Exposure to Estrone Impairs Oogenesis and Spawning in Zebrafish (Danio rerio)

Estrone (E1) is one of the major natural estrogens influencing the reproductive system of vertebrates. However, it contaminates aquatic environments due to runoff from livestock waste and the discharge of domestic sewage. The aim of this study was to investigate the impact of exposure to environmentally relevant concentrations of E1 on oogenesis and spawning in zebrafish (Danio rerio). Fish were exposed, in duplicate, to 20, 200, and 2000 ng/L of E1 for 49 days for oogenesis analysis evaluating histology, sex steroids, morphometry, cell proliferation, apoptosis, immunohistochemistry for insulin-like growth factor (IGF) and inducible nitric oxide synthetase (iNOS). 17β-estradiol (E2) and 11-ketotestosterone (11-KT) were assessed by ELISA assay. Nitric oxide production was evaluated indirectly by quantifying nitrite. Results revealed that E1 exposure altered the proportions of ovarian follicles and somatic components, with significant increase of oogonia, follicular atresia and inflammatory infiltrate, and decrease of follicular cells. Levels of E2, and immunoreaction for IGF1 increased in E120 and 200 ng/L groups, but 11-KT, IGF2 and IGF1R were not altered. Nitrite concentrations in ovaries were significantly elevated in E1 200 and 2000 ng/L groups, but iNOS immunoreaction was not altered. These changes led to reduced egg production in all groups exposed to E1 and significatively decreased fertilization rates at 200 and 2000 ng/L. Overall, the findings demonstrate that E1, even at concentrations commonly found in surface waters, has detrimental effects on ovarian development, gametogenesis and reproductive success in zebrafish. Monitoring environmental estrogens levels in aquatic environments is therefore essential for fish conservation.