Lack of class I major histocompatibility antigens on trophoblast of periimplantation blastocysts and term placenta in the pig.

In this study, the pattern of expression of class I major histocompatibility (MHC) antigens and mRNA on periimplantation blastocysts and term placental tissue was determined for the pig. Class I MHC antigens could not be detected immunohistochemically either on extra-embryonic membranes or on the embryonic portion of Day 14, 16, 22, and 25 blastocysts. Nor could class I MHC antigens be detected on the outer trophoblast epithelium and inner endodermal surface of the chorioallantoic membrane or on the outer and inner surfaces of the amnion at term. However, MHC class I antigens were detected on the vascular mesoderm found in both the chorion and amnion at term, and in Day 25 extra-embryonic membranes. Uterine endometrial cells and tissues and maternal peripheral blood leukocytes stained strongly for class I MHC antigens. There was a large difference in the intensity of class I MHC mRNA signal, detected by Northern blot analysis, in embryo/fetus-derived tissues compared to that in maternal tissues. The embryos appeared to express even less class I MHC mRNA than did the extra-embryonic membranes. In addition, in situ hybridization of Day 16 blastocysts indicated class I MHC mRNA to be ubiquitously expressed at low levels in embryos and extra-embryonic tissues compared to uterine endometrial tissue controls. Taken together, these results indicate that class I MHC antigens are either not expressed on the surface of the extra-embryonic/fetal membranes during gestation in the pig or are expressed at very low levels, and that specific mRNA is expressed at correspondingly low levels.