title: Septation and differentiation of the embryonic human cloaca.
abstract: OBJECTIVE
  Limitations in methodologies have fostered controversy regarding the septation of the human embryonic cloaca. The aim of this study was to evaluate the septation of the human embryonic cloaca.
  
  
  METHODS
  Using the Carnegie Embryological Collection and specimens at Johns Hopkins, Baltimore, MD, the authors studied 12 embryos and five fetuses. Embryo photomicrographs were reconstructed using three-dimensional modeling.
  
  
  RESULTS
  In Carnegie stage 13 the authors observed a cloaca, distinct primitive urogenital sinus, and anorectum separated by the urorectal septum. The primitive urogenital sinus and anorectum enter the cloaca separated from the amniotic space by the cloacal membrane. As the embryo becomes a fetus it lengthens, grows, expands and rotates through a process called transformation. Transformation gives rise to a loss of caudal curvature and a decrease in distance between the septum and membrane, but these structures do not fuse. Disintegration of the cloacal membrane produces openings for the urogenital sinus and anorectum.
  
  
  CONCLUSIONS
  The observations suggest that the urogenital sinus and anorectum form early and are separated by the urorectal septum as a passive structure. There does not appear to be septation or differentiation of the cloaca itself.
authors.affiliation: Department of Pathology, The Johns Hopkins Medical Institutions, Baltimore, MD, USA.
authors.forename: C N
authors.initials: CN
authors.lastname: Paidas
authors.forename: R F
authors.initials: RF
authors.lastname: Morreale
authors.forename: K M
authors.initials: KM
authors.lastname: Holoski
authors.forename: R E
authors.initials: RE
authors.lastname: Lund
authors.forename: G M
authors.initials: GM
authors.lastname: Hutchins
mesh_terms: D000482:Allantois / Q000196:embryology; D002454:Cell Differentiation; D002988:Cloaca / Q000196:embryology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D012007:Rectum / Q000196:embryology; D015017:Yolk Sac / Q000196:embryology