Fig. 3. Expression of Fgf8 and Shh in the urethral epithelium of wild-type and Fgfr2-IIIb^-/- embryos. (A,B) Fgf8 expression in the distal urethral epithelium of male wild-type (A) and Fgfr2-IIIb^-/- (B) genital tubercles at E13.5. (C,D) Shh expression in the urethral epithelium of male wild-type (C) and Fgfr2-IIIb^-/- (D) genital tubercles. The Shh domain bifurcates proximally in the Fgfr2-IIIb^-/- mutant (D), where the urethral plate has opened. (E,F) Transverse sections through the distal genital tubercle of E15.5 wild-type embryo, showing histological organization (E) and Shh expression (F) in urethral epithelium (boxed). (G,H) Transverse sections through the distal genital tubercle of E15.5 Fgfr2-IIIb^-/- mutant embryo reveal the lack of a histologically distinct urethral epithelium (G) and the absence of Shh expression (H).