Effect on survival of micromanipulating the zona pellucida of bovine embryos

Day 7 bovine embryos collected from superovulated cows were subjected to standard incision in the zona pellucida before subsequent in vitro culture for 72 h or direct transfer. The aim of this simple microsurgical treatment of fresh or frozen-thawed embryos of different quality was to facilitate the process of hatching. Non-treated embryos were used as controls. Culture of zona-slit fresh grade 1 embryos yielded a higher hatched blastocyst percentage than culture of non-treated controls [67.5% (54/80) vs. 57.6% (38/66), p > 0.05]. An even stronger effect of zona slitting was observed in grade 2 and grade 3 embryos [36.4% (20/55) vs. 10.3% (7/68), p < 0.01]. Culture of zona-slit frozen-thawed grade 1 embryos yielded a higher hatched blastocyst percentage than culture of non-treated controls [48.7% (37/76) vs. 25.5% (12/47), p < 0.025]. The same applied to the effect of zona slitting of grade 2 embryos [14.4% (13/90) vs. 2.3% (2/86), p <0.005]. The effect of zona treatment of fresh grade 1 embryos intended for immediate transfer was nonsignificant [64.6% (42/65) survival for zona-slit vs. 63.9% (30/61) survival for non-teated], but became apparent in the transfer of grade 2 or grade 3 embryos. pregnancy rates for the transfer of zona-slit and non-treated embryos were 58.7% (37/63) and 47.9% (35/73), respectively. Pregnancy rates after transfer of frozen-thawed grade 1 embryos were higher in the group of recipients of zona-slit embryos than in controls receiving non-treated embryos [57.5% (46/80) vs. 45.3% (39/86), p > 0.05]. No effects of zona slitting were demonstrable after transfer of frozen-thawed grade 2 embryos, however [30% (18/60) vs. 33.3% (15/45)]. The simple zona pellucida slitting procedure before in vitro embryo culture increased the survival rate in both fresh and frozen-thawed embryos. The effect was more pronounced in frozen-thawed grade 1 and in fresh grade 2 and 3 embryos.