Influence of estrogen and progesterone on the incorporation of 35S methionine by blastocysts in ovariectomized mice

Incorporation of 35S methionine by mouse blastocysts in vivo, as detected by radioautography, provides a presumptive measure of the blastocysts' protein synthesizing activity. Normally, there is a marked increase in 35S methionine incorporation between 95 and 99 hours post coitum. To determine whether or not ovarian hormones influence 35S methionine incorporation by the blastocysts, mice were ovariectomized on day 4 of pregnancy and injected with progesterone and/or estradiol 17‐β and given a single injection of 35S methionine. The results demonstrate that 35S methionine is incorporated in mouse blastocysts in vivo only after combined progesterone and estrogen treatment; 12 to 14 hours are required for the maximum response. Further, blastocysts were transferred (101–103 hours post coitum) from intact donors to ovariectomized, pseudopregnant recipients that were untreated or treated with progesterone or estradiol. The blastocysts incorporated 35S methionine, thus suggesting that hormonally induced changes in the blastocysts are responsible for initiating protein synthesis. These findings provide a tentative explanation for the mechanism involved in initiating normal implantation and for terminating the embryonic diapause in the delayed implantation of lactation.