During early pregnancy, placentation occurs in a relatively hypoxic environment which is essential for appropriate embryonic development. Intervillous blood flow increases at around 10-12 weeks of gestation and results in exposure of the trophoblast to increased oxygen tension (PO2). Prior to this time, low oxygen appears to prevent trophoblast differentiation towards an invasive phenotype. In other mammalian systems, oxygen tension effects are mediated by hypoxia inducible factor-1 (HIF-1). We found that the ontogeny of HIF-1 alpha subunit expression during the first trimester of gestation parallels that of transforming growth factor-beta(3) (TGF beta(3)), an inhibitor of early trophoblast differentiation. Expression of both molecules is high in early pregnancy and falls at around 10 weeks of gestation when placental PO2 levels are believed to increase. Antisense-induced inhibition of HIF-1 alpha inhibited the expression of TGF beta(3), and stimulated extravillous trophoblast (EVT) outgrowth and invasion. Of clinical significance we found that TGF beta(3) expression was increased in pre-eclamptic placentae when compared to age-matched controls. Significantly, inhibition of TGF beta(3) by antisense oligonucleotides or antibodies restored the invasive capability to the trophoblast cells in pre-eclamptic explants. We speculate that if oxygen tension fails to increase, or trophoblasts do not detect this increase, HIF-1 alpha and TGF beta(3) expression remain high, resulting in shallow trophoblast invasion and predisposing the pregnancy to pre-eclampsia. Effective fetal-maternal interactions during early placentation are critical for a successful pregnancy. Optimal placental perfusion requires the controlled invasion of trophoblast cells deep into the decidua to the spiral arteries. Trophoblast stem cells, also referred to as cytotrophoblast cells, reside in chorionic villi of two types, floating and anchoring villi. Floating villi, which represent the vast majority of chorionic villi, are bathed in maternal blood and primarily perform gas and nutrient exchange for the developing embryo. During early placentation, cytotrophoblast cells in the floating villi proliferate and differentiate by fusing to form the multinucleate syncytiotrophoblast layer. Cytotrophoblast cells in anchoring villi either fuse to form the syncytiotrophoblast layer, or break through the syncytium at selected sites and form multilayered columns of non-polarized extravillous trophoblast cells, which physically connect the embryo to the uterine wall (Figure 1). The extravillous trophoblast cells invade into the uterine wall as far as the first third of the myometrium and its associated spiral arteries, where they disrupt the endothelium and the smooth muscle layer and replace the vascular wall. This results in the conversion of the narrow calibre arteries into distended uteroplacental arteries, thereby increasing blood flow to the placenta and allowing an adequate supply of oxygen and nutrients to the growing fetus. The invasive activity of the extravillous trophoblast cells is at a maximum during the first trimester of gestation, peaking at around 10-12 weeks and declining thereafter. Insufficient invasion contributes to the development of pre-eclampsia, which often results in fetal intrauterine growth restriction, maternal hypertension and proteinuria. In contrast, unrestricted invasion is associated with premalignant conditions, such as invasive mole, and with malignant choriocarcinoma. Invading trophoblast cells undergo striking and rapid changes in cellular functions that are temporally and spatially regulated along the invasive pathway (Figure 1) (Cross, Werb and Fisher, 1994. The formation of the anchoring villi is accompanied by changes in synthesis and degradation of extracellular matrix proteins and their receptors, and changes in the spatial distribution of extracellular matrix proteins, as well as changes in the expression of adhesion molecules (Damsky, Fitzgerald and Fisher, 1992; Bischof et al., 1993). For example, during invasion the expression of laminin is lost and the expression of fibronectin is acquired. Extravillous trophoblast cells lose the expression of E-cadherins, downregulate the expression of alpha(6)beta(4) integrin (a laminin receptor) and acquire the expression of alpha(5)beta(1) integrin (a fibronectin receptor). Subsequently extravillous trophoblast cells differentiate to acquire an invasive phenotype, which is indicated by the expression of markers such as gelatinase B/MMP9 and alpha(1) integrin (a collagen/laminin receptor) as well as markers typical of a vascular adhesion phenotype. Thus, specific changes in extracellular matrix proteins and their receptors are associated with the acquisition of an invasive phenotype by the extravillous trophoblast cells. (C) 2000 IFPA and Harcourt Publishers Ltd.
