Fig. 4. Disorganization of the urethral plate in Fgfr2-IIIb^-/- mutants. Transverse histological sections through the genital tubercle of wild-type (A,C) and Fgfr2-IIIb^-/- mutant (B,D) genitalia. Urethral epithelium is shown within the boxes and at higher magnification on right sides of A and B. (A) At E13.5, cells in the wild-type urethral plate are stratified and have a complex squamous organization. (B) Urethral plate of Fgfr2-IIIb^-/- mutant is a thin, unstratified plate of cuboidal cells. (C) Proximal transverse section through wild-type phallus at E15.5 showing separation of the bilaminar urethral plate to form a urethral tube. Boxed area indicates urethral epithelium. (D) Proximal transverse section through phallus of Fgfr2-IIIb^-/- mutant showing that by E15.5, epithelial organization of the urethra has been lost.