Epithelial Framework Reorganization during Human Thymus Involution

Abstract

Background: The thymus undergoes age-related (physiological) involution in the course of normal ontogenetic development. In addition to this chronic involution, the thymus can also undergo an acute (age-independent) regression, defined as spontaneous, transient involution. This process is induced by either exogenous or endogenous factors, including some infections (infection-type involution). Objective: The purpose of the present work was to undertake a comparative study of the epithelial framework organization and cytokeratin immunoreactivity of human thymic epithelial cells during age-related and infection-induced involution. Methods: Routine methods for light and transmission electron microscopy, as well as indirect immunoperoxidase staining and immunogold electron microscopy, were applied. Results: The epithelial thymocyte microenvironment was of a generally similar cellular composition in both chronically and acutely involuted thymus. Structurally, aged thymus glands and infection-affected thymus glands displayed a large mass of adipose tissue containing scattered islands composed of epithelial cells, lymphocytes and reticular connective tissue. Correlation between thymus involution and regional peculiarities in the presence and distribution of cytokeratin-immunopositive cells and their intermediate filaments was investigated. Conclusion: The epithelial framework of the thymus undergoes reorganization during both age-related and infection-induced thymus involution. The involutionary processes demonstrated essential regional and intracellular (structural and immunocytochemical) differences. The epithelial cell rearrangement and cytokeratin modulation that we observed might be involved in thymic microenvironment plasticity and reorganization in the course of these processes.