The age changes of the tissue structure and the cellular composition of the human thymus

With the use of histological, morphometric and immunohistochemically methods, it is established that the age transformations of the tissue structure of the thymus are characterized by the development of fibrous connective and adipose tissue in cortical partitions, which leads to fragmentation of the cortex, and then to the disjunction of fragments of the cortical substance and the marrow substance. The greatest progression of the processes of involution in the thymus is noted during the second mature age period from 36 to 60 years. In old and elderly people remain small islands of parenchyma without differentiation into cortical and brain matter, surrounded by fat tissue. In the process of age-related involution, the lymphocyte count decreases, their mitotic activity decreases, and the proportion of cells capable of proliferation decreases, both in cortical and in brain substance. This, apparently, is due to both a lack of inflow into the thymus of the marrow progenitors, and a decrease in the number of epithelial reticular cells creating a microenvironment for T-lymphocytes. Age changes are accompanied by a decrease in the number of thymic bodies, among which the share of mature forms increases. In the thymus of people of the senile age, thymic bodies are not found. The rate of processes of age involution is individual and varies widely, which is consistent with the life expectancy of people.