dPCM [11] while Six2 is enriched in vPCM (Fig. 1M ). Consistently, Six1-positive lineages are predominantly localized at the ventral side of the genital tubercle (Fig. 9) [11]. We have also shown that Six1 and Six2 coordinately control proliferation and survival of PCM progenitors, potentially through candidate signal molecules (Fig. 8), and that genetic deletion of Six1 and Six2 results in agenesis of the perineum and severe hypoplastic genitalia. These data suggest that patterning along the dorsoventral axis is required for completion of cloacal division, as well as outgrowth and patterning of the genital tubercle. Shh is expressed in the cloacal endoderm and is required for all stages of genitourinary tract development [30,38,39]. Shh signaling controls cell cycle kinetics of mesenchyme [42]. It is worth noting that Six6, a homology of Six1, is directly involved in modulating cell cycle of retinal progenitor [43]. Shh is maintained in Six1 and Six2 compound mutants (data not shown) and Eya1 mutant [11], raising a possibility that Shh maybe an upstream regulator. A key future question would be to understand intrinsic and 
extrinsic mechanism underlying the asymmetric growth and patterning of the cloacal mesenchyme. The proposed cloacal occlusion model is supported by the unexpected origin of the perineum discovered here and previously [10,11]. Seifert et al., reported previously that the midline epithelium of the perineum has an endodermal origin [10]. Of the various models put forth, the cloaca occlusion model best accounts for the observations of the shape (a narrow line) and asymmetric positioning (midline caudal surface) of the endoderm remnant (Fig. 9A and B). As illustrated in Figure 9A, occlusion of the cloaca results in displacement of the cloaca duct and formation of the perineum. On the other hand, the Rathke’s fold model predict that any surviving endodermal cells would be randomly distributed and embedded in the perineum stromal layer [1,2]. The Tourneux’s f.El: patterning of cloacal mesoderm leads 1516647 to occlusion of the cloaca and outgrowth of the genital tubercle. (A and B) Asymmetric growth and patterning along the rostrocaudal axis (A) and dorsoventral axis (B) causes occlusion and division of cloaca into urinary and digestive tracts. The process also displaces the cloacal duct (CD), remnant of the cloacal epithelium, to the surface of perineum as a thin epithelial lining. (C and D) Midline sagittal diagrams of genital tubercle at e11.5 (C) and e17.5 (D). Continuous growth of peri-cloacal mesenchyme leads to remodeling and opening of the anal canal and urethra, and of the digestive and urinary outlets, respectively. Peri-cloaca mesenchymal progenitors contribute to most, if not all, stromal tissues of genital tubercle and perineum. Asterisk, juxtaposition of ICM, dPCM and the cloacal membrane; A, anus; C, cloaca; CD, cloacal duct; CM, cloacal membrane; ICM, intro-cloacal mesenchyme; PCM; peri-cloacal mesenchyme; dPCM, dorsal PCM; vPCM, ventral PCM; Per, perineum; R, rectum; T, tail; TG, tail gut; U, urethra; UGS, urogenital sinus; UM, urethral meatus. doi:10.1371/journal.pone.0055587.glocalized cell death likely retards growth of the dPCM, thereby causing asymmetric growth along the dorsoventral axis and a ventral shift of the cloacal membrane, as proposed by van der Putte [6]. Asymmetric expression patterns of Six1 and Six2 suggest that PCM is indeed patterned along the dorsoventral axis, as Six1 is highly enriched in the dPCM [11] while Six2 is enriched in vPCM (Fig. 1M ). Consistently, Six1-positive lineages are predominantly localized at the ventral side of the genital tubercle (Fig. 9) [11]. We have also shown that Six1 and Six2 coordinately control proliferation and survival of PCM progenitors, potentially through candidate signal molecules (Fig. 8), and that genetic deletion of Six1 and Six2 results in agenesis of the perineum and severe hypoplastic genitalia. These data suggest that patterning along the dorsoventral axis is required for completion of cloacal division, as well as outgrowth and patterning of the genital tubercle. Shh is expressed in the cloacal endoderm and is required for all stages of genitourinary tract development [30,38,39]. Shh signaling controls cell cycle kinetics of mesenchyme [42]. It is worth noting that Six6, a homology of Six1, is directly involved in modulating cell cycle of retinal progenitor [43]. Shh is maintained in Six1 and Six2 compound mutants (data not shown) and Eya1 mutant [11], raising a possibility that Shh maybe an upstream regulator. A key future question would be to understand intrinsic and extrinsic mechanism underlying the asymmetric growth and patterning of the cloacal mesenchyme. The proposed cloacal occlusion model is supported by the unexpected origin of the perineum discovered here and previously [10,11]. Seifert et al., reported previously that the midline epithelium of the perineum has an endodermal origin [10]. Of the various models put forth, the cloaca occlusion model best accounts for the observations of the shape (a narrow line) and asymmetric positioning (midline caudal surface) of the endoderm remnant (Fig. 9A and B). As illustrated in Figure 9A, occlusion of the cloaca results in displacement of the cloaca duct and formation of the perineum. On the other hand, the Rathke’s fold model predict that any surviving endodermal cells would be randomly distributed and embedded in the perineum stromal layer [1,2]. The Tourneux’s f.