Fig. 8. Model for the dual mode of Shh signaling in the developing GT. Red regions indicate endodermal regions shown as cross-sectioned circular regions (A, urogenital sinus at the outgrowth initiation period; B, urethral plate) and green lines represent ectoderm (A,B). Blue regions represent mesenchymal region and yellow portions for epithelia that normally express Shh (A, the epithelium of the outermost part of urogenital sinus at 10.5 dpc; B, the urethral plate epithelium emanating Shh at about 13 dpc). Altered expression of Fgf8 and Bmp4 are illustrated associated with the Shh mutation, which may account for the lack of GT outgrowth (A). Mesenchymal Fgf10 gene may be a candidate responding gene for the epithelial derived Shh (B).