Human placental villi are surfaced by the syncytiotrophoblast (STB), with a layer of
cytotrophoblasts (CTB) positioned just beneath the STB. STB in normal term pregnancies is
exposed to maternal immune cells in the placental intervillous space. Extravillous
cytotrophoblasts (EVT) invade the decidua and spiral arteries, where they act in conjunction
with natural killer (NK) cells to convert the spiral arteries into flaccid conduits for maternal
blood that support a 3–4 fold increase in the rate of maternal blood flow into the placental …