Haemostatic mechanisms in human placenta.

The placenta is a unique organ with dual blood circulation functioning throughout fetal development. The architecture and functions of the placenta, where maternal blood flows into the intervillous space, present haemostatic problems, mainly the risk of haemorrhage. Placental trophoblasts express and produce coagulation components, participating not only in haemostasis but also in placental vascular development and differentiation. The expression of tissue factor, membrane phosphatidylserine and fibrin render the trophoblasts pro-coagulant, thus compromising the risk of bleeding while exposing the placenta to pro-thrombotic risks. Local inhibitory mechanisms-TFPI-1 and TFPI-2, thrombomodulin, annexin V and the fibrinolytic system-limit coagulation activation and fibrin deposition. Pregnancy complications have been associated with abnormalities in the functions of these inhibitors. Haemostatic processes in placental cells change throughout gestation and are affected by the changing requirements of the organ.