Fig. 1. Recombinant GREM1 protein is able to induce ectopic epithelial buds and restore branching in Grem1-deficient kidney primordia. Wild-type and Grem1-deficient kidney primordia expressing the Hoxb7-GFP transgene in their Wolffian duct and ureteric epithelium (Srinivas et al., 1999) were isolated from mouse embryos of 40-44 somites (E11.0-E11.25) and cultured for up to 96 hours. Panels show from left to right: cultures at 0 hours, 24 hours, 48 hours, 72 hours and 96 hours (time: ±2-3 hours). White asterisks indicate ureteric buds, red asterisks indicate ectopic epithelial buds and blue asterisks ectopic branches. The right-most panels are schematic tracings of the Wolffian duct and epithelial branching pattern at the developmental time points indicated (96 hours for Grem1 mutants; 72 hours for wild type). Wolffian duct and ureteric bud are shown in white; ectopic epithelial buds and/or outgrowth in yellow; first branch in red; second branch in blue; third branch in green; fourth branch in purple; fifth branch (A only) in orange. (A,B) Kidney primordia cultured in control medium. (A) Wild-type control. (B) Grem1-deficient metanephros; outgrowth and branching are blocked. (C-E) Grem1-deficient (C,D) and wild-type (E) kidney primordia cultured in medium supplemented with recombinant GREM1 (5 µg/ml). The Grem1-deficient metanephric kidney primordia shown in C and D are representative of the variability observed.