Fig. 7. Pre-patterning in the intermediate mesoderm directs pronephric kidney cell fate. (A) Expression patterns of wt1 (orange), pax2.1 (blue) and sim1 (brown) at the eight- to ten-somite stage. The notochord (N) and somites (S) are shown as axial references. Glomerular progenitors, bordered by yellow lines, lie in the anterior-most nephrogenic mesoderm and express wt1; tubule progenitors, bordered by green lines, are found caudally in a zone that expresses both wt1 and pax2.1; and duct cells, horizontal lines, develop from cells that express pax2.1 and sim1. (B) Diagram of the mature pronephric kidney (unilateral) indicating the final position of the segment. The midline glomerulus shown in yellow, the tubule drawn in green and the nephric duct diagrammed using horizontal lines. Anterior is at the top of the figure.