MATERNAL RECOGNITION OF PREGNANCY

Maternal recognition of pregnancy in domestic farm animals evolved as a physiological mechanism by which the conceptus inhibits luteolysis through alteration in endometrial prostaglandin F2-alpha (PGF2-alpha) synthesis or movement during the establishment of early pregnancy. Luteolysis in ruminants, swine and horses results from the pulsatile release of PGF2-alpha from the endometrium during late diestrous. Current evidence indicates that the timing of luteolysis is controlled by down regulation of nuclear progesterone receptors within the uterine epithelium during mid-luteal phase of the estrous cycle. Loss of progesterone regulation on uterine epithelium may permit the synthesis of oxytocin receptors necessary for establishing luteolytic pulses of PGF2-alpha as proposed for the ewe and cow of allow synthesis of PGF2-alpha essential for regressing the corpus luteum. Mechanism(s) by which the conceptus of each species gains control over the uterine epithelium to protect the corpora lutea from luteolysis, most likely evolved through pathways which were best suited to meet the requirements of placental growth, development and attachment. Ruminants have adapted a specific trophoblast Type I interferon (IFN) to inhibit prostaglandin synthesis and possibly stabilize the uterine environment while trophoblast expansion and attachment occur. Although the pig conceptus secretes a trophoblast IFN, its endometrium is refractory to IFN stimulation of 2', 5'-oligoadenylate synthetase as occurs in ruminants. Estrogen secretion by the porcine conceptus alters endometrial PGF2-alpha movement and allows estrogen stimulated endometrial growth and secretion necessary for establishment of pregnancy in this polytocous species. The earlier evolution of the horse, which retains a spherical chorio-vitelline placenta during maternal recognition of pregnancy, developed a method of intrauterine conceptus migration to inhibit luteolysis through secretion of an unknown prostaglandin synthetase inhibitor.