Evidence that "embryonic estrogen" is a factor which controls the development of the mouse preimplantation embryo

4- to 8-cell mouse PIE's (preimplantation embryos) were cultured in vitro for 48 hr in media containing various concentrations of the anti-estrogen CI-628. Graded effects of the various concentrations were observed with the highest concentration (1.5 microng/ml) being 100% effective in blocking development to the blastocyst stage. The deleterious effects of the drug were prevented to some extent by including estradiol-17beta in the medium. Our interpretation is that the effect of CI-628 was due to its anti-estrogenic properties. If this interpretation is correct, then the results support our hypothesis that the development of mouse PIE's depends upon estrogen which originates in the PIE.