Does the urorectal septum fuse with the cloacal membrane?

Purpose: Traditional theories of cloacal embryogenesis assume that the uror ectal septum fuses with the cloacal membrane before the anal membrane disin tegrates. However, recent observations in humans and other species raise do ubt about this assumption. We determined whether urorectal septum fusion oc curs in rats. Materials and Methods: Rat embryos were harvested at specific times between days 11 and 16 of gestation. We evaluated the morphology, growth and relat ionship of the urorectal septum to the cloacal membrane on serial histologi cal sections. Results: The urorectal septum consistently fused with the cloacal membrane on day 15 of gestation before the cloacal membrane began to disintegrate. Conclusions: In rats the urorectal septum fuses with the cloacal membrane, after which the urogenital membrane and anal membrane disintegrate by a pro cess of apoptosis.