Histological features of the human anorectal lining at certain stages of embryonic development

BACKGROUND: Contemporary studies on the histological structure of the rectum mainly focus on the characteristics of its layers in adults under normal and pathological conditions and are largely devoted to anatomical and morphogenetic processes. However, there is a lack of information regarding the histological features of the epithelial lining of the anorectum during the first half of human embryogenesis. Meanwhile, according to observations by clinicians and pathologists, anatomical regions where genetically and morphologically distinct epithelia are in direct contact are often prone to malignant transformation. Therefore, a detailed study of their embryonic development is necessary to identify possible histogenetic prerequisites for such disorder.

AIM: This work aimed to characterize the histological features of the epithelial lining of the human anorectum at specific stages of embryonic development.

METHODS: We used histological specimens of human embryos at 7–8 and 17–18 weeks of gestation obtained from the archive of the Department of Histology with a Course in Embryology, Kirov Military Medical Academy. The sections were stained with hematoxylin and eosin, amidoblack 10B, by the Shabadash method, and by the Feulgen reaction. Karyometric analysis of epithelial cells of the anorectal canal was performed.

RESULTS: For the first time, the characteristics of the stratified epithelium of the anal invagination in a 7–8–week human embryo are presented. Histological features of the lining of the intermediate portion of the human anorectum at 17–18 weeks of embryonic development were identified for the first time: in the region of anal sinus formation, clearly demarcated areas corresponding to three morphologically distinct epithelial types were observed. Between the stratified nonkeratinized squamous epithelium of ectodermal origin and the simple columnar epithelium of endodermal origin, a previously undescribed bilayered, pseudostratified, urothelium-like epithelium was identified. We suggest that this epithelium represents a site of an evolutionarily developed mechanism responsible for the division of the anorectal canal into three segments (proctodeum, urodeum, and coprodeum) in animals and serves as a provisional structure.

CONCLUSION: The findings may contribute to understanding the mechanisms underlying the development of anorectal malformations, to identifying additional criteria for the microscopic diagnosis of reactive changes in the anorectal region, and may also further refine the concept of critical periods of embryogenesis.