41 THE EFFECT OF ULTRARAPID VITRIFICATION FOR SHEEP OOCYTE VIABILITY

Abstract

Advances in reproduction technologies, such as in vitro maturation, IVF, and in vitro culture, have stimulated research for efficient cryopreservation techniques for mammalian oocytes. It is well known that the oocyte is the largest cell of an animal’s body and as such, is full of water and, in many species, fat, making it difficult to cryopreserve. The objective of this work was to study the effect of vitrification for cryopreservation of the metaphase II plate (MPII) of sheep oocytes. In our experiment, we used the Vit-Master™ (MTG, Bruckberg, Germany). Ovaries from 19 ewes of Kazakh Arkharo-Merino breed were acquired after slaughter and maintained at 37°C in TCM-199. The maturation medium was TCM-199, containing 1 mM of glutamine, 10% fetal bovine serum, 5 μg mL⁻¹ FSH, 5 μg mL⁻¹ LH, 1 μg mL⁻¹ oestradiol, 0.3 mM sodium pyruvate, and 100 mM cysteamine. The oocytes were incubated in 400 μL of medium in 4-well dishes covered with mineral oil. The IVM conditions were 5% CO₂ in humidified air at 39°C for 24 h. Then, oocytes were placed for 10 min in medium with Hoechst 33342 (3 μg mL⁻¹) and cytochalasin B (7 μg mL⁻¹) to facilitate enucleation of the MPII with a minimum volume of ooplasm. The MPII plates were divided into 2 groups: the vitrification group was exposed to vitrification media containing 1.12 M ethylene glycol (ET) + 0.87 M ME₂SO for 5 min and was exposed in vitrification media containing 2.24 M ET + 1.75 M ME₂SO for 5 min, and then in vitrification solution containing 4.48 M ET + 40% ME₂SO + 0.25 M sucrose for 30 s. Oocytes were loaded into a cryoloop and using negative pressure of liquid nitrogen in the chamber for freezing with the VIT-Master. Oocytes were thawed in a 25°C water bath and then placed in TCM-199 at 20% fetal bovine serum. After 15 min of incubation, the oocytes were activated for extrusion of the second polar body in 1 mg mL⁻¹ Ca ionophore for 5 min and washed for 5 min followed by 4 h in 6-DMAP (0.12 mM) + cycloheximide (0.6 μg mL⁻¹). After activation, the MPII were washed and cultured for 20 h. The control group received the same treatment but were not vitrified. Differences between the experimental groups were tested using Chi-squared test. Our research showed that expulsion of the second polar body after activation was observed in more than 59.7% of the MPII that were not vitrified (control group), whereas 37.7% of vitrified plates had expulsion of polar bodies (P < 0.05). These preliminary studies showed that it is possible to vitrify MPII plates. On the other hand, the drastic reduction of the volume of the sheep oocytes might make cryopreservation possible with greater efficiency.