Fig. 3. The expression of Bmp7, its target genes, Msx1 and Msx2, and its antagonist, Nog, are affected in Hoxa13^GFP-mutant embryos. (A) Expression of Bmp7 in the UPE and lateral shelf mesenchyme (LSM; black arrow) in E12.5 control embryos. Black bracket shows sites of gene expression, white portion of brackets show sites where gene expression is absent. White arrow denotes the thin layer of epidermis. (B) Absence of Bmp7 expression in pUPE of E12.5 male homozygous mutants, as well as in the later shelf mesenchyme (black arrow). Note the thickened epithelial layer (white arrow) in the distal genital tubercle. (C) Expression of Bmp7 in E14.5 male wild-type embryos. Note high levels of Bmp7 expression in the developing meatus (black arrow), as well as in the urethral epithelium which is covered by mesenchyme (black arrowhead) and the developing preputial gland (white arrowhead). (D) Malformation of the meatus is concomitant with reduced Bmp7 expression in E14.5 male homozygous mutants (black arrow). Note that Bmp7 expression is reduced in the preputial gland (white arrowhead) and absent in the epithelial chord (black arrowhead), which is not covered by mesenchyme. (E) Nog is normally expressed in the lateral shelf mesenchyme (LSM) of E11.5 wild-type embryos. Black arrowhead in E denotes position of ectopic Nog expression seen in the mesenchyme flanking pUPE in E11.5 male homozygous mutants (F). (G) In E12.5 control embryos, Nog is expressed at low levels in mesenchyme flanking the pUPE (black arrowhead); dUPE (black arrow). (H) E12.5 homozygous mutants exhibit strong ectopic Nog expression in the mesenchyme adjacent to the medial-proximal UPE (red arrowhead), as well as in a narrow band in the GT mesenchyme (black arrowhead). Nog expression was reduced in the mesenchyme flanking the dUPE in these same mutant embryos (black arrow). Inset (H) reflects Bmp4 expression in the mesenchyme flanking the medial-proximal UPE. (I) In E11.5 wild-type embryos, Msx1 expression is localized to the mesenchyme (M) flanking the entire UPE. Black bracket marks the rostral-caudal extent of Msx1 expression. (J) In E11.5 homozygous mutants, Msx1 expression is absent in the mesenchyme flanking the pUPE. Black portion of bracket represents Msx1 expression in the rostral mesenchyme. White portion of bracket represents caudal portion of mesenchyme lacking Msx1 expression. (K) In E12.5 embryos, Msx1 is predominantly expressed in the distal GT where the glans will eventually form (black arrow); in homozygous mutants (L) this is noticeably reduced in size (black arrow). (M) Msx2 is strongly expressed in the rostral mesenchyme, as well as in the UPE and sinus epithelium in E11.5 wild-type male embryos. Black bracket denotes Msx2 expression along the rostral-caudal axis of the genital tubercle. (N) Homozygous mutants also exhibit strong Msx2 expression in the UPE and mesenchyme flanking the dUPE (black portion of bracket); however, no expression could be detected in pUPE as denoted by the white portion of the bracket. (O) In E12.5 embryos, Msx2 is expressed in the developing glans mesenchyme (G) as well as in the UPE. Black bracket reflects continuous UPE expression of Msx2. (P) In E12.5 mutants, Msx2 expression is also seen in the dUPE (black portion of bracket) and glans (G), whereas no Msx2 expression is seen in the dUPE (white portion of the bracket).

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