ULTRASTRUCTURE OF THE CORTICAL EPITHELIUM OF THE RAT THYMUS AFTER IN-VIVO EXPOSURE TO 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known for inducing corti cal atrophy in the rat thymus. The present study was conducted to prov ide ultrastructural evidence for the cortical epithelium to be a targe t for TCDD in vivo. Juvenile male Wistar rats were orally intubated on ce with either 50 or 150 mug/kg TCDD and killed 4 or 10 days thereafte r. Major changes were found in the cortical thymic epithelium. First, a relative shift occurred from ''pale'' to darker cortical epithelial cell types, as judged by their nuclear and cytoplasmic electron densit y. This effect was most prominent at 10 days after exposure to 150 mug /kg TCDD. The increased electron density of the cortical epithelium in dicates an altered state of cellular differentiation. Secondly, at the 150 mug/kg dose level focal epithelial cell aggregates were seen both at day 4 and day 10 after administration. This aggregation may either be compound induced or represent a secondary event to the collapse of the thymic stroma. Thirdly, increased vacuolation of cortical epithel ial cells was apparent. This effect is interpreted as a consequence ra ther than a cause of thymocyte depletion from the cortex. This study i ndicates that TCDD exposure affects the cortical epithelium of the rat thymus at a high dose level. Electron microscopy reveals that the dif ferentiation of epithelial cells is altered. In addition, epithelial c ell aggregates are formed.