We investigated age-related changes in immunocytochemical localisation of cytokeratin 16 (CK16) in thymuses of female Wistar rats at various stages of adult life (months 1, 3, 6, 12). Within the 1st month of life, distribution of CK typical for individual subsets of thymic epithelial cells (TEC) was observed. The most numerous CK16+ TEC were observed in the outer region of medulla, in the outer cells of Hassall's corpuscles and in the superficial epithelial layer neighbouring the connective tissue of the capsule, septa and vessels of the thymus. In the 3rd month of life, increased intensity of CK16 reaction in superficial TEC was accompanied by increased numbers of CK16+ TEC in the outer region of the medulla. Age-related alterations in the distribution of the studied markers were evident beginning from the 6th month of life and involved increased expression of CK16 in the superficial layer of TEC, which at the interface with the septa formed stratified epithelium. In parallel, decreased numbers of CK16+ TEC were observed in the outer region of the medulla. Changes in CK16+ TEC distribution of a similar type developed in 12-month old rats and they probably reflected altered functions of some TEC populations and decreased or increased biological activity of other TEC populations.
