Embryo-endometrial relationship in the mouse during activation of the blastocyst by oestradiol.

Blastocysts that have been activated for implantation by an injection of oestrogen become separated from the uterine surface by a layer of secretion. By 8 hr after the injection, the trophoblast cytoplasm contains several glycogen granules, groups of ribosomes, and mitochondria of a condensed type. These ultrastructural changes are signs of the increased metabolic activity exhibited by the trophoblast at activation. It is suggested that the uterine epithelium keeps the trophoblast inactive by delivering only a minimal nutritive medium and that the epithelium activates the trophoblast by making more nutrients, probably some carbohydrate, available as a consequence of an oestrogen-induced increase in the secretory activity.