Expression, regulation and proposed function of OPN produced by the uterine LE of pregnant pigs. A) As porcine conceptuses (Trophoblast) elongate they secrete estrogens for pregnancy recognition. These estrogens also induce the synthesis and secretion of OPN (osteopontin) from the uterine LE (luminal epithelium) directly adjacent to the conceptus undergoing implantation. The implantation cascade is initiated when progesterone from CL down-regulates Muc 1 on the surface of uterine LE. This exposes integrins on the LE and trophoblast surfaces for interaction with OPN, and likely other ECM proteins, to mediate adhesion of trophoblast to LE for implantation[58, 59, 64]. B) In vitro experiments have identified the αvβ6 integrin receptor on trophoblast, and the αvβ3 integrin receptor on LE as binding partners for OPN. OPN may bind individually to these receptors to act as a bridging ligand between these receptors. Alternatively, OPN may serve as a bridging ligand between one of these receptors and an as yet unidentified integrin receptor expressed on the opposing tissue.