Effects of vitrification of partially denuded bovine immature oocytes

In spite of many trials on bovine oocyte cryopreservation, their efficiency remains unsatisfactory, regardless of method used. Although a births of about 15 calves obtained after transfer of embryos developed from frozen or vitrified oocytes were reported by now, a total efficiency of oocyte cryopreservation, in terms of blastocyst rate obtained, reaches approximately a half of those reported for fresh oocytes. The authors' former research on mature bovine oocyte vitrification showed a possibility of obtaining a very high oocyte survival rate and development of up to 29.6% of blastocysts. In the present paper, reported and discussed are results of similar vitrification procedure, but employed for germinal vesicle stage (GV) of bovine oocytes. Immature cumulus-oocyte complexes (COCs) obtained from ovarian follicles were vortexed for short time to strip off a portion of cumulus cells. Vitrification was performed using microdroplet method in VS14 solution, after 12-15 min pre-equilibration and 30 or 45 s equilibration periods. Warming was accomplished in warm dilution medium, without sucrose. After in vitro maturation and fertilization, presumptive zygotes were cultured in vitro up to day 10. Depending on pre-equilibration parameters 5.3-8.4% blastocysts developed from vitrified oocytes, that is significantly less compared to blastocyst ratio obtained from control, fresh oocytes (40.9%, P <= 0.01, Fisher test). Although the ratio of blastocysts obtained from vitrified GV oocytes presented in this paper does not differ from the best results reported elsewhere, the autors suggest a need of further extensive research on bovine immature oocyte vitrification to obtain replicable, satisfactory efficiency.