Figure 1. Differentiation of a mouse metanephric nephron (according to Saxén, 1987). Grey, stem cells; red, pretubular condensates; brown, secretory nephrons; yellow, stroma; green, ureteric bud and its derivatives (at and after day 14, the branches are known as collecting ducts).

Figure 2. The circulus vitiosus (dotted circle) of the inductive events in nephrogenesis. Once the ureteric bud has formed, it gives an inductive signal to a cluster of nephrogenic mesenchyme cells, which then start to differentiate into epithelial cells. The induced condensates of the mesenchyme upregulate GDNF expression which, in turn, induces further branching of the ureteric tree. These reciprocal inductive events are repeated at the tips of the ureteric tree until the last mesenchymal cells have been induced to epithelialize (according to Sariola and Sainio, 1997). TGF1 and hepatocyte growth factor (HGF) may be involved in tubule elongation, and Wnt-4, and Wnt9B, in the epithelial differentiation of the nephrogenic mesenchyme.

Figure 3. Micrographs of developing E11 mouse metanephros cultivated in vitro. Note the bright, round areas in the mesenchyme; these represent the pretubular condensates that will become secretory nephrons. The E11 kidney rudiment is approximately 0.3 mm in the longest diameter.