Histological features of the lining of the human anorectum at some stages of embryonic development

Modern studies on the histological structure of the rectum focus on anatomical shaping processes, characteristics in an adult normally and in various pathological conditions and there is no information about the histological features of the epithelial lining of the anorectum in the first half of human embryogenesis. Meanwhile, according to the observations of clinicians and pathologists, anatomical areas where there are areas of direct contact of genetically and morphologically different epithelium are often malignized, therefore a detailed study of their embryonic development is required in order to establish possible histogenetic prerequisites for this pathology. The aim of the work is to characterize the features of the epithelial lining of the human anorectum at some stages of embryonic development. The work was performed on histological preparations of 7/8 and 17/18-week-old human embryos from the archive of the Department of Histology with the course of Embryology of the Military Medical Academy named after S.M. Kirov, stained with hematoxylin and eosin, amidoceric 10 B, according to the Shabadash method and the Felgen method. The karyometric characteristics of the anorectal canal epithelial cells were measured. In our work, we presented for the first time the characteristics of the multilayer epithelium of anal invagination in a human embryo of 7/8 weeks of development. For the first time, histological features of the lining of the intermediate part of the human anorectum were established at week 17/18 of embryonic development, in the zone of formation of anal sinuses, clearly marked areas corresponding to three morphological varieties of epithelium were identified. For the first time, a two-layered multi-row urothelium-like epithelium was revealed between a multilayer flat non maturing epithelium of the ectodermal type and a single-layer columnar epithelium of the endodermal type. We assume that this epithelium is a site of an evolutionary formed mechanism for dividing the anorectal canal into three part (proctodeum, urodeum, coprodeum) in animals and carries the function of a temporary formation. The obtained data can help clarify the mechanisms of formation of malformations of the anal region of the rectum, search for additional criteria for microscopic diagnosis of its reactive changes, and can also serve as an addition to the theory of critical periods of embryogenesis.