199 Nanoplastics are incorporated by the bovine cumulus–oocyte complex and form a potential treat for oocyte competence

The widespread presence of micro- and nanoplastics (MNPs) in the environment has raised increasing concerns. The detection of plastic particles in human blood implies that MNPs are circulating in our body and are able to reach organs, including the reproductive tract (Leslie et al. 2022 Environ. Int. 163, 107199). However, the impact of nanoplastics (NPs) on oocyte development in mammals remains largely unknown, with most studies focusing on aquatic animals. This research aimed to investigate the uptake and impact of NPs on oocyte development using a bovine cumulus–oocyte complex (COC) model. This model is superior to study human reproduction compared with other models, due to the large similarities between bovine and human reproduction during oocyte and embryo development (Sirard 2017 Animal Models and Human Reproduction, 127–144). To determine the uptake of NPs by the cumulus cells and oocyte, bovine COCs collected from slaughterhouse ovaries were in vivo matured (IVM) in medium (NaHCO₃-buffered M199 supplemented, with 100 IU mL⁻¹ penicillin-streptomycin, 0.05 IU mL⁻¹ FSH, 0.1 µM cysteamine, and 10 ng mL⁻¹ epidermal growth factor) containing 10 µg mL⁻¹ of 50 nm or 200 nm fluorescently labelled pristine polystyrene (PS) NPs spheres (CD Bioparticles; Polysciences Europe GmbH) for 23 h at 39°C and 5% CO₂ in air. The dose–response effect on oocyte maturation was studied by exposing COCs during IVM (n ≥ 5 runs) to 50 nm (1426 COCs in total) or 200 nm non-fluorescently labelled pristine PS NPs (1468 COCs in total) (Polysciences Europe GmbH) at the concentrations of 0, 0.3, 1, 3, 10, and 30 µg mL⁻¹. After IVM, the uptake and oocyte nuclear maturation stage, defined by the metaphase-II stage of meiosis, was examined via Olympus IXplore SpinSR. One-way analysis of variance followed by Holm-Sidak multiple comparisons test was used for statistical analysis. A P-value < 0.05 was considered statistically significant. Exposure of COCs to 50 nm and 200 nm PS-NPs during the 23 h maturation period resulted in the uptake of NPs in cumulus cells. Interestingly, only the smaller NPs of 50 nm were incorporated by both the cumulus cells and the oocyte. Exposure to 3 µg mL⁻¹ of the 50 nm NPs led to a significant decrease in oocyte nuclear maturation rate (66.1%) compared with the control group (81.0%, P = 0.01), whereas there was no effect on the nuclear maturation at the other concentrations or after exposure to 200 nm NPs during the 23-h maturation. In conclusion, NPs of 200 nm and 50 nm are taken up by cumulus cells and 50 nm NPs are able to enter oocytes, and affect nuclear maturation at a concentration of 3 µg mL⁻¹. The presence of increasing amounts of NPs in the environment stresses the necessity for further research on the potential impact of NP exposure on oocyte and embryo development.