The impacts of laser zona thinning on hatching and implantation of vitrified-warmed mouse embryos.

Embryo vitrification has advantages in assisted reproduction yet it also induces zona hardening. Laser zona thinning (LZT) is considered as a solution yet its efficacy and security have not been well studied. In this study, we used vitrified-warmed morulae from 2-month-old and 10-month-old ICR female mice as model to investigate the impacts that LZT treatment brings to the in vitro hatching process and implantation by analyzing hatching rate, implantation rate, and blastocyst quality. The results showed that the fully hatched rate was significantly higher after LZT treatment for both young (25.7% vs. 16.2%, P < 0.05) and aged (36.6% vs. 13.2%, P < 0.01) mice. For zona-thinned morulae in young mice, its onset of hatching occurred earlier (28.6% vs. 8.8%, P < 0.01) at D4 and with a greater percentage of U-shaped hatching at D5 (48.3% vs. 33.0%, P < 0.05). LZT treatment did not induce expression change of apoptosis-related genes in all groups (P > 0.05), but for young mice, the total cell number of day 5 blastocyst in zona-thinned group was significantly less than that of the control group (40.6 ± 5.1 vs. 59.9 ± 14.5, P < 0.01). At last, there was an increasing implantation rate in zona-thinned compared to the control group for young (63.8% vs. 52.5%, P > 0.05) and aged (55.6% vs. 47.2%; P > 0.05) mice after embryos were bilaterally transferred in the same recipient. In conclusion, the significant increase of fully hatched rate after LZT treatment is related to the advanced onset of hatching as well as the enhancement of superior hatching structure, and LZT also lead to a better implantation after embryo transfer.