Cloacal membrane and imperforate anus

The gut tube, the tube that runs the length of the body, has a blind pouch (a dead-end sac) at the head end and another at the tail end. Membranes keep the pouches shut: the cloacal membrane at the tail end and, at the head end, its counterpart the buccopharyngeal membrane.

At the tail end, the hindgut (the last part of the gut tube) opens into a shared chamber called the cloaca, which also communicates with the developing genitourinary tract (the urinary and genital organs). The cloacal membrane is a thin sheet of tissue that closes the cloaca on the outside. The urorectal septum, a wall of tissue that grows downwards, divides the cloaca into a bowel part and a urogenital part. The membrane then breaks down (ruptures), so that the gut opens to the outside.

If the membrane does not rupture, it persists and the bowel has no outlet. The result is an imperforate anus (imperforate means not pierced through, so a simple memory aid is no hole). The newborn fails to pass meconium (the first stool), the abdomen swells, and surgical correction is needed. With a persisting membrane, the rectum above the block has formed normally and there is no fistula (an abnormal channel between two organs).

Anorectal agenesis, where the anus and rectum do not develop at all, is a different and more severe defect than a missing outlet. In anorectal malformations, a fistula is an abnormal connection between the bowel and a neighbouring passage such as the bladder, and arises from a defect in the septum that divides the cloaca. A persisting cloacal membrane is the classic explanation of an imperforate anus, although the exact developmental mechanism of anorectal malformations is still debated.