Fig. 5. Loss of urethral progenitors and arrest of cell division in Fgfr2-IIIb^-/- mutant genitalia. Transverse sections through phalli of wild-type (A,C,E) and Fgfr2-IIIb^-/- mutant (B,D,F) mice. Genitalia of male embryos are shown in A,B,E; genitalia of female embryos are shown in C,D,F. (A) Immunolocalization of keratin 14 (K14) at E16.5 reveals the presence of urethral epithelial progenitor cells in the basal layer of the wild-type urethra. (B) K14 staining is absent from the urethral region of the Fgfr2-IIIb^-/- mutant phallus. (C,D) At E13.5, the proliferation marker Ki67 is seen in the urethral plate of wild type (C) and Fgfr2-IIIb^-/- mutant (D) embryos. (E) Ki67 staining showing sustained proliferation of urethral epithelial cells in wild-type mouse at E15.5. (F) Ki67 staining is absent from the urethral region of Fgfr2-IIIb^-/- mouse. Boxes indicate position of urethral epithelium.