FIGURE 4.

Schematic representation of a human placenta at term. The amniotic membrane is the layer closest to the fetus and is attached to the chorionic plate mesenchyme from which large stem villi reach into the intervillous space. The villous trees are fully differentiated and have a large number of terminal villi where enlarged capillaries, sinusoids, allow a higher exchange rate between maternal and fetal blood. Anchoring villi are still connected to the uterine wall, while trophoblast cell columns are exhausted. Spiral arteries invaded by endoarterial trophoblast and uterine veins invaded by endovenous trophoblast can still be found in the placental bed allowing the constant flow of maternal blood into the placenta and the drainage back into the maternal circulation (red arrows in artery and vein). Histological images of (A) term amniotic membrane with epithelium and avascular mesenchyme, (B) term chorionic plate covered by the amniotic membrane, (C) placental villi of a term placenta with a sinusoid in a terminal villus and a neighboring mature intermediate villus, (D) anchoring villus that is attached to the uterine wall where interstitial trophoblast can be found, (E) vessel in the basal plate of a term placenta. The vessel is surrounded by interstitial trophoblast. A, uterine spiral artery; AM, amniotic membrane; AV, anchoring villus; CP, chorionic plate; GA, gestational age; IT, interstitial trophoblast; MIV, mature intermediate villus; SI, sinusoid; STB, syncytiotrophoblast; TV, terminal villus; V, uterine vein.