At the time of the discovery of the physiological changes of spiral arteries in the pregnant uterus, Brosens and colleagues suggested that these changes result from the destructive action of the invading trophoblasts on the vascular smooth muscle and the elastic membrane. This chapter reiterates the main findings regarding the successive spiral artery remodeling steps. It seems appropriate to relate the time-course of the vascular remodeling process to the new insights in uteroplacental flow changes during this pregnancy period. In preeclampsia, trophoblast-associated remodeling is restricted to decidual spiral arteries throughout the placental bed. Spiral artery conversion is obviously important for safeguarding an adequate maternal blood supply to the placenta. Deep trophoblast invasion and spiral artery remodeling of the inner 'junctional zone' myometrium is a feature of normal human pregnancy, while in preeclampsia and maybe in other pregnancy complications this process may be seriously impaired.

This chapter provides a general overview of the two major components of pregnancy-associated spiral artery remodeling, the maternal (decidual) and the fetal (trophoblastic). During placentation the mother comes in close contact with semi-allogeneic fetal trophoblastic cells which play a key role in maternal-fetal physiological exchange. Focusing on spiral artery invasion, there is little direct evidence for the occurrence of intrinsic trophoblastic defects in early pregnancy. Impaired trophoblast invasion in spiral arteries may not only be due to an intrinsic defect in invasive properties, but may also be induced by maternal cells. For a long time trophoblast invasion was thought to be controlled by a restrictive action of the decidua. A stimulatory role of decidua for trophoblast invasion may also apply to the endovascular invasion of the spiral arteries. Decidua-associated vascular remodeling not only occurs in the decidua but also in the junctional zone myometrial compartment.