The development of the male genitourinary system. I The development of the male genitourinary system. I. The origin of the urorectal septum and the formation of the perineum  P.J Hynes, J.P Fraher  British Journal of Plastic Surgery  Volume 57, Issue 1, Pages 27-36 (January 2004) DOI: 10.1016/j.bjps.2003.08.019

Fig. 1 (a) Reconstructed mid-sagittal section of an E11 specimen (cloacal region) as viewed from a right lateral perspective. The body stalk (umbilicus) and tail regions are to the left and right, respectively. The ventral surface of the embryo faces upwards. (b,c) Light micrographs of transverse sections through the cloacal region of an E11 specimen. Their levels are indicated in (a). Sections were cut transversely in a caudocranial direction. As the cloaca is traced rostrally (b to c), its width increases. (d) Scanning electron micrograph (SEM) of an E11 embryo's infra-umbilical surface. In this, the plane of sectioning of E11 specimens is indicated. The right limb bud has been removed and the body stalk (upper left) and tail (lower right) have been transected where they approach the cloacal region. On the ventral surface of the cloaca, the cloacal folds bulge ventrally on either side of the median cloacal membrane, which forms the floor of the cloacal groove. Key to labels in Figures: ac, amniotic cavity; bs, body stalk; cd, cloacal duct; cf, cloacal fold; cg, cloacal groove; cm, cloacal membrane; dw, dorsal body wall; hg, hindgut; ic, intraembryonic coelom; iuw, infra-umbilical ventral wall; lb, limb bud; lcf, lateral cloacal fold; lw, lateral body wall; mc, membranous cloaca; md, mesonephric duct; pc, phallic cloaca; ps, preputial swelling; rc, rostral cloaca; rmgt, rostral mesoderm of genital tubercle; t, tail; tg, tail gut; ud, urogenital duct; up, urethral plate; urs, urorectal septum; us, urogenital sinus. Arrowheads indicate apoptosis. Note: in Figs. 1(a)–6(a), red, yellow and grey represent ectoderm, endoderm and mesoderm, respectively. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)

Fig. 2 (a) Reconstructed mid-sagittal section of a late E11 specimen. The URS is dividing the rostral cloaca (rc) and the urogenital sinus (us) ends blindly in the base of the body stalk (bs). The size of the tail gut (tg) is decreased. (b–d) Light micrographs of transverse sections of a late E11 specimen. Their levels of section are indicated in (a). (e) Enlargement of the URS, a section semiserial to (d) showing mitotic figures (arrows) in the lateral mesoderm and apoptotic bodies (arrowheads) in the endoderm lining the hindgut. The mesodermal cells appear to be streaming medially towards the URS. Key to labels are given in caption of Fig. 1. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)

Fig. 3 (a) Reconstructed mid-sagittal section of an E12 specimen. With ventral growth of the RMGT the genital tubercle begins to form and the membranous cloaca (mc) comes to extend ventrally along its caudal aspect as the phallic cloaca (pc). The tip of the URS has not yet reached the membranous cloaca. E12 and older specimens were sectioned in the frontal (coronal) plane so that the genital tubercle was sectioned transversely from distal to proximal. (b–d) Light micrographs of transverse sections of an E12 specimen. Their levels are indicated in (a). The phallic cloaca is broad (b) but the cloacal duct (cd) is narrow (c). Key to labels are given in caption of Fig. 1. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)

Fig. 4 (a) Reconstructed mid-sagittal section of a late E12 specimen. The tip of the URS has entered the membranous cloaca. (b,c) Light micrographs of transverse sections of a late E12 specimen. Their levels are indicated in (a). At this stage the cloacal duct is narrow. The tip of the urorectal septum is level with the cloacal folds. Key to labels are given in caption of Fig. 1. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)

Fig. 5 (a) Reconstructed mid-sagittal section of E13. The tip of the URS is closer to the dorsal part of the cloacal membrane (cm). (b) Light micrograph of a transverse section of an E13 specimen. Its level is indicated in (a). The cloacal duct is being displaced onto the surface and the dorsal part of the cloacal membrane is about to disrupt. Key to labels are given in caption of Fig. 1. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)

Fig. 6 (a) Reconstructed mid-sagittal section of an E14 specimen. The tip of the URS has reached the surface. (b) Light micrograph of a transverse section of an E14 specimen. Its level indicated in (a). The dorsal part of the cloacal membrane has disappeared and the cloacal duct remnant forms a shallow median groove on the surface of the URS. SEM of an E14 genital tubercle (caudal surface). The plane of sectioning of E12 and older specimens is indicated. Key to labels are given in caption of Fig. 1. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)

Fig. 7 The tail gut disappears at late E11. The lengths of the rostral cloaca and URS increase at a similar rate during E11 and E12. But as the URS extends beyond the rostral cloaca and into the membranous cloaca at late E12 its length comes to exceed that of the rostral cloaca. During the formation of the genital tubercle the length of the cloacal membrane increases. With the disruption of its dorsal part and the incorporation of its distal (ventral) part into the urethral plate at late E13 its length decreases thereafter. Key to labels are given in caption of Fig. 1. British Journal of Plastic Surgery 2004 57, 27-36DOI: (10.1016/j.bjps.2003.08.019)