Antigen expression by villous and extravillous human trophoblast populations at discrete anatomical sites has been reviewed. The various different antigenic phenotypes have been highlighted using a panel of monoclonal antibodies reactive with characteristic trophoblast membrane antigens, a trophoblast-leucocyte common antigen, class I MHC antigens, epithelial cell cytokeratin and epithelial membrane markers. This approach has allowed three separate fetal trophoblast populations to be identified within term amniochorionic membranes, and also has facilitated further definition of trophoblast populations in maternal uterine tissues. Furthermore, antigenic alterations have been noted in the maternal uterine gland epithelium in pregnancy leading to the expression of a trophoblastic phenotype, thereby suggesting a mechanism of extrinsic regulation of gene expression in these tissues. The possible involvement in the immunoregulatory control of maternal responses in pregnancy of MHC-linked gene products expressed by trophoblast has been discussed.
Normal trophoblast of the human placenta elaborates at least two major protein hormones, chorionic gonadotrophin (hCG) and placental lactogen (hPL). Molar and choriocarcinoma tissues characteristically synthesize large amounts of hCG and small quantities of hPL. To examine the role of trophoblast differentiation in the expression of the hCG and hPL genes, we studied the cytological distribution of their mRNAs in tissue sections of human hydatidiform mole and choriocarcinoma by in situ hybridization. Histologically, these tissues are in different stages of cellular differentiation. In normal placenta, hCG mRNA can be localized to some cytotrophoblasts and primarily to the syncytium, whereas hPL mRNA appears only in the syncytial layer. In hydatidiform mole, which still retains placental villous morphology, the hPL gene and hCG and genes are expressed but are poorly localized because of the admixture of cyto- and syncytiotrophoblasts. By contrast, choriocarcinoma, which is devoid of placental villous pattern but in which the cyto- and syncytiotrophoblast-like components are distinguishable, expresses hCG and in the syncytial-like areas but little, if any, hPL. These results suggest that a certain level of trophoblast differentiation, such as villous formation, is associated with hPL expression, while the hCG gene and, the hCG gene can be expressed in more disorganized tissues which contain cytotrophoblastic elements.
Pregnancy-specific -glycoprotein (SP ) was identified by peroxidase-antiperoxidase (PAP) immunohistochemistry in the placenta of inbred strains of rat between 10 and 21 days of gestation. SP was located predominantly in basal zone trophoblast and in intravascular trophoblast of decidual vessels, but it was absent from labyrinthine trophoblast. No perivascular cells were identified by SP staining, but occasional clusters of interstitial trophoblast stained for SP . The results suggest that basal and labyrinthine trophoblast are functionally different.
Immunofluorescence and radiobinding studies have shown that native human α -macroglobulin (α M), but not α M-trypsin complexes, binds to isolated human placental syncytiotrophoblast microvillous plasma membrane vesicles. Inhibition studies have indicated tha α M may bind to a trophoblast surface protease. This interaction is of significance for the control of trophoblast invasiveness and haemostasis within the placental bed.