Fig. 1. Expression of wt1 in the developing kidney. wt1 first appears as bilateral stripes in the intermediate mesoderm at the two- to three-somite stage. (A) An eight-somite stage embryo showing wt1 expression in the anterior intermediate mesoderm. (B) Co-staining for alpha-tropomyosin indicates that the anterior border lies at the level of the first somite and extends caudally to the level of the fourth somite. (C) Two-color in situ hybridization with wt1 (orange) and pax2.1 (purple) labeling the intermediate mesoderm; the anterior-most cells express only wt1 (arrow). Posteriorly, we find a cell population that expresses both wt1 and pax2.1. (D) At the 11-somite stage, wt1 expression medial to the two original stripes is observed (arrow). (E,F) wt1 staining in pronephric cell aggregates ventral to the somites at the 23 somite stage in (E, arrow) and shown in cross-section in F. (G,H) Expression of wt1 is observed in the paired pronephric primordia at the 36 hpf stage (G), shown in cross-section in H. (I) wt1 (orange) localizes to the glomerulus while pax2.1 (purple) marks the pronephric tubule and anterior duct at the 48 hpf stage, a higher magnification of the same photograph is shown in J. (K) Cross-sections of a 48 hpf embryo with midline glomerular wt1 staining ventral to the dorsal aorta. (L) Co-staining with wt1 (orange) and pax2.1 (purple) at 76 hpf reveals that while wt1 still localizes to the glomerulus, pax2.1 expression is only observed in the tubules, a higher magnification of the same photograph is shown in M. (N) wt1-stained 76 hpf embryo shown in cross-section, note the absence of wt1 transcripts in the pronephric tubule and the restriction to the glomerulus. Anterior is towards the left in all panels except D, where it is at the top of the panel. (A,B,D,E,G) Dorsal views; (C) Lateral view; (I,J,L,M) Dorsolateral views. g, glomerulus; pp, pronephric primordia; pt, pronephric tubule. Arrowhead in E indicates where the wt1 stripes begin to fade.