An intact zona pellucida is not necessary for successful mouse embryo cryopreservation

To determine the developmental potential of mouse embryos that underwent cryopreservation after micromanipulation of the zona pellucida. Gaps were produced in the zona pellucida of mouse oocytes or two-cell embryos by zona drilling with acid Tyrode’s solution. Zona-drilled oocytes were fertilized in vitro and cultured to the two-cell stage. Two-cell embryos were frozen, thawed, and cultured to the expanded blastocyst stage. There was no difference in the rate of embryo survival post-thaw (248/318, 77% versus 288/345, 83.4%), or in the rate of development to the expanded blastocyst stage (91/248, 36.7% versus 88/288, 30.6%), between embryos that were zona drilled as oocytes and unmanipulated controls. Similarly, there was no difference in the rate of cryosurvival (206/217, 94.9% versus 168/187, 89.8%) or development to the blastocyst stage (154/206, 74.7% versus 132/168, 78.6%) between embryos that were fertilized in vivo and zona drilled at the two-cell stage and embryos that were unmanipulated. These findings indicate that small gaps in the zona pellucida, such as those that result from micromanipulation, do not significantly alter the ability of embryos to withstand cryopreservation.