In vivo hatching phenomenon of mouse blastocysts during implantation.

PURPOSE: To determine whether the blastocyst zona shedding process within the murine uterine cornus in vivo is due to a global lytic process caused by uterine proteolytic enzyme, or is triggered by the blastocyst hatching process as observed in vitro.
METHODS: Fifty-one female ICR mice aged 5-8 weeks were used for this study. From 8:00 p.m. of the 4th day postcoitus to 7:00 p.m. of the 5th day postcoitus, the uterine cornua of 51 mice were isolated at 30-min intervals. Blastocysts within the uterine cornua were flushed out with a balanced solution under the dissecting microscope. The stages of blastocyst development and the proportion of hatching or hatched blastocysts and the discarded zona pellucida (ZP) were inspected and counted.
RESULTS: A total of 672 blastocysts were recovered from the uterine horns of the 51 mice. They were divided into six groups according to the blastocyst developmental stages (before or after ZP escape; before or after the initiation of implantation). Group I represents the earliest embryonic stage and Group VI represents the most advanced blastocyst developmental stage during the peri-implantation period. The empty ZP recovery rates (number of discarded ZP/all hatched blastocysts) were 52.3%, 21.5%, 17.2%, 6.6%, 1.6%, and 0% in Groups I-VI, respectively. Five hatching blastocysts out of 199 embryos in Group I were found and a 100% of ZP recovery rate was obtained from 6 of 19 mice in Groups I and II.
CONCLUSIONS: The present study shows that active blastocyst hatching occurs in vivo because both hatching blastocysts and empty ZP can be found within the uterine cornua of ICR mice before implantation. The empty ZP recovery rate declined significantly, along with a progression of embryo development and implantation, implying that intrauterine zona lytic activity occurs during the peri-implantation stage.