Trophoblast invasion is a major feature of hemochorial placentation, notably in the human where this invasion is exceptionally deep compared with the few other primate species studied so far (Ramsey et al., 1976). It is assumed that pre-eclampsia/eclampsia, which occurs almost exclusively in the human species, is in some way associated with problems related to this deep invasion. So far, only a few case reports have been published suggesting a similar complication in pregnant gorillas (Baird, 1981; Thornton and Onwude, 1992). Unfortunately nothing is known about trophoblast invasion in this species.
It is now common knowledge that during pregnancy extensive vascular alterations take place in the spiral arteries which supply maternal blood to the placenta. In this chapter we will first sketch briefly the historical context of the discovery of structural changes in placental bed spiral arteries and the role of invading extravillous trophoblast. The importance of this “physiological change” of spiral arteries is highlighted by its restricted occurrence in pre-eclampsia, and therefore we will discuss next the evidence for impaired trophoblast invasion preceding the onset of clinical symptoms of pre-eclampsia. In the third part the structural features of invaded and non-invaded spiral arteries will be described in some detail, and finally we will evaluate the occurrence of physiological changes and vascular lesions in other disorders of pregnancy.
Impaired physiological change in spiral arteries
Although pre-eclampsia/eclampsia has been recognized for a long time in history as an important pregnancy complication (Lindheimer et al. 1999), a possible histopathological basis for this condition was only identified in the late 1960s.