Lysis of the zona pellucida and attachment of embryos to the uterine epithelium in ovariectomized mice treated with oestradiol-17 beta and progesterone.

Preimplantation mouse embryos with intact zonae pellucidae were transferred into the uteri of ovariectomized females treated with progesterone, oestradiol-17 beta plus progesterone, or oestradiol-17 beta alone; the disappearance of zonae from the uterine lumen was used to determine the presence of 'zona-lytic' activity in situ. Lysis of zonae did not occur in animals treated with progesterone or oestradiol-17 beta alone. However, lysis did not occur when oestradiol-17 beta was combined with progesterone; 'zona-lytic' activity reached peak levels within 12 to 24 h, then decreased. Attachment of embryos to the uterine epithelium occurred only in animals treated with oestradiol-17 beta plus progesterone and was initiated at about the time of peak 'zona-lytic' activity. It is suggested that the chymotrypsin-like enzyme activity present in uterine fluid is involved with the initiation of implantation.