Blastocyst attachment and early invasion during oestradiol-induced implantation in the mouse

Summary Blastocysts were recovered from mice in experimentally delayed implantation 16 hrs after an injection of estrogen. When obtained from uterine horns with Pontamine Blue-positive sites, the blastocysts possessed a smooth embryonic pole and a rough abembryonic one as seen with SEM. In the embryonic pole, most of the trophoblast cells were flat while in the abembryonic region, the cells showed irregular cell extensions and rough imprints. TEM of Pontamine Blue-positive sites demonstrated that the embryonic pole had a rather smooth borderline to the epithelium, while the borderline at the abembryonic pole was more irregular with small parts of the uterine epithelium indenting into the trophoblast. The cytoplasm of the trophoblast contained a well developed endoplasmic reticulum, several mitochondria, large vacuoles, many multivesicular bodies and lipid granules. The difference in structure between the two poles of the blastocyst is presumably related to the differences in enzymatic activity between the poles as demonstrated by histochemistry and to the greater invasive capacity of the abembryonic trophoblast.