Geisert R D, Yelich J V
Animal Science Department, Oklahoma State University, Stillwater 74078, USA.
J Reprod Fertil Suppl. 1997;52:133-49.
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.
家猪的着床/胎盘形成之前，孕体合成雌激素以在整个妊娠期维持功能性黄体，并且孕体从球形迅速发生形态转变，成为长丝状的线状结构。孕体最初的伸展长度可达1米，这不仅划定了胎盘附着的表面积，还提供了将雌激素传递至子宫角各处胎盘形成所需信号事件的机制。猪体内孕体基因表达以诱导滋养层伸展和附着的时间，在时间上与子宫上皮内孕酮受体的下调以及雌激素受体的增加相关。在子宫腔范围内，猪的孕体通常不会侵蚀或穿透子宫上皮表面。然而，猪的孕体具有广泛的蛋白水解活性，因为它在猪子宫腔外具有高度侵袭性。伸长的猪孕体最初释放的雌激素会诱导子宫内膜释放细胞因子和多种蛋白酶抑制剂。最近，在孕体伸长和附着期间，猪子宫内膜中检测到了蛋白酶抑制剂间胰蛋白酶抑制剂（IαI）家族的子宫内膜表达。IαI可能起到抑制滋养层侵袭作用，还可能作为整合素和肝素等黏附分子的靶点，以帮助胎盘附着于子宫上皮。
