COMPARATIVE MORPHOLOGICAL STUDY OF QUANTITATIVE RATIOS OF LYMPHOCYTES AT DIFFERENT MATURITY STAGES IN THYMUS OF IMMATURE VERTEBRATES

The main function of thymus, a central organ of lymphoid system, is to form a pool of immunocompetent autotolerant thymocytes. The process of lymphocyte differentiation within thymus largely depends on organization level of the vertebral animal and its environment (habitat). So far, variability of morphological parameters of lymphoid component in thymus have been poorly studied for distinct representatives of various terrestrial vertebrate, due to the lack of comparative morphological studies. Therefore, the main purpose of our work was to identify this kind of dependences. In this study, on the example of prepubertal representatives belonging to four classes of vertebrate animals (Chordata, Vertebrata) including humans, we conducted a comparative morphological study of variability for main micromorphological parameters of lymphoid components in thymus gland. We have studied 212 preparations of thymus glands from 4 classes of vertebrate animals: Amphibia, Reptilia, Aves, Mammalia. The changes in total amount of thymocytes were studied for cortical and medullary substance of thymus gland. Percentage of thymocytes at different maturity stages, and mitotic thymocyte indexes were also studied.
On the basis of these data, a conclusion was made about differences in intensity of processes, connected with maturation and proliferation of lymphoid cells in thymus gland of distinct animals which are different in organization level and habitat could be determined by with different influence of adaptive changes which occurred in the course of evolution of vertebrates during. It was found that the morphological parameters reflecting the characteristics of the differentiation processes and maturation of lymphocytes in thymus are largely determined by adaptive changes that arose in the vertebrate evolution during the formation of true terrestrial and warm-bloodedness. Comparison of amphibians and reptiles, as well as cold-blooded beings with warm-blooded animals revealed differences in the number of thymocytes at different maturity levels in cortical and medullary zones, as well as differences in thymocyte division rates. In turn, comparison of mitotic index for the thymocytes of all vertebrates, within this morphological range, showed that intensity of thymocyte proliferation primarily depends on the energy capacity of the given organism. However, specific adaptations associated with evolution to a distinct certain living form (for example, adaptation of birds to flight) also exerts a significant effect upon rates of thymocyte division. Morphological features characteristic to the human thymus were determined, and possible causes of such differences were identified. It was found, that morphological parameters of the lymphoid component in human thymus are more similar to those of thymus from cold-blooded vertebrates which is already evident from early stages of ontogeny. We guess that this finding may be explained by exposure of humans to numerous unfavorable anthropogenic factors, which lead to some deviation of thymic morphology from the values, which are observed in mammals from the natural habitat. The obtained data are of interest for theoretical and practical biology and medicine.

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