46 VITRIFICATION OF IMMATURE AND MATURE BOVINE OOCYTES

Abstract

While vitrification has become a valuable system used in oocyte and embryo preservation, there is still much to be learned in optimizing this protocol. Both mature and immature oocytes can be vitrified but each presents challenging aspects. Mature oocytes have microfilaments that are not yet developed in immature oocytes, which are fragile and may be disrupted by ice crystal formation during freezing. Further, currently many different cryoprotectants are used in different concentrations, most being combinations of dimethyl sulfoxide (DMSO), glycerol, and ethylene glycol. This study aimed to determine if vitrification solutions composed of ethylene glycol and either dimethyl sulfoxide or glycerol resulted in more-competent post-thaw oocytes, and to determine if maturation stage affected optimal vitrification solution. As validation of the IVF protocol, fresh mature oocytes from a commercial source were fertilized and proportion, with pronuclei formation 48 h post-IVF was recorded. Two experiments evaluated 2 cryoprotectant solutions by analysing post-vitrification and thaw competence of in vitro-fertilized oocytes to form pronuclei. Oocytes in both studies were exposed to 2 sequential vitrification solutions containing 10% DMSO or glycerol, 10% ethylene glycol and 0.5 M sucrose, and then 20% DMSO/glycerol and ethylene glycol and 0.5 M sucrose, before vitrification on cryolocks. In the first study, immature bovine oocytes (n = 200) were vitrified. Following thawing and IVM, they were analysed for pronuclei formation, with 8.49% and 0% fertilization following vitrification in DMSO and glycerol, respectively (P < 0.01). In the second study, mature oocytes were vitrified (n = 200), thawed, and fertilized using the same methods as in study 1. In total, 12.62% and 3.4% of the mature oocytes were successfully fertilized following vitrification in DMSO and glycerol, respectively (P < 0.05). Fisher’s exact test was used for all statistics in both studies. These results suggest that DMSO in combination with ethylene glycol may be superior to glycerol for vitrification of both immature and mature bovine oocytes.