Vitrification of equine oocytes is an essential practice for advancing assisted reproductive technologies however, its efficiency remains limited due to the lack of stage and species-specific information on membrane permeability parameters. In this study, water (Lp) and CPA permeability (Ps) for dimethyl sulfoxide (Me₂SO) and ethylene glycol (EG) were measured in in vitro matured (MII) equine oocytes. Cumulus oocyte complexes were obtained from abattoir ovaries or by ovum pick-up and matured in vitro for 30 h at 6% CO2. Oocytes followed ideal osmometer behavior principles, with an osmotically inactive volume of 27%. Lp increased with temperature from 0.941 ± 0.082 µm min−1 atm−1 at 25 °C to 1.462 ± 0.084 µm min−1 atm−1 at 38.5 °C in Me₂SO, and from 0.889 ± 0.094 to 1.613 ± 0.066 µm min−1 atm−1 in EG. Ps also increased significantly with temperature: PsMe₂SO rose from 0.175 ± 0.024 µm s−1 to 0.353 ± 0.022 µm s−1 and PsEG from 0.138 ± 0.020 µm s−1 to 0.349 ± 0.014 µm s−1. Activation energies (Ea) for Lp were 6.03 and 8.15 kcal mol−1, and for Ps were 9.60 and 12.69 kcal mol-1 for Me₂SO and EG, respectively, measured at 25 °C and 38.5 °C. In silico predictions closely matched in vitro observations. Simulations predicted that oocytes recovered their original volume after 7 min 42 s at 38.5 °C and at 25 °C after 17 min 8 s. This study provides the first stage and species-specific permeability values for MII equine oocytes, supporting improved vitrification modeling.
Gago et al. (Fri,) studied this question.