Evaluation of morphokinetic characteristics of zona pellucida free mouse pre-implantation embryos using time-lapse monitoring system

The mammalian zygote cleaves and develops to blastocyst within the zona pellucida (ZP) in vivo. The presence or absence of ZP may affect the characteristics of the embryo, including blastomere alignment, cell-cell junction, and compaction.This study aimed to comparethe morphokinetic characteristics of ZP-intact and ZP-free mouse pre-implantation embryos with a time-lapse monitoring system. Mouse 2-cell embryos were collected 1.5 days post coitum (dpc), and their ZPs were removed by treatment with acidTyrode's solution. All embryos were cultured in vitro up to the outgrowth stage at 7.5 dpc. In this study, ZP did not influence the cumulative times from 2-cell to further stages and blastulation. Interestingly, ZP-free embryos at 4-cell stage have three patterns of blastomere alignment according to the number of contact points between blastomeres. However, blastomere alignment did not lead to any differences in morphokinetic comparisons. Regardless of the presence or absence of ZP, embryos compacted after the 8-cell stage took shorter time to become blastocysts than embryos compacted pre-8-cell stage. Nevertheless, cell-cell junction proteins required for successful compaction were similarly expressed between ZP-intact and ZP-free embryos. ZP-intact embryos compacted post-8-cell stage had a higher rate of reaching blastocysts than compacted ZP-intact embryos before 8-cell stage, while the outgrowth/blastocyst rate was similar. In this study, the presence or absence of ZP did not influence embryonic development and expression of cell surface glycoproteins, whereas compaction timing may be one of the criteria for evaluating embryo quality. ZP-free embryos may become an alternative for overcoming cases with ZP problems in human ART programs.