The Structure of the Human Placenta: Implications for Initiating and Defending Against Virus Infections.

The architecture of the human placenta permits an extensive and intimate association between the maternal and fetal circulations. The fetal component consists of the elaborately branched villous tree, and this is bathed directly by maternal blood circulating within the intervillous space. Whilst this arrangement may favour metabolic exchange, it poses considerable risks for the vertical transmission of pathogens. Some features of placental structure can be considered potential impediments to transmission, such as the syncytial nature of the outer villous covering, the syncytiotrophoblast, and the ability of this tissue to secrete both nitric oxide and interferons. Other features may facilitate vertical transmission, including the lack of expression of MHC Class 1 antigens by the syncytiotrophoblast, and its vesicular and immunoglobulin transport pathways. More importantly, it is known that physical defects occur in the trophoblast layers at all stages in gestation. Whilst repair processes have been identified it must be assumed that pathogens or infected maternal white cells have access to the trophoblastic basement membrane, albeit transiently. The universal nature of these defects suggests that the trophoblast cannot be of paramount importance in the prevention of transmission. Rather, the defence mechanisms must lie either at the level of the basement membrane or within the villous core. There they may be represented by the resident macrophage population or the capillary endothelial cells and their junctional complexes. Consequently, the placenta should be viewed as an active rather than a passive barrier. Copyright 1997 by John Wiley & Sons, Ltd.