Effect of an estrogen antagonist on development of blastocysts and implantation in the hamster

Abstract
CI 628 citrate, an estrogen antagonist, was given as a single intraluminal injection (5 μg) on day 3 to ovariectomized, progesterone-treated hamsters. This significantly reduced embryo cleavage rate, transformation of morula into blastocyst, and completely inhibited implantation. The effects of the drug could be reversed by estradiol-17B (1 μg) but not estradiol-17α (1 μg) injected intraluminally with CI 628 citrate. Our finding suggests a role of estrogen present in hamster preimplantation embryo in the triggering of embryonic differentiation and implantation of the blastocyst.