Storage of human oocytes in the vapor phase of nitrogen

Objective: To evaluate the effectiveness of long-term vapor-phase nitrogen storage of vitrified human oocytes as a strategy for preventing the risk of cross-contamination due to direct contact with the liquid nitrogen (LN).

Design: Prospective randomized study.

Setting: Private infertility center, IVI, Valencia.

Patient(s): Oocyte donors (n = 44) and recipients (n = 46).

Intervention(s): Vitrification by the Cryotop method. Storage of vitrified oocytes in a vapor-phase nitrogen storage freezer and a traditional LN storage tank. Donation of the surviving oocytes and evaluation of fertilization, embryo development, and clinical results.

Main outcome measure(s): Survival, fertilization, and cleavage rates. Embryo quality and clinical outcome.

Result(s): Survival was 95.3% (vapor-phase nitrogen) and 94.5% (LN). Fertilization rates (73.1% and 71.7%) or cleavage on day 2 (95.6% and 94.7%), day 3 (84.5% and 79.9%), and blastocyst formation (54.7% and 53.9%) were similar between vapor-phase nitrogen and LN. Implantation, clinical, and ongoing pregnancy rates were similar for vapor-phase nitrogen (40.5%, 58.1%, and 48.8%, respectively) and LN groups (33.7%, 53.3%, and 46.6%, respectively).

Conclusion(s): The vapor-phase nitrogen system permits the storage of oocytes vitrified, maintaining their potential to develop into competent embryos in a similar manner as those stored in a traditional LN freezer. This approach represents a practical alternative that prevents cross-contamination during the storage of vitrified samples.