Anal and ano-urogenital malformations: a histopathological study of "imperforate anus" with a reconstruction of the pathogenesis.

Histopathological information about "anorectal malformations" is scarce and the pathogenesis still controversial. Autopsy specimens of 20 human fetuses and newborns with "main" types of the disorder were studied histologically. Supplemented with surgical-anatomical data from the literature and with information from our own and earlier embryological research in animal models as well as from recent observations on the normal development of the human perineum, the study allowed for a new reconstruction of the pathogenesis of the disorder. The histological analysis of the malformations in human fetuses and newborns showed a ventralward deviation of the anal canal as the principal deformity. Ano-urogenital communications and differently structured ectopic anocutaneous canals issued from anywhere between the bladder and the vestibular/urethral orifice (female urethra excluded) and between the orifice and the usual site of the anus, respectively, or they ended blindly, but with a suggestion of lost communication. They occurred isolated or in association with other primary or secondary regional anomalies. Patho-embryological data from animal models revealed that the deviation was caused by defective development of the dorsal cloaca and not by disturbances in a series of fusion processes inside and outside the cloaca, as is currently believed. This mechanism fits well into recent adjustments of ideas about the normal development of the perineum. The cause of the defect is still obscure, but a malfunctioning of cells ingressing from an end-stage primitive streak that affects the dorsal side of the prospective cloaca appears most likely. The data collected permit a new reconstruction of the pathogenesis of anal and ano-urogenital malformations.