Fig. 1. Reciprocal expression of plexin B1 and Sema4d during early stages of branching morphogenesis during kidney development. Boxed areas are magnified in adjacent panels in A-F. Images represent mRNA in situ hybridization (A-E) or lacZ-staining (F,G). (A,B) At E13.5, plexin B1 mRNA is expressed in ureteric tips (arrow in A) whereas Sema4d mRNA is expressed in the condensing mesenchyme (arrowhead in B). (C,D) At E15.5, plexin B1 begins to bt expressed in developing glomeruli (arrow in C) and Sema4d is found in mesenchymal epithelium-derived-S/comma-shaped tubular bodies (asterisk in D). (E) In situ mRNA hybridization for plexin B1 (blue-purple stain) and co-immunostaining with an antibody against WT1 (red) reveal expression of plexin B1 in ureteric tips complementary to WT1-stained condensing mesenchyme at E13.5. (F) Heterozygous reporter mice expressing lacZ via the mouse locus for plexin B1 (Plxnb1lacZ/+) reveal β-galactosidase-stained ureteric tips in the outer cortex of developing kidneys at E17.5 (arrows) in addition to tubular structures in the medulla. Kidneys from wild-type littermates do not show β-galactosidase staining (G). Scale bars: 10 µm in A-G.