Clinical validation of a closed vitrification system in an oocyte-donation programme

Controversy exists about the risk of microbiological contamination from direct contact with unsterile liquid nitrogen during oocyte vitrification. The aim of this observational study was to evaluate the effectiveness of oocyte vitrification using a high-security closed vitrification system in a donation programme. Oocyte vitrification was performed using CBS High Security closed straws (Cryo Bio System) with DMSO/ethylene glycol/sucrose as the cryoprotectant (Irvine Scientific freeze kit). A total of 123 vitrified metaphase-II oocytes were warmed in 20 recipient cycles (6.2 warmed oocytes per recipient); of these, 111 oocytes (90.2%) survived vitrification and warming. All surviving oocytes were microinjected and 86 (77.5%) were normally fertilized, of which 53 (61.6%) developed up to good-quality day 3. Ten embryo transfers resulted in a clinical pregnancy (50.0%) and an ongoing clinical pregnancy rate of 45%. Five revitrified embryos were warmed in three warming cycles (survival rate 100%). These transfers resulted in an additional ongoing twin pregnancy, leading to a cumulative ongoing pregnancy rate per patient of 50% (10/20). The ongoing implantation rate per warmed oocyte and per injected oocyte was 10.6% (13/123) and 11.7% (13/111). The present data demonstrate that oocyte vitrification using a closed vitrification device yields excellent oocyte survival, fertilization and embryo development.Progress in oocyte vitrification has led to successful implementation of the technique for different clinical applications. These excellent results have been achieved with open carriers, allowing direct contact of the oocyte with the liquid nitrogen. However, controversy exists about the risks of microbiological contamination in case of direct contact with the unsterile liquid nitrogen. This validation trial aimed to evaluate oocyte vitrification using a high-security closed vitrification system in a donation programme. In this study, a total of 123 vitrified metaphase-II oocytes were warmed in 20 recipient cycles resulting in 111 surviving oocytes (90.2%). Intracytoplasmic sperm injection resulted in 86 (77.5%) 2-pronuclear embryos, of which 53 (61.6%) developed to good quality by day 3. Ten embryo transfers resulted in a clinical pregnancy (50.0%) and an ongoing clinical pregnancy rate of 45%. The ongoing implantation rate including the outcome of revitrified embryos per warmed oocyte, and per injected oocyte is 10.6% (13/123) and 11.7% (13/111). The present data demonstrate that oocyte vitrification using a closed vitrification device yields excellent oocyte survival, fertilization and embryo development comparable to open vitrification. The evaluated closed vitrification device guarantees aseptic vitrification without requiring sterilized liquid nitrogen or the addition of a sealed container for storage.