Establishment of uteroplacental circulation.

Haemochorial placentation, as it occurs in the human and other primate and rodent species, requires a connection of the placenta with supplying maternal (uteroplacental) arteries. Very little is known of the initial stages but endovascular trophoblast invasion seems to represent an essential element for further elaboration of an adequate uteroplacental circulation. In the human, endovascular trophoblast arrives in myometrial segments of spiral arteries only from about 15 weeks of pregnancy. This is preceded by an interstitial type of cytotrophoblast invasion which seems to be associated with regressive changes in spiral artery walls. It is possible that the latter forms an essential priming factor to allow subsequent endovascular migration. Endovascular trophoblast invasion has been documented in different laboratory animals, including the rat and the golden hamster. Especially in the latter case a sequence of changes in the maternal component, i.e. the maternal cellular elements in the spiral artery walls, precede the arrival of trophoblast. Besides, there is also some evidence of haemodynamical factor(s) influencing trophoblast migration. Because of the importance of this phenomenon in the establishment of an adequate uteroplacental circulation, it is essential to develop further experimental models for studying pathological situations as exist in human pregnancy.