An ultrastructural study of implantation in the golden hamster. I. Loss of the zona pellucida and initial attachment to the uterine epithelium.

Forty five blastocysts from 14 golden hamsters were examined in situ with light and electron microscopy between 3 and 4 days of pregnancy (post-ovulation). At 3 days blastocysts remained free within the uterine lumen with their zonae pellucidae intact. At various intervals between 3 days and 3 days 6 hours a gradual thinning of the zona occurred, indicating the possible presence of a lytic factor which aids in the removal of the zona pellucida. Blastocysts at 3 days 8 hours had lost their zonae pellucidae, and by 3 1/2 days they were completely enclosed by the uterine pithelium. By 4 days a more intimate association between the juxtaposed trophoblast and uterine epithelium occurred, leaving intermittent deposits of electron-dense material between the adjoining plasma membranes.