ABSTRACT
In the studies on the hatching mechanisms in mammals, many investigators focused on the embryonic intrinsic factor(s) in in vitro culture, but the uterine environment as the extrinsic factor(s) is thought to play an important role in hatching mechanism. Therefore, to evaluate the effect of uterine environment on the hatching event in vivo, the immature(GV) and ovulated(MII) oocytes, and the late 2-cell embryos of mouse were transferred to pseudopregnant foster mother's uterus during peri-implantation period. So it was verified whether there would happen hatching by only uterine environment independently on embryonic stage. The ultrastructural changes of the zona surface of transferred group were compared with those of in vivo and vitro group by SEM. 36 hrs after transfer, the immature and ovulated oocytes almost degenerated, and the late 2-cell embryos developed to various embryonic stages. However, the embryos which didn't develop to blastula stage did not hatch. The ultrastructural network of ZP in transferred group seemed to be smoothed uniformly, which was different from in vitro group. In conclusion, it is suggested that the uterine environment during peri-implantation period enhances the embryo hatching by provoking the structural change of ZP.