Phylogenetic aspects of cloacal epithelium development in vertebrates

BACKGROUND: The study of embryonic tissue and organ development from the perspective of comparative evolutionary histology enables the tracing of gradual changes in histogenetic processes and their continuity among vertebrate species. Data on the phylogenetic aspects of cloacal epithelium development remain fragmentary. Data on the histological structure of the epithelial lining of the anorectal canal (ARC) and cloacal subdivisions in embryogenesis across vertebrates (amphibians, birds, mammals) and humans are scarce in the scientific literature, with only a few published studies available. Research in this area is of particular relevance as it contributes to refining the theory of critical periods of embryogenesis and understanding the mechanisms of congenital malformations in humans. AIM: To provide a comparative morphological characterization of the epithelial lining of the cloaca in cloacal vertebrates and the epithelial lining of the anorectal part of the digestive tract in rat and human embryos, based on the concept of heteromorphy. METHODS: A single-center, observational, retrospective, uncontrolled study was conducted. The study material included the caudal region of the ARC in cloacal vertebrates (adult frogs, 13-day-old chicken embryos) and mammals (rat embryos at 9, 15, and 18 days of development). Additionally, histological specimens of human embryos (7–8 weeks of intrauterine development) were analyzed. Samples were fixed, dehydrated in alcohol, embedded in paraffin, and sectioned for Hematoxylin and Eosin staining, as well as Feulgen staining. Statistical analysis was performed, including calculations of the mean (M) and standard error of the mean (m). RESULTS: The study revealed that different parts of the cloaca in vertebrates exhibit morphometric differences in their epithelial lining. The urinary part of the cloaca in vertebrates with a cloaca and chicken embryos is characterized by a heteromorphic structure. In mammalian embryogenesis (rat embryos), the formation of the cloacal membrane involves interacting histological systems derived from various embryonic primordia, including the peridermal epithelium, the allantoic epithelium adjacent to the periderm, and the gut-type epithelium of the hindgut. In human antenatal development, ARC formation is marked by the emergence of interaction zones between epithelial layers, which differ in structure and tinctorial properties. CONCLUSION: Comparative analysis indicates that the embryogenesis of vertebrates, particularly in rats and humans, exhibits specific similarities in the composition of the epithelial lining of the ARC and the cloacal epithelium of amphibians and birds. The bilayered urothelium-like epithelium serves as a transient structure during embryonic development.