Septation and differentiation of the embryonic human cloaca

Background/Purpose: 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.