Today, the normal and abnormal development of the hindgut is still a matter of speculation. However, as the result of recent studies in appropriate animal models, most embryologic events that finally lead to abnormal hindgut development are better known than in the past: (1) the process of maldevelopment starts in early embryonic stages; (2) the cloacal membrane is always too short in its dorsal part, thus, the dorsal cloaca is missing; and (3) as a result, the hindgut remains attached to the sinus urogenitalis, forming the recto-urethral fistula. In the past, an impaired process of septation was believed to be the main cause of abnormal hindgut development. In contrast to this, our results indicate that the development of the septum is more passive than active. Furthermore, the results of our studies in normal and abnormal development indicate that (1) the embryonic cloaca never passes through a stage that is similar to any form of anorectal malformation in neonates, including the so-called "cloacas" in female embryos, and (2) to explain abnormal development, studies in abnormal embryos are mandatory.
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