Comparison of the Efficacy of Conventional Slow Freezing and Rapid Cryopreservation Methods for Bovine Embryos

Day 7 bovine morulae and early blastocysts were randomly assigned to one of four cryopreservation methods: (i) a modified conventional controlled slow freezing and stepwise dilution after thawing; and three methods which enable direct transfer of the embryo into the recipient upon thawing: (ii) conventional controlled slow freezing and a modification of a one-step procedure, (iii) vitrification with 6.5 M glycerol plus 6% BSA (w/v), and (iv) vitrification with 25% glycerol (v/v) and 25% propanediol (v/v). In a comparative in vitro study, the percentage of grade 1 and 2 embryos developing into expanded blastocysts in culture for cryopreservation methods 1-4 were, respectively, 53% (29/55), 33% (20/61), 44% (26/59), and 51% (17/33). Method 2 yielded a significantly lower survival rate than methods 1 (P 0.1) when compared to method 1. Method 3 has considerable promise in providing a successful method for the cryopreservation of bovine embryos that (i) reduces the time required for equilibration and cooling, (ii) provides for simple and rapid one-step dilution of cryoprotectant after thawing, and (iii) enables more embryos to be thawed and transferred per unit time.