Successful pregnancy following blastocyst cryopreservation using super-cooling ultra-rapid vitrification

Background: Blastocysts were cryopreserved by a new two-step ultra-rapid cooling in super-cooled liquid nitrogen (-205°C). Methods: There were 308 mouse blastocysts collected from fertile B6CBF1 mice and 249 human blastocysts collected from 51 couples treated with IVF. The blastocysts were super-cooled by a Vit-Master™ and cryoloops after treatment in 50 and 100% vitrification solution (VS) for 2min and 30s, respectively. The 100% VS was composed of 20% ethylene glycol, 20% dimethylsulphoxide and 0.5 mol/l sucrose in human tubular fluid medium with 20% human serum albumin. The embryos were warmed after treatment in 0.25 and 0.125 mol/l sucrose for 2 and 3 min, respectively. The survival of embryos was observed after re-swell. Results: The survival rate (SR) and hatching rate (HR) of mouse blastocysts in the super-cooled, the cryosolution-treated and control groups were not significantly different (SR, 87, 95.5 and 100%; HR, 50, 33 and 44.6%, respectively; P > 0.05). After 96 super-cooled human blastocysts were warmed, 60 survival blastocysts were transferred into 13 patients. The successful SR and pregnancy rate (PR) for the super-cooled blastocyst group were 77.1% (74 out of 96) and 53.8% (seven out of 13). Conclusion: The ultra-rapid vitrification of blastocysts with a successful SR and PR could be used to replace classical slow cooling.