The cloaca epithelial layer heteromorphy in the grass frog Rana temporaria

In amphibians, the cloaca is formed during embryogenesis and functions throughout life. The cloaca as a temporary (provisional) formation also exists in early human embryogenesis, and its developmental disturbances lead to a number of congenital pathologies, one of which is persistent cloaca. Understanding the histogenetic causes of congenital anomalies can be achieved by taking into account phylogenetic studies. The starting point for them can be the identification of histogenetic features of the formation of the cloaca, its epithelial layer and derivatives using amphibians (Grass frog, Rana temporaria), birds, mammals and humans as an example. The purpose of the study is to characterize the epithelial layer of the cloaca in amphibians. The object of the study was the caudal section of the gastrointestinal tract of adult individuals of the species Rana temporaria, n=10. Morphometric analysis of epithelial cells of three parts of the cloaca - cutaneous, urinary and intestinal - was performed on paraffin sections stained with hematoxylin and eosin. The following karyometric parameters were measured: long and short axes of the nucleus, nuclear area, and nuclear elongation. As a result of morphometric analysis, it was established that the length of the frog cloaca parts varies. In the cutaneous part it is 893±82,2 μm, in the urinary part - 320±29,2 μm, in the intestinal part - 1092±110 μm. The epithelial layer of each of the three parts of the cloaca is characterized by cell heteromorphism. The skin part is represented by a multilayered mucous epithelium, which breaks off along the line of contact with the epithelium of the urinary part and then the layer continues with an epithelium of a different structure and morphometric parameters. In the area of contact with the intestinal epithelium, a clear demarcation line is also observed. The epithelial layer of each of the three parts of the cloaca is characterized not only by differences in morphology, but also by differences in the karyometric indices of the cells, which indicates different embryonic sources of development. Morphometric analysis of the cellular composition of the epithelial layer of different parts of the frog cloaca allows for differential diagnostics of different parts of the anorectal tract in the embryogenesis of higher vertebrates.