Bulk vitrification of in vitro produced bovine zygotes without reducing developmental competence to the blastocyst stage.

Cryopreservation of mammalian zygotes can be advantageous since it enables their flexile use in time and space for alternative purposes such as genome editing. Here we report a simple, quick and inexpensive vitrification protocol for in vitro produced bovine zygotes which enables their bulk preservation. Slaughterhouse-derived oocytes were subjected to in vitro maturation and fertilization (IVF). Ten h after IVF, cumulus-enclosed zygotes were equilibrated in 2% (v/v) ethylene glycol + 2% (v/v) propylene glycol for 13-15 min then vitrified in groups of 52-100 in 2 μL microdrops of 17.5% (v/v) ethylene glycol + 17.5% (v/v) propylene glycol supplemented with 0.3 M sucrose and 50 mg/mL polyvinylpyrrolidone. The presence of cumulus cells is important for the success of the process. Therefore, we applied a modified IVF protocol using a short (30 min) co-incubation interval which allowed zygote culture with attached cumulus cells until vitrification and even reduced polyspermy rates without affecting the total fertilization rate. Vitrified zygotes were similar to their non-vitrified counterparts in terms of survival, post-warming development to the blastocyst stage and blastocyst quality measured by cell numbers and cryo-survival. In conclusion, our vitrification protocol integrated with the modified IVF system enabled the quick cryopreservation of bovine zygotes in large groups without reducing their developmental competence to the blastocyst stage.