Structural interactions of trophoblast and uterus during hemochorial placenta formation.

It is argued that the successful placental arrangements are those that decrease the exposure of the trophoblast to the efferent side of the cellular immune response. Examples are taken from three different groups with hemochorial placentation, to show that their placental morphology places most of the trophoblast in contact with maternal blood, not maternal connective tissue. In addition, maternal cells of the junctional area are modified either before contact with trophoblast (rat) or just after trophoblast invasion (primate), or the region of contact is limited as in the armadillo, in which maternal blood sinuses are expanded to form intervillous spaces.