Phylogenetic aspects of the development of the epithelial lining of the vertebrate cloaca
Authors: Комарова Anastasiya , Odintsova Irina A 
Published: February 2025

BACKGROUND: the study of the embryonic development of tissues and organs from the perspective of comparative evolutionary histology allows us to assess the gradual change in the main histogenetic processes and the continuity of these changes in a number of vertebrates. Knowledge of the development of histological structures of vertebrates allows us to extrapolate this knowledge to the embryonic development of tissues and organs in humans. Information about the histological structure of the epithelial lining of the anatomical sections of the anorectal canal as well as the cutaneous, urinary and intestinal parts of the cloaca in embryogenesis in various representatives of vertebrates (amphibians, birds, mammals) and humans representatives in the scientific literature is sketchy. Research in this area is relevant, as it contributes to clarifying the theoretical and critical periods of embryogenesis and explaining the mechanisms of the formation of human malformations.
AIM: from the point of the doctrine of heteromorphism to present a comparative characteristic of the epithelial lining of the cloaca in cloacal vertebrates and the epithelial lining of the anorectal part of the digestive tube in rat and in human embryos.
METHODS: an observational single-center retrospective uncontrolled study was conducted. The study material was the caudal section of the anorectal canal in cloaca vertebrates (post-larval stage of the frog, 13 day chicken embryos), mammals (9 day, 15 day, 18 day rat embryos), histological preparations of 7–8 weeks of human embryos. After fixation the material was dehydrated in alcohols, poured into paraffin, prepared slices and stained with hematoxylin and eosin, and according to the Felgen method. The obtained data were subjected to statistical processing, the arithmetic mean (M) and standard error (m) were calculated in each measurement zone.
RESULTS: in the course of the study, it was found that the epithelial lining of the cloaca in cloaca vertebrates has its own morphometric structural features in different parts of it. The urinary part of the cloaca of cloaca vertebrates and chicken embryos is characterized by a heteromorphic structure. Interacting systems of histological elements from different embryonic rudiments participate in mammalian embryogenesis during the formation of the cloacal membrane in rat embryos: the peridermal epithelium, the allantois epithelium adjacent to the periderm and the intestinal epithelium of the posterior intestine. The embryonic development of the anorectal canal in humans is characterized by the formation of epithelial interaction zones that differ in structure and tinctorial properties of cells.
CONCLUSION: a comparative study of the embryonic development of vertebrates indicates that in rats and humans, the composition of the epithelial lining of the anorectal canal has certain similarities with the epithelial lining of the cloaca of amphibians and birds. Histogenetic feature of the lining consists in the fact that the two-layered urothelium-like epithelium in its composition is a temporary formation