Biochemical interactions between blastocyst and endometrium in the large domestic animals

In pigs, the blastocyst begins to elongate from a sphere to a long filamentous thread around day 10.5 of pregnancy. At about this time the endometrium secretes large quantities of protein into the uterine lumen. The synthesis of this material which is believed to be required for nutritional support of the conceptus is under the control of progesterone. The release of secretory protein appears to be triggered by the production of estrogens by the elongating blastocyst. Blastocyst estrogens are also involved in the phenomenon of maternal recognition of pregnancy in swine, and their interaction with the maternal system, by a mechanism as yet unknown, prevents a return to reproductive cyclicity. Maternal recognition of pregnancy in the sheep and cow occurs at around the time of blastocyst elongation. Here estrogens do not appear to be involved, and protein products secreted by the conceptus have been implicated. One product of the sheep, ovine trophoblast protein-1, which is produced only during a brief period (days 13–21) of pregnancy, has been purified. It appears to be a hormone whose target tissue is the uterine endometrium.