Placental structure and fetal membranes. (A) Hematoxylin and eosin, term placenta. Villi of the labyrinth (Lab), supplied by maternal arteries (Ma) from the uterus (Ut). (B) Hematoxylin and eosin, term. Detail of the maternal vessels (Mv) in the labyrinth and the cytotrophoblast clusters (circle and detail). (C,D) Immunohistochemical staining for vimentin and cytokeratin, respectively, near term. The mesenchyme of the fetal blood vessels (Fv) was strongly reactive for vimentin, in contrast to the maternal vessels (Mv). Cytokeratin in trophoblastic cells (arrows) near the maternal vessels (Mv). (E-F) PCNA, near term. In E: Proliferative trophoblastic cells (arrows) in the labyrinth, near the fetal vessels (Fv). Mv = maternal vessels. In F: Proliferation of the chorioallantoic branches (arrow) inside the labyrinth. (G,H) Transmission electron microscopy, mid-gestation. Cytotrophoblast (Ct) and syncytiotrophoblast (St) near a maternal vessel (Mv) and a fetal vessel with endothelium (En).