchyme to form pre-tubular aggregates, which are stimulated to undergo mesenchymal to epithelial transformation (E,F) through the formation of comma-shaped (E) and S-shaped (F) bodies to form components of the nephron (G): renal tubules (proximal and distal) and the epithelial component of the glomerulus. (B) Nephron endowment is thought to be largely determined through branching of the UB. (Left) The UB adopts a strategy of lateral branching followed by bifurcation of a stem into two daughter branches (terminal bifid branching) to form the collecting system of the kidney. Nephrons are induced at UB tips but are also formed around the stem of elongating branches during the later branching iterations (arcades) and late-phase lateral branching. (Right) The segments of the collecting system proximal to the ureter (the renal pelvis and calyces) are formed from early branching segments of the UB that have dilated.