Er they promote apoptosis [36]. Because Six12/2;Six2+/2 mutants displayed increased Bmp signaling (Fig. 8A , Q) and apoptosis (Fig. 6), we therefore examined the expression level of Dkk1 and Dkk2. At e13.5, Dkk1 transcripts were detected in mesenchymal cells lateral to the urethral plate, and expression of Dkk1 was slightly upregulated in the Six12/2;Six2+/2 mutants (Figs 8M ). Consistently, both Dkk1 and Dkk2 genes were significantly upregulated in the mutant genital tubercle at e11.5, based on a quantitative PCR analysis of micro-dissected tissues (Fig 8Q). Six1 is required for Fibroblast growth factor (Fgf8) expression order BI-78D3 during cardiac and craniofacial development [22]. Exogenous Fgf8 promotes genital tubercle outgrowth in organ cultures [37], and its expression in the distal urethral plate depends on both Shh and Wnt/?catenin signaling pathways [29,30,38,39]. However, conditional deletion of Fgf8 has no obvious genital tubercle defect [40]. On the other hand, a mutation in murine Fgf10 results in a 
hypospadias-like phenotype [41]. We detected reduced expression of Fgf8 in Six12/2;Six2+/2 mutants at e12.5 (Figs. 8I ), but increased expression of Fgf10 (Fig. 8Q), suggesting that downregulation of Fgf8 might be compensated by upregulation of Fgf10. Indeed, expression of dual specificity protein phosphatase 6 (Dusp6), which is downstream of the Fgf signaling pathway [11,40], was not affected (Fig. 8Q). Taken together, these candidate gene expression analyses suggest that deletions of both Six1 and Six2 disrupt dynamic expression patterns of several critical signal molecules required for normal development of urogenital structures.DiscussionOur findings uncover that PCM progenitors are the unexpected source of perineum and urogenital organs. We show for 
the first time that Six1 and 11967625 Six2 are asymmetric and complementarily expressed in the PCM progenitors, where they are required for proliferation and survival of these progenitors. These observations are suggestive that a process reminiscent to vascular occlusion underlies the partitioning of cloaca and remodeling of urogenital structures.Cloaca Septation and Urogenital DevelopmentAsymmetric growth of mesenchyme is the major driving force that transforms cloaca into urinary and digestive tracts (Fig. 9). Therefore, patterning of the cloacal mesoderm is a central issue of cloaca morphogenesis. Along the rostrocaudal axis, cloaca is surrounded by mesenchyme at the rostral ICM cells and lateral PCM cells but not the caudal cloacal membrane, which is devoid of mesenchyme (Fig. 9A). Thus, an intrinsic asymmetry is established because of the absence of mesenchyme in the cloacal membrane. A rapid increase in both PCM and ICM cells occludes the cloacal cavity and GNF-7 chemical information separates the hindgut (rectum and anal canal) and urogenital sinus (bladder and urethra). The process also pushes the cloacal duct, the remnant of cloaca, caudally towards the surface of the perineum. Consequently, independent digestive and urinary tracts are established, and the cloaca duct persists at the midline surface of perineum epithelium. Unlike the intrinsic asymmetry of rostrocaudal axis, cloaca is surrounded at all sides by the PCM progenitors along the dorsoventral axis (Fig. 9B and C). It is not immediately clear how asymmetric gene expression and growth along dorsoventral axes are established. An intriguing observation is the high levels of apoptosis at the dPCM and tail gut region (Fig. 6) [24,25]. ThisFigure 9. A working mod.Er they promote apoptosis [36]. Because Six12/2;Six2+/2 mutants displayed increased Bmp signaling (Fig. 8A , Q) and apoptosis (Fig. 6), we therefore examined the expression level of Dkk1 and Dkk2. At e13.5, Dkk1 transcripts were detected in mesenchymal cells lateral to the urethral plate, and expression of Dkk1 was slightly upregulated in the Six12/2;Six2+/2 mutants (Figs 8M ). Consistently, both Dkk1 and Dkk2 genes were significantly upregulated in the mutant genital tubercle at e11.5, based on a quantitative PCR analysis of micro-dissected tissues (Fig 8Q). Six1 is required for Fibroblast growth factor (Fgf8) expression during cardiac and craniofacial development [22]. Exogenous Fgf8 promotes genital tubercle outgrowth in organ cultures [37], and its expression in the distal urethral plate depends on both Shh and Wnt/?catenin signaling pathways [29,30,38,39]. However, conditional deletion of Fgf8 has no obvious genital tubercle defect [40]. On the other hand, a mutation in murine Fgf10 results in a hypospadias-like phenotype [41]. We detected reduced expression of Fgf8 in Six12/2;Six2+/2 mutants at e12.5 (Figs. 8I ), but increased expression of Fgf10 (Fig. 8Q), suggesting that downregulation of Fgf8 might be compensated by upregulation of Fgf10. Indeed, expression of dual specificity protein phosphatase 6 (Dusp6), which is downstream of the Fgf signaling pathway [11,40], was not affected (Fig. 8Q). Taken together, these candidate gene expression analyses suggest that deletions of both Six1 and Six2 disrupt dynamic expression patterns of several critical signal molecules required for normal development of urogenital structures.DiscussionOur findings uncover that PCM progenitors are the unexpected source of perineum and urogenital organs. We show for the first time that Six1 and 11967625 Six2 are asymmetric and complementarily expressed in the PCM progenitors, where they are required for proliferation and survival of these progenitors. These observations are suggestive that a process reminiscent to vascular occlusion underlies the partitioning of cloaca and remodeling of urogenital structures.Cloaca Septation and Urogenital DevelopmentAsymmetric growth of mesenchyme is the major driving force that transforms cloaca into urinary and digestive tracts (Fig. 9). Therefore, patterning of the cloacal mesoderm is a central issue of cloaca morphogenesis. Along the rostrocaudal axis, cloaca is surrounded by mesenchyme at the rostral ICM cells and lateral PCM cells but not the caudal cloacal membrane, which is devoid of mesenchyme (Fig. 9A). Thus, an intrinsic asymmetry is established because of the absence of mesenchyme in the cloacal membrane. A rapid increase in both PCM and ICM cells occludes the cloacal cavity and separates the hindgut (rectum and anal canal) and urogenital sinus (bladder and urethra). The process also pushes the cloacal duct, the remnant of cloaca, caudally towards the surface of the perineum. Consequently, independent digestive and urinary tracts are established, and the cloaca duct persists at the midline surface of perineum epithelium. Unlike the intrinsic asymmetry of rostrocaudal axis, cloaca is surrounded at all sides by the PCM progenitors along the dorsoventral axis (Fig. 9B and C). It is not immediately clear how asymmetric gene expression and growth along dorsoventral axes are established. An intriguing observation is the high levels of apoptosis at the dPCM and tail gut region (Fig. 6) [24,25]. ThisFigure 9. A working mod.
