Endotheliochorial

Endotheliochorial is a type of placentation in which the fetal trophoblast invades the maternal endometrium to the extent that the maternal epithelium and much of the underlying connective tissue are eroded away, leaving the chorionic villi in direct contact with the maternal endothelium. This places the placental barrier between fetus and mother between that of epitheliochorial and hemochorial types.

As a result, the placental barrier is thinner than in epitheliochorial placentation but not as permeable as in hemochorial placentation. The interface is formed where fetal chorionic tissue sits against the maternal endothelium, with maternal epithelial layers largely absent at the site of placental contact. Maternal blood remains within endometrial vessels that are exposed to fetal tissue.

Occurrence of endotheliochorial placentation is most commonly described in carnivores, including domestic dogs and cats. It represents a moderate level of invasiveness, bridging the gap between minimal invasion (epitheliochorial) and maximal invasion (hemochorial) seen in other mammalian lineages.

Functional and comparative notes: The endotheliochorial arrangement allows closer fetal-maternal contact than epitheliochorial placentas, potentially facilitating nutrient transfer and exchanges at the maternal–fetal interface, while still reducing direct exposure of maternal epithelium to fetal tissues relative to hemochorial placentas. This placental type has implications for immunological interactions and placental stability during pregnancy across species that exhibit it.