ULTRASTRUCTURE OF DIFFERENT TYPES OF THYMIC EPITHELIAL-CELLS IN NORMAL AND CYCLOSPORINE-A-TREATED RATS

Transmission electron microscopy was used to study the ultrastructural features of thymic epithelial cells of intact and cyclosporin-A-treat ed adult male Wi star 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 we re predominantly situated in the cortex (type 4 - ''dark'' cells - occ asionally penetrated into the medullar region) and three were position ed within the thymic medulla. All subsets of thymic epithelial cells w ere markedly changed after the application of cyclosporin A. The promi nent 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 cell s. All other subsets of cortical epithelial cells were enlarged (thoug h 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 nu mber of epithelial cells, with the prevalence of phenotypically immatu re subsets, was evident within the residual islands of thymic medullar y tissue.