Ultrastructure of different types of thymic epithelial cells in normal and cyclosporin-A-treated rats

Transmission electron microscopy was used to study the ultrastructural features of thymic epithelial cells of intact and cyclosporin-A-treated adult male Wistar rats. The animals received a peroral daily dose of 30 mg of cyclosporin A per kg body weight for 21 consecutive days. On the basis of ultrastructural features, seven subsets of epithelial cells were distinguished within the intact rat thymus. Four subsets were predominantly situated in the cortex (type 4 - "dark" cells - occasionally penetrated into the medullar region) and three were positioned within the thymic medulla. All subsets of thymic epithelial cells were markedly changed after the application of cyclosporin A. The prominent enlargement of cytokeratin contents was registered in type 1 - "subcapsular/paraseptal/perivascular" - epithelial cells. At the same time these cells acquired the morpho-functional aspect of resting cells. All other subsets of cortical epithelial cells were enlarged (though in different ways) in comparison to the corresponding subsets of the control thymus, whereby their subcellular organization was suggestive of increased synthetic and secretory activity. The decrease in the number of epithelial cells, with the prevalence of phenotypically immature subsets, was evident within the residual islands of thymic medullary tissue.