Abstract
Anorectal malformations (ARMs) are congenital diseases of the rectum and anus in which the development of the anorectum is affected to varying degrees. The exact mechanism of anorectal development in the early embryonic period remains contradictory.
Two options are envisaged. According to the first, the cloaca, the basis of the future anorectum, is divided by the descent of the growing urorectal septum into the anterior urogenital section and the posterior anorectal section.
The urorectal septum also divides the cloacal membrane into anterior urogenital membrane and posterior anal membrane. Both membranes later disappear and an urogenital orific and aperture ani are formed.
The lower edge of the urorectal septum is the basis of the perineum. The mesenchyme abounds at the edges of the anal membrane and its edges rise in the ectoderm-covered anal lashes.
The middle part of this area deepens into the well, the proctodeum, and corresponds to the future anal opening. According to this theory, ARMs are explained as a disorder in the development of the urorectal septum, differentiation of the proctodeum and in the persistence of the anal membrane.
The second theory about the importance of the septum in the development of the anorectum doubts and points to the influence of the so-called cloacal disc, consisting of a multilayered solid epithelium and separating the cloac from the amniotic cavity. The cloacal disc is located dorsally from the base of the tail process and ventrally extends to the genital hump.
The urorectal septum is, in fact, part of the mesenchyme with two tubular formations that are the basis of the future anorectum and urogenital sinus. By regression of the cloacal disc, both outlets of the tubular bases of the urogenital sinus and the anorectum open.
At the same time, the mass of the mesenchyme between the anal orifice and the outlet of the urogenital sinus increases, and the outlets of the two devices move away from each other. The location of the urogenital tract depends on the position of the genital hump, the result of the anorectum on the regression of the dorsal part of the cloacal disc.
In ARM, the cloacal plate does not reach the base of the tail groove and is interrupted by the mesenchyme. The amount of mesenchyme that has replaced the cloacal plate epithelium probably determines the length of the atretic region.