During a normal pregancy the maternal spiral arteries that nourish the placenta are continuosly pulled into the lacunar system.
These structural adaptations are accompanied by an edema, the dissolution of the endothelium and destruction of the tunica media and the membrana elastica interna, which are replaced by fibrous tissue. Through these alterations the arteries are removed from neuro-vascular control and the influence of the tone-producing vessel mediators (prostaglandin, nitrous oxide, endothelin). Thus, a larger blood flow is allowed in the placenta.
The migration of the trophoblast cells stands under a strict temporal-spatial control, an alteration of which can provoke a disorder in placental function. This ranges from a preeclampsia (characterized by insufficient penetration by the trophoblast) to a chorion carcinoma (characterized by an excessive trophoblast invasion).