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 cumulusoocyte 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 5.38.4% 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.