Fig. 10. A model for Hoxa13 function during development of the external genitalia. (A) Fgf8 and Bmp7 are co-expressed in the UPE at E11.5, whereas Hoxa13^GFP is expressed in the UPE as well as the GT mesenchyme. (B,D) Hoxa13 is essential for normal expression of Fgf8 and Bmp7 in the UPE, as well as for the mesenchymal repression of Nog. In combination, these signals control cell proliferation and PCD to establish normal formation of the meatus and urethral tube closure. (C,E) In the absence of Hoxa13 function, Fgf8 and Bmp7 expression is reduced in the UPE, whereas Nog expression is ectopic, which, in combination, causes decreased cell proliferation, reduced PCD and capillary vessel enlargement, as well as defects in urethral tube closure and meatus development. Asterisks indicate sites of programmed cell deaths. Red triangle in A indicates regions of colocalized Fgf8, Bmp7 and Hoxa13 expression. The developing meatus in D is indicated by the purple symbol.

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