Toxic effects of in vitro exposure to p-tert-octylphenol on bovine oocyte maturation and developmental competence

Alkylphenolic compounds are a widespread family of xenoestrogens. High concentrations of these substances are present in sewage sludge that is spread on arable land and pasture as fertilizer. Because of their known endocrine system-disrupting activity, alkylphenols represent a potential risk for the reproductive health of farm animals. In this study, the impact of p-tert-octylphenol (OP) on the developmental competence of bovine oocytes was evaluated. Endocrine activity of OP was investigated for its effect on estrogen receptors α and β (ERα and ERβ) and progesterone receptor (PR) mRNA levels. Cumulus-oocyte complexes (COCs) were exposed during in vitro maturation to serial concentrations of OP (1-0.0001 μg/ml) and were compared with vehicle-treated controls and a group of COCs treated with 17β-estradiol (E2). A dose-related decrease in the percentage of oocytes that completed maturation after 24 h and in oocyte fertilization competence was observed at doses of OP as low as 0.01 μg/ml. Groups treated with ≥0.001 μg/ml OP showed impaired embryo development. No adverse effects of E2 were observed. In the E2-treated COCs, ERα mRNA was decreased but PR mRNA was upregulated compared with controls. Treatment with 0.001 and 0.0001 μg/ml OP induced a decrease in ERα mRNA, but ERβ and PR mRNA were not affected. Treatment with 0.01 μg/ml OP did not produce changes in the expression of any of the mRNAs studied. OP impairs meiotic progression and developmental competence of bovine oocytes without demonstrating clear estrogen-mimic activity.