Toxicity of TCDD in European flounder (Platichthys flesus) with emphasis on histopathology and cytochrome P450 1A induction in several organ systems.

The present study is part of a series of experiments, set up to elucidate the impact of aquatic pollution on fish health in the marine and estuarine environment. In the Dutch coastal and estuarine waters, European flounder (Platichthys flesus) showed a relatively high prevalence of (pre)neoplastic liver lesions and lymphocystis virus disease. The hypothesis of a causal relationship between pollution and these diseases was supported by semi-field experiments. Therefore a series of laboratory experiments was performed to further substantiate causality and identify the xenobiotics that may play a major role in the field. Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are important environmental pollutants. They are highly persistent, highly lipophilic, and have shown to induce several toxic effects in mammalian and non-mammalian species at relatively low concentrations. This report describes a study in which European flounder were orally exposed to the most toxic PCDD congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or to harbor sludge extract under controlled laboratory conditions. The effects on several organs (liver, gills, gastro-intestinal tract, thyroid gland, gonads, spleen and mesonephros) were examined microscopically. Induction and localization of cytochrome P4501A (CYP1A) immunoreactivity, and effects on hepatocyte-proliferation were visualized immunohistochemically. Effects on thymus size were examined by morphometric analysis. Oral exposure of flounder to 0.0125 or 0.3125 µg TCDD/kg bw, or to 0.3125 µg TEQ/kg bw of a harbor sludge extract, weekly for 8 weeks, induced a significant increase in CYP1A immunoreactivity in hepatocytes. Single administration of higher doses (20, 100 and 500 µg/kg bw) of TCDD also induced a significant increase CYP1A immunoreactivity in the endothelium in all organs examined, and in the epithelium of the digestive tract, liver, and mesonephros. Remarkably, strong immunoreactivity was noted in a distinct cell population of the hematopoietic tissue in the mesonephros and spleen, which has not been described in fish previously. Moreover, oral exposure to 20 µgTCDD/kg bw resulted in an increased mitotic activity, and an increased hepatosomatic index was found after exposure to 500 µgTCDD/kg bw. In the thymus only a trend in size reduction was noted, again in the highest dose group. Nevertheless, no marked pathology was detected even in fish exposed to a single dose of 500 µg TCDD/kg body weight. The present experiments show that, under the actual experimental conditions, European flounder is relatively insensitive to the toxic effects of TCDD. However, we assume that exposure to TCDD (and related substances) may promote the development of tumors in the field.