Septation of the anorectal and genitourinary tracts in the human embryo: crucial role of the catenoidal shape of the urorectal sulcus.
BACKGROUND
Previous studies of the tracheoesophageal sulcus and the sulci of the developing heart have suggested that the catenoidal or saddle-shaped configuration of the sulcus had mechanical properties that were important to developmental processes by causing regional growth limitation. We examined the development of the human perineal region to determine if a similar configuration exists in relation to the urorectal septum. We wished to re-examine the controversial issue of the role of the urorectal sulcus in the partitioning of the cloaca.


METHODS
Digitally scanned photomicrographs of serial histologic sections of embryos from Carnegie stages 13, 15, 18, and 22, obtained from the Carnegie Embryological Collection were used. Each image was digitally stacked, aligned, and isolated using image-editing software. Images were compiled using 3-D image-visualization software (T-Vox), into full 3-D voxel-based volume renderings. Similarly, digital models were made of the urogenital sinus, anorectum, cloaca, allantois, mesonephric ducts, ureters, and kidneys by isolating their associated epithelium in each histologic section and compiling the data in T-Vox. Methods were developed to create registration models for determining the exact position and orientation of outlined structures within the embryos.


RESULTS
Models were oriented and resectioned to determine the configuration of the urorectal sulcus. The results show that the urorectal sulcus maintains a catenoidal configuration during the developmental period studied and, thus, would be expected to limit caudal growth of the urorectal septum.


CONCLUSIONS
The observations support the concept that the urorectal septum is a passive structure that does not actively divide the cloaca into urogenital and anorectal components.
D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008953:Models, Anatomic; D008954:Models, Biological; D012007:Rectum / Q000196:embryology*; D013997:Time Factors; D014566:Urogenital System / Q000196:embryology*