Role of progesterone in implantation in the sheep.

Abstract

Merino ewes were ovariectomized on Days 4 8 or 12 of pregnancy and injected daily from that time with 1- 4- or 16-mg progesterone in oil. The ewes were killed on Day 20 and examined for embryos. Sections of uteri were examined histologically. There was a clear effect of dose of progesterone on the number and viability of embryos. Of the ewes receiving 1- 4- and 16-mg progesterone there were 0/12 2/11 and 11/14 with viable embryos on Day 20. The effect of increasing progesterone was also seen in significantly increased glandular and luminal epithelial cell heights in ewes receiving 16 mg day. Plasma progesterone was measured daily during Day 1-5 and twice daily during Days 6-15 of pregnancy of of the estrous cycle. The number of corpora lutea and embryos in these animals was verified at laparotomy on Days 16-18. Mean plasma progesterone in pregnant and nonpregnant ewes did not differ significantly until Day 16. The individual pattern of progesterone levels was characterized by a large (up to 16 ng/ml) and transient peak in the period from Days 11-14. Different ewes had peak values at different times so that the mean progesterone values took the form of a variable plateau of 2-3 ng/ml lasting several days. Peak levels of progesterone appear in the plasma at times consistent with the idea that this hormone is responsible for the rapid preimplantation growth phase of the embryo during Days 11-15.(FULL TEXT)