Effects of endocrine-disrupting chemical exposure on zebrafish ovarian follicles

The process of oocyte maturation involves both nuclear and cytoplasmic events that are essential for the production of viable eggs and subsequent fertilization in sexually reproducing animals. In zebrafish, oocyte nuclear maturation (meiotic resumption) is marked by a transition from opaque to translucent ovarian follicles. This transition is called clearing and results from germinal vesicle breakdown (GVBD) and the transformation of vitellogenin into yolk proteins. Oocyte maturation can be perturbed by endocrine-disrupting chemicals (EDCs). These EDCs have been broadly shown to interfere with reproductive development. Many herbicides and pesticides used agriculturally act as EDCs. These chemicals can enter aquatic ecosystems via runoff and erosion. To evaluate the effects of EDCs on zebrafish oocyte maturation, zebrafish ovarian follicles were cultured and exposed to various concentrations of either atrazine, 2,4-D, or diazinon. Follicles were evaluated at 2, 3, and 4 hours for size and viability, but clearing was assessed only at 4 hours post-exposure. Atrazine and diazinon significantly reduced follicle clearing at the tested concentrations. No effect was observed with 2,4-D exposure. At low enough concentrations, none of the chemicals produced endocrine-disrupting effects. The results suggest that at or above environmentally relevant concentrations, some EDCs impair oocyte maturation in exposed zebrafish follicles. The results support the need to monitor EDC exposure to prevent harmful effects not only on aquatic organisms, but also on humans and wildlife that are also exposed to these contaminants