Comparison between vitrification and slow freezing on the post-thaw development of bovine embryos produced in vitro

We evaluated the post-thaw development of in vitro-produced (IVP) bovine embryos cryopreserved using vitrification and slow-freezing techniques on different days after in vitro fertilization (IVF). Nine replicates of IVP were performed. Embryos at the expanded blastocyst stage with quality grades 1 and 2 (according to the International Embryo Technology Society (IETS) manual) were selected on days 7, 7.5, and 8 after IVF. Embryos (n = 472) were randomly divided and cryopreserved using slow freezing (n = 257) or vitrification (n = 215). The embryos were organized into six groups according to the cryopreservation technique and the day: 1) Group DT7 (embryos subjected to slow freezing on D7, n = 140); 2) Group DT7.5 (embryos subjected to slow freezing 12 hours after D7, on D7.5; n = 61); 3) Group DT8 (embryos subjected to slow freezing on D8, n = 56); 4) Group VIT7 (embryos vitrified on D7, n = 127); 5) Group VIT7.5 (embryos vitrified 12 hours after D7, on D7.5; n = 49); and 6) Group VIT8 (embryos vitrified on D8, n = 39). Data were arcsine-transformed and analyzed using analysis of variance and the GLIMMIX procedure in SAS (SAS 9.2), with P < 0.05. The re-expansion and embryo hatching rates were higher in vitrified embryos than in embryos subjected to slow freezing (P < 0.05). Embryo quality grade did not influence the total developmental rate (P > 0.05). However, grade 1 embryos re-expanded more rapidly (within 24 hours) than grade 2 embryos. Grade 1 embryos showed better results with vitrification than with slow freezing. The experimental groups represented a technique x day interaction and did not differ in terms of re-expansion and hatching rates (P > 0.05).