Early pregnancy signals in domestic animals

Large early embryonic death losses occur in all domestic animals and these losses are temporally related to early pregnancy recognition signals. Binucleate trophoblastic cells, which migrate to the endometrial endothelium, are a potential vehicle for early (day 15-20) communication between the mother and the embryo in ruminant animals. In cattle, the first pregnancy recognition signal, which results in increased progesterone secretion by the corpus luteum as early as day 10, appears to be a small (less than 10,000 Mr) heat-labile lipid-soluble molecule that can be adsorbed by dextran-coated charcoal. Although this substance has not yet been identified, there is a possibility that it is embryo-derived platelet-activating factor (EDPAF). EDPAF appears to influence progesterone synthesis by causing the release of luteotropic factors (arachidonic acid metabolites and serotonin) from activated platelets. The second pregnancy recognition factor in cattle and sheep appears to be a trophoblastic peptide having a molecular weight of 22,000-24,000 daltons (bTP-1, cattle) or 17,000 daltons (oTP-1, sheep). These compounds do not have direct luteotropic effects, but are thought to exert their antiluteolytic effects by inhibiting the production of luteolytic eicosanoids (PGF2 alpha) by the endometrium. A third early pregnancy signal in cattle is an hCG-like protein that appears in allantoic fluid on day 25, just after the disappearance of bTP-1. There is as yet no clear evidence that steroids produced by the conceptus act as early pregnancy signals in ruminants. However, estrogens appear to provide the essential early signal for corpus luteum maintenance in the pig.