haemochorial

Haemochorial, also spelled hemochorial, describes a type of placentation in which the chorion (fetal tissue) is in direct contact with maternal blood within the intervillous space, with only trophoblastic tissue separating the two circulations. In this arrangement the maternal epithelium and much of the decidua are breached by placental invasion, enabling close exchange between the maternal and fetal blood.

This placentation type occurs in humans and many other primates, as well as several rodent species, and is characterized by a highly invasive interface between fetal and maternal tissues. During placentation, the trophoblast invades the maternal endometrium and remodels the spiral arteries, creating blood-filled lacunae that bathe the chorionic villi. The placental barrier is formed primarily by trophoblast layers (syncytiotrophoblast and cytotrophoblast) and, at the level of fetal capillaries, fetal endothelial cells; there is no intact maternal epithelial barrier at the interface.

Functionally, haemochorial placentation supports efficient transfer of gases, nutrients, and wastes between mother and fetus. The invasive nature of this type fosters substantial maternal–fetal exchange but also greater exposure of fetal antigens to maternal immune factors, influencing immune tolerance and potential antibody transfer.

Compared with epitheliochorial placentation, where maternal tissues largely remain intact and the barrier is thicker, haemochorial placentation generally achieves higher transfer efficiency but involves a closer, more immune-active interaction between mother and fetus. The term derives from Greek haima (blood) and chorio (chorion).