Regulation of conceptus development and attachment in pigs.

Implantation/placentation in domestic pigs is preceded by synthesis of oestrogen by the conceptus to maintain functional corpora lutea throughout pregnancy and a rapid morphological transformation of conceptuses from spherical to long filamentous thread-like structures. Initial conceptus expansion, reaching a metre in length, not only delineates the surface area for placental attachment, but also provides the mechanism for delivery of oestrogen to signal events necessary for placentation throughout the uterine horn. Timing for conceptus gene expression to induce trophoblast expansion and attachment in pigs is temporally associated with downregulation of progesterone receptors and increase in oestrogen receptors within the uterine epithelium. Within the confines of the uterine lumen, pig conceptuses normally do not erode or invade through the uterine epithelial surface. However, the pig conceptus possesses extensive proteolytic activity as it is highly invasive outside the uterine lumen of the pig. Initial release of oestrogen by the elongating pig conceptus induces endometrial release of cytokines and a variety of protease inhibitors. Recently, endometrial expression for the inter-trypsin inhibitor (I alpha I) family of protease inhibitors has been detected in the pig endometrium during conceptus elongation and attachment. It is possible that I alpha Is may function to inhibit trophoblast invasion and also serve as targets for adhesion molecules, such as integrins and heparin, to aid in placental attachment to the uterine epithelium.