Morphofunctional organization of the thymus of human fetuses in the late stages of the fetal period

The aim of the study was to identify the features of the morphofunctional organization of the thymus of human fetuses based on a quantitative assessment of subpopulations of T-lymphocytes with the CD4+ and CD8+ phenotype. Material and methods. The material for the study was the thymus of stillborn children (n=16) aged 35-40 weeks who died as a result of asphyxia, the cause of which was not established. All sectional material was carefully selected according to the anamnesis in order to exclude causes that could affect the structure of the thymus The study was carried out using standard methods of light microscopy, histology and immunohistochemistry. The following characteristics of the thymus were studied: corticomedullary and mitotic indices, the numerical density of thymocytes in the cortex and medulla, the relative area occupied by the blood-stream, fibrous connective, adipose, and lymphoid tissues on a section of the thymus lobes. The relative number of thymic bodies, as well as the quantitative ratio of thymocytes with the immunophenotypes of cytotoxic (CD8+) and helper cells (CD4+) were identified. Results. In the thymus of human fetuses, the values of the corticomedullary and mitotic indices were 4.8±0.63 and 2.69±0.36%, respectively. Young thymic corpuscles (TT I) quantitatively predominate over mature (TT II) and aging corpuscles (TT III), 1.5 and 3 times, respectively. A histological section of the thymus did not reveal fatty degeneration; the relative amount of fibrous connective tissue was 5.22 (3.8464; 5.6330)%. The cortical-cerebral boundary had clear outlines. T lymphocytes, together with cells of the epithelial microenvironment, occupied more than 90% of the area of the histological section. In the thymus cortex, a high numerical density of thymocytes was found (679.95±46.9), among which cells expressing the CD8+ coreceptor on the surface of the membrane predominated (72.64±5.05%). In the medulla, the number of T-lymphocytes with CD4+ and CD8+ phenotypes was the same (28.81±4.37% and 26.10±5.61%, respectively), the numerical density of thymocytes was 403.61±28.9 cells. Conclusion. The state of the thymus gland in human fetuses at 35–40 weeks of the fetal period indicates a high level of its functional activity. The absence of noticeable signs of accidental involution of the thymus indicates that asphyxia, not aggravated by infectious influences, and also not accompanied by developmental anomalies, due to its transience, does not have a significant effect on the morphology of the thymus and its functions. The quantitative ratio of thymocyte subpopulations with the CD8+ and CD4+ phenotype may indicate different rates of migration of these cells to the periphery, or indicate a different nature of the interaction of developing T lymphocytes with epithelial cells carrying MHC class I and II complexes.