The role of progesterone and conceptus-derived factors in uterine biology during early pregnancy in ruminants1

This review integrates established and new information on the role of progesterone, interferon tau (IFNT), and prostaglandins in uterine biology of ruminants. Establishment of pregnancy in ruminants encompasses growth of the posthatching blastocyst, elongation of the conceptus (embryo and extraembryonic membranes), and suppression of the endometrial luteolytic mechanism to maintain progesterone production by the ovary. Conceptus elongation involves exponential increases in length of the trophectoderm for pregnancy recognition signaling, implantation, and establishment of pregnancy. Pregnancy recognition signaling is accomplished by IFNT from the trophectoderm that has a paracrine antiluteolytic effect to inhibit upregulation of oxytocin receptors in the endometrial epithelia, thereby inhibiting production of luteolytic PGF2α pulses by the uterus. Survival and growth of the preimplantation blastocyst and elongating conceptus clearly requires embryotrophic factors (AA, carbohydrates, proteins, lipids, and other substances) in the uterine lumen. Individual, interactive, and coordinated actions of progesterone, IFNT, and prostaglandins regulate expression of elongation- and implantation-related genes in the endometrial epithelia that, in turn alter the uterine luminal histotroph and govern conceptus survival and growth. An increased knowledge of progesterone biology and conceptus-endometrial interactions is necessary to understand and elucidate the causes of pregnancy loss and provide a basis for new strategies to improve pregnancy outcome and reproductive efficiency in ruminants.