Effect of assisted hatching by partial zona pellucida dissection on mouse embryos in vitro.

The zonae pellucida of mouse embryos were dissected to examine the effect of partial zona dissection (PZD) at the four-cell stage on embryonic development and the hatching process in vitro. Micropipettes were used to create a gap of one-sixth the zona circumference, to avoid damage to the blastomere. A total of 484 embryos at the four-cell stage (defined as day 2 embryos) were randomly divided into two groups, the PZD group (n = 238) and the control group (n = 246). Embryos of both groups were cultured in vitro under identical conditions and examined on the following day. The hatching rates were as follows: day 3, 11/238 (4.6%) in the PZD group and 0/246 (0%) in the control group; day 4, 105/238 (44.1%) in the PZD group and 54/246 (22.0%) in the control group; day 5, 71/238 (29.8%) in the PZD group and 36/245 (14.5%) in the control group. On day 4 the rates of complete hatching were 7/238 (2.9%) in the PZD group and 2/246 (0.8%) in the control group. On day 6 they were 88/238 (37.0%) in the PZD group and 66/246 (26.8%) in the control group. A significantly higher percentage of PZD embryos than controls initiated and completed hatching in similar culture conditions. The present study indicates that the creation of an opening in the zona pellucida is not harmful to embryonic development and may facilitate hatching. Whether the enhanced hatching ability of embryos in vitro caused by PZD can improve the implantation rate requires further study.