Fig. 6. Ectopic application of Fgf8 rescues reduced GT proliferation in Hoxa13^GFP-homozygous mutants. (A) Expression of Fgf8 in the genital tubercle of E11.5 wild-type male embryos. Note that Fgf8 expression is seen in both dUPE and pUPE (black arrow) of E11.5 wild-type male embryos. Genital Shelf Mesenchyme, GSM. (B) Fgf8 expression is absent in the pUPE (arrow) of E 11.5 Hoxa13^GFP-homozygous mutants. (C) Section analysis of Fgf8 expression in the urethral plate epithelium (UPE) of E11.5 wild type male embryos. Note that Fgf8 expression is present in both the proximal (arrowhead) and distal UPE. (D) Fgf8 expression is restricted to the dUPE in E11.5 homozygous mutants. Note the complete loss of Fgf8 expression in the pUPE (arrowhead). (E,G) Implantation of heparin beads (arrow) treated with BSA into the UPE of E11.5 homozygous mutants had no effect on the reduced proliferation seen in the GSM. (F,H) Implantation of beads treated with 0.1 mg/ml Fgf8b (arrow) stimulated proliferation of the GSM in age-matched homozygous mutant embryos. (I,J) Typical levels of cell proliferation in the E11.5 GT of normally developing heterozygous male embryos. Arrow denotes normal thickening of the UPE. (K,L) Cell proliferation in the GT of a E11.5 Hoxa13^GFP homozygous male mutant. Arrow denotes the earliest detection of the abnormally thickened UPE. Note how Fgf8 applications alter cell proliferation in the mutant GT to resemble proliferation levels exhibited by heterozygous littermates (compare H with I), whereas mutant embryos treated with BSA maintain reduced levels of cell proliferation (compare G with K). Scale bars: 50 µm.