Fig. 1

Human placental villous cytotrophoblast (CTB) differentiation at the maternal-fetal interface. a Anatomy of the human placenta. Chorionic villi are the functional units. The histology of the boxed area is shown in the panel to the right. b Depiction of the maternal-fetal interface at the cellular level. The mononuclear cytotrophoblasts (CTB) of the (early gestation) chorionic villi fuse to become multinuclear syncytiotrophoblasts (STBs), which form the surface of the placenta. Floating villi are perfused by maternal blood. Anchoring villi give rise to invasive interstitial CTBs (CTBi) that emigrate from the chorionic villi via cell columns that attach the placenta to the maternal unit and infiltrate the uterine wall. Maternal cells in this region include the decidua, remodeled uterine blood vessels, which are lined by cytotrophoblasts (CTBe), and immune cells. During vascular invasion, the cells breach both veins and arteries, but they have more extensive interactions with the arterial portion of the uterine vasculature. Here, they replace the endothelial lining and intercalate within the smooth muscle walls of the spiral arteries, producing hybrid vessels that are composed of both embryonic/fetal and maternal cells. Vascular invasion connects the uterine circulation to the intervillous space where maternal blood perfuses the chorionic villi. Immunoreactivity of molecular biomarkers was evaluated in five zones (I-V) corresponding to different stages of CTB differentiation: I) CTB progenitors in floating villi (FV); II) CTBs of the proximal (AVp) and III) distal (AVd) anchoring villi; IV) invading interstitial CTBs (CTBi); and V) endovascular CTBs (CTBe) that remodel the uterine vasculature. Image modified from Maltepe and Fisher, 2015; Damsky et al., 1992 (Damsky et al., 1992; Maltepe and Fisher, 2015)