As the human embryo grows and caudal folding continues, the urorectal septum divides the cloaca into a ventral urogenital sinus and dorsal anorectal canal.
2.
Before the urorectal septum has an opportunity to fuse with the cloacal membrane, the membrane ruptures, exposing the urogenital sinus and dorsal anorectal canal to the exterior.
3.
Other entities associated with an imperforate anus are trisomies 18 and 21, the cat-eye syndrome ( partial trisomy or tetrasomy of a maternally derived number 22 chromosome ), Baller-Gerold syndrome, Currarino syndrome, caudal regression syndrome, FG syndrome, Johanson-Blizzard syndrome, McKusick-Kaufman syndrome, Pallister-Hall syndrome, short rib-polydactyly syndrome type 1, Townes-Brocks syndrome, 13q deletion syndrome, urorectal septum malformation sequence, and the OEIS complex ( omphalocele, exstrophy of the cloaca, imperforate anus, spinal defects ).
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