Early Pregnancy in the Ewe: Effects of Oestradiol and Progesterone on Uterine Metabolism and on Embryo Survival

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Abstract
The hormonal regulation of embryo development during early pregnancy in the ewe was examined. Ovariectomized ewes received injections of estradiol (E2) and progesterone (P) according to schedules designed to simulate endogenous ovarian secretion during the luteal phase of the previous estrous cycle (priming P), around the time of estrus (estrous E2) and during early pregnancy (maintenance P, maintenance E2). Embryos were transferred to the ewes on the 4th day after induced estrus, and ewes were killed at 6 or 13 days after transfer to assess embryo development. Cytosol concentrations of estradiol and progesterone receptors and RNA and protein metabolism in the endometrium and amounts of protein in uterine flushings were examined on the day of embryo transfer and 6 days after transfer. Twenty one of 27 ewes which received maintenance P, estrous E2 and priming P carried normal embryos at the time of killing. Omitting maintenance E2 had no effect on the proportion of ewes in which embryos developed normally. When either priming P or estrous E2 was omitted embryos ceased to develop normally within 1-2 days of the time of transfer. Omitting estrous E2 reduced the amount of protein in the uterine lumen and the RNA:DNA ratio, rate of synthesis of protein and amounts of estradiol and progesterone receptors in the endometrium at the time of embryo transfer. Omitting priming P caused a small decrease in the concentration of progesterone receptor but no other significant changes in the endometrium or uterine flushings at the time of transfer. It is suggested that estrous E2 controls embryo development by regulating endometrial sensitivity to the progesterone of pregnancy. The mechanism by which priming P affects embryo development remains unknown.