The placenta consists of the placental disk, the extraplacental free membranes, and the umbilical cord. The fetal surface of the disk is the chorionic plate (Figure 2), and the cotyledons and basal plate constitute the maternal surface (Figure 3). The fetal surface, the chorionic plate, is covered by amnion, and normally the cord is inserted here. The free membranes are normally inserted at the margins of the disk. On microscopic examination, the following structures are noted:

(1) Placental disk: from the fetal to the maternal surface (Figure 4);

(a) Chorionic plate (fetal surface) (Figure 5): amnion, chorion, subchorionic fibrin, larger branches of umbilical blood vessels in the chorion, and smaller branches in the stem villi;

(b) Placental parenchyma (Figure 6): stem villi with smaller branches of the umbilical

blood vessels, intermediate villi, and terminal villi, and the intervillous space; (c) Placental septa and extravillous trophoblast cell islands (Figure 7): during

embryogenesis most of the trophoblast contributes to the development of the villi; however, the extravillous trophoblast forms other chorionic portions of the placenta, i.e. the chorionic plate, chorion leve, septa, and cell islands. The septa arise from the basal plate and subdivide the placenta into grossly detectable cotyledons; the septa are identifiable as irregular grooves on the maternal surface. (As a point of clarification, a

cotyledon, therefore, does not actually correspond to a true, functional unit of the placenta, since a single cotyledon may be supplied by a number of decidual arteries.) The septa rarely reach the fetal surface and contain extravillous trophoblasts (the so-called ‘X’ cells and intermediate trophoblasts) and a few decidual cells. Cell islands, composed primarily of extravillous trophoblast, fibrinoid, and possibly a few decidual cells, are connected to the villous tree or chorionic plate;

(d) Basal plate (Figure 8): fibrin, extravillous trophoblast cells, decidua basalis, and maternal blood vessels.