Comparative features of thymus morphology in human and vertebrate animals (<i>Chordata</i>, <i>Vertebrata</i>)

BACKGROUND: It is well known that the thymus structure in the vertebrate phylogeny is characterized by a combination of conservative and highly plastic features. However, the question remains about the causes and patterns of evolutionary similarities and differences in human and animal thymus structure depending on level of organization, habitat, and adaptability. AIM: The aim of the study was to identify the main patterns of change in the thymus microscopic structure in phylogeny by comparing the thymus structure in humans and various Chordata species. MATERIALS AND METHODS: Light microscopy was used to determine the cortical/medullary and mitotic indices as well as the area of fibrous tissue, lymphoid tissue, and adipose tissue in thymus sections from 19 vertebrate species and humans. Thymocytes, thymic corpuscles, and the number and area of microcirculatory vessels were counted per conventional area unit. The study was conducted on immature animals of each species as well as on animals that had reached the second stage of maturity. RESULTS: Comparative analysis shows that immature animals have predominantly similar thymus structures. Significant differences were observed in the parameters of age-related involution, which is characterized by significant magnitude and total fat degeneration in humans compared to animals. The morphological features of the thymus associated with thymocyte migration and maturation have the highest conservatism and include cortical/medullary and mitotic indices, the numerical density of thymocytes in the cortex, the total area of microcirculatory vessels, the relative area of lymphoid tissue. Human thymus, regardless of age, has a higher relative amount of fibrous tissue than vertebrates. In addition, some specific morphological features of the thymus corpuscles also vary. CONCLUSIONS: The structural features of the human thymus were determined that changed in adaptation to specific conditions of the anthropogenic environment. The revealed morphological differences in human thymus are consistent with immunological hypothesis explaining the causes of age-related thymic involution. They correspond to the main points of Academician A.A. Zavarzin’s theory of parallel development of homologous tissues in vertebrate phylogeny.