Structural Organization of the Thymus in 7-10-Month-Old Human Fetuses

Morpho- and immunogenesis result from the complex interaction of precursor cells of thymocytes and their immature forms with different structural components of the stroma which allows forming a microenvironment for T-lymphocytes. The response of the immune system to the antigenic effects depends on the morphofunctional maturity of the immune-competent organs. However, information on the features of the microscopic fetal structure of the thymus has not been systematized in the sources of scientific literature and this area needs further studying. The purpose of the study was to analyze the features of the thymus histotopography development in 7-10 month-old human fetuses. Material and methods. The study involved 18 series of histological sections of thymus of human fetuses with 231.0-375.0 mm of crown-rump length. Results and discussion. From the middle of the 7th month of intrauterine development (fetuses with 250.0-260.0 mm of crown-rump length) there is an increase in the area of the peripheral part of the thymus lobe- the cortical substance. The thymus lobes are well-formed, limited by stromal connective tissue with blood vessels. In the thymus there are large-sized particles with a large area of cerebrospinal fluid, with the connective tissue layers and with small vessels in their cortical substance. In the medullary substance of the thymus there are Hassall's corpuscles among which there are single large thymic corpuscles. When the Hassall's corpuscles are magnified enough you can clearly distinguish cells (nuclei and cytoplasm), some layers of cells (layered corpuscles). In the 8th month of intrauterine development (fetuses with 271.0 - 310.0 mm of crown-rump length), the structural organization of the thymus is similar to the previous term. In the fetuses with 320.0-330.0 mm of crown-rump length, the cortical substance of the thymus lobes is infiltrated by T-lymphocytes, which densely fill the lumens of the reticulate epithelial shell resulting in this part of the lobe having a characteristic appearance and dark color on the specimens. The brain substance of the thymus lobe has a lighter color, because it contains less lymphocytes compared with the cortical substance. In the brain substance of the thymus are the bodies of Hassalle. The medullary substance of the thymus is lighter in color as it contains fewer lymphocytes than the cortical substance. The medullary substance of the thymus contains Hassall's corpuscles. At the end of the fetal period of human ontogenesis, an increase in the area of the cortical substance of the thymus lobes is observed. In the thymus there are large-sized particles with a large area of medullary substance with layers of the connective tissue with small vessels in their cortical substance. The medullary substance of the thymus contains a moderate number of Hassall's corpuscles. Conclusions. In fetuses with 250.0-260.0 mm of crown-rump length there is an increase in the area of the thymus cortical substance, the medullary substance contains Hlassall's corpuscles, among which there are single large thymic corpuscles. In the fetuses with 320.0-330.0 mm of crown-rump length in the thymus lobes the cortical and medullary substances are clearly differentiated. Cortical substance of the thymus lobes is infiltrated by T-lymphocytes, which densely fill the lumens of the reticulate epithelial shell. Compared to the cortical substance, the medullary substance of the thymus lobe contains fewer lymphocytes. The medullary substance contains Hassall's corpuscles, many of which are found in large lobes of the thymus. In the fetuses with 365.0-375.0 mm of crown-rump length the thymus lobes are formed, they are limited by stromal connective tissue with vessels, the area of the cortical substance of the lobes increases. In the thymus there are large-sized particles with a large area of medullary substance, containing a moderate number of Hassall's corpuscles.