Clarification of the process of separation of the cloaca into rectum and urogenital sinus in the rat embryo

Background/Purpose: The normal process of division of the cloaca into a rec tum and urogenital tract is still not fully understood. The main controvers ies relate to how the urorectal septum (URS) divides the cloaca and whether the URS fuses with the cloacal membrane. This study used a 3-dimensional r econstruction technique, combined with histologic correlation, to observe t he developmental and septational processes of the cloaca of the normal rat embryo from gestational days 11 to 16. Methods: Normal rat embryos from gestational days 11 to 16 were sectioned s erially both transversely and sagittally and stained with H&E. 3-dimensiona l reconstructions were performed on embryos younger than day 13.5. The rele vant structures were examined in a temporo-spatial sequence. Results.-The tailgut started to regress by apoptosis on day 12 in a cranio- caudal direction. The URS, first evident in day-12.5 embryos, extended and fused with the cloacal membrane on day 15 of gestation, completing the sepa ration of the cloaca into rectum and bladder. Regression of the tailgut and ventral protrusion of the urogenital sinus markedly changed the configurat ion of the cloaca. The cloacal membrane did not break down until after it h ad fused: with the URS. Conclusions: These findings clarify the relative contributions made by acti ve septation of the cloaca by the URS and configurational changes of the cl oaca to produce a rectum and bladder. The URS fuses with the cloacal membra ne before the anal and urogenital membranes break down. J Pediatr Surg 35:1 810-1816. Copyright (C) 2000 by W.B. Saunders Company.