Protein synthesis by blastocysts in the uteri and oviducts of intact and hypophysectomized mice

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Abstract
Two possible explanations for the stimulatory effect of estrogen and progesterone on protein synthesis by preimplantation mouse embryos are that the hormones: (1) stimulate the embryos directly; or (2) induce changes in the uterus that in turn affect the embryos.To determine which of these possibilities is correct, mouse blastocysts were retained in one oviduct by a ligature at the uterotubal junction; the mice were then either left intact or hypophysectomized. 35S methionine was injected and the tubal and uterine blastocysts were recovered and prepared for radioautography.Blastocysts in utero incorporate 35S methionine only if the pituitary is present. On the other hand, blastocysts in the oviducts incorporate 35S methionine regardless of the hormonal status of the mother. This suggests that estrogen and progesterone influence the level of protein synthesis in uterine blastocysts by inducing changes in the uterus rather than by stimulating the embryos directly.In addition, blastocysts were recovered from oviducts or uteri between day 5 and day 10 post coitum and transferred to pseudopregnant females to test their ability to develop into normal fetuses. Uterine blastocysts retained the ability to develop normally while the percentage of tubal blastocysts capable of development dropped sharply between day 5 and day 10. Therefore, it is suggested that the dormant condition of mouse blastocysts in utero during delayed implantation may serve to protect the embryos from damage.