In vivo hatching phenomenon of mouse blastocysts during implantation

Purpose: To determine whether the blastocyst zona shedding process within t he murine uterine cornus in vivo is due to a global lyric process caused by uterine proteolytic enzyme, or is triggered by the blastocyst hatching pro cess 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 postc oitus, the uterine cornua of 51 mice were isolated at 30-min intervals. Bla stocysts within the uterine cornua were flushed our with a balanced solutio n under the dissecting microscope. The stages of blastocyst development and the proportion of hatching or hatched blastocysts and the discarded zona p ellucida (ZP) were inspected and counted. Results: A total of 672 blastocysts were recovered from the uterine horns o f the 51 mice. They were divided into six groups according to the blastocys t developmental stages (before or after ZP escape; before or after the init iation of implantation). Group I represents rite earliest embryonic stage a nd Group VI represents the most advanced blastocyst developmental stage dur ing the peri-implantation period. The empty ZP recovery rates (number of di scarded ZP/all hatched blastocysts) were 52.3%, 21.5%, 17.2%, 6.6%, 1.6%, a nd 0% in Groups I-IV, respectively. Five hatching blastocysts out of 199 em bryos in Group I were found and a 100% of ZP recovery rate was obtained fro m 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 r ate declined significantly, along with a progression of embryo development and implantation, implying that intrauterine zona lytic activity occurs dur ing the peri-implantation stage.