Alterations in the cortical thymic epithelium of rats after in vivo exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): an (immuno)histological study.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces thymic atrophy in rats. The present study was initiated to provide (immuno)histological data on the mechanism of action. Juvenile male Wistar rats were orally intubated once with 50 or 150 micrograms/kg TCDD. They were euthanized 4 or 10 days thereafter, or were allowed to stay alive until Day 20 or 26. Growth retardation occurred rapidly in all TCDD-treated animals. Lethality was demonstrated within 20-21 days after administration. At Days 4 and 10 after intubation, thymic atrophy was shown by reduction of thymic weight and cortex/medulla ratio. Staining patterns for T-cell markers in the atrophic thymuses coincided with the reduction of cortical areas. There was no evidence indicating that the effects were indirectly caused by stress. TCDD-induced thymic atrophy persisted until Day 26 after administration. Immunohistochemical analysis revealed prominent changes in the cortical thymic epithelium at the 150-micrograms/kg dose level. First, in the cortex epithelial cell aggregates were observed both at Day 4 and at Day 10 after administration. Apparently, the architecture of the epithelium had changed in these animals. Second, at 10 days after administration epithelial cells were found with the simultaneous expression of markers that in the normal uninvoluted thymus only occur either in the subcapsular/medullary area or in the cortex. This phenotype points to an unusual stage of differentiation. We conclude that TCDD exposure affects the cortical epithelium of the rat thymus at a high dose level. Apparently, it disturbs the epithelial network and interfers with the differentiation of epithelial cells.