The placenta is a disc-shaped organ that serves for the exchange of materials between the fetal and maternal circulations during pregnancy. It develops primarily from embryonic tissue, the chorion frondosum. One side of the placenta is embedded in the uterine wall at the basal plate. The other side faces the amniotic cavity that contains the fetus. After birth, the placenta separates from the wall of the uterus and is discharged along with the contiguous membranes of the amniotic cavity. The umbilical cord connects the fetus to the placenta. It contains two arteries that carry blood from the fetus to the placenta and a vein that returns blood from the placenta to the fetus. The umbilical arteries have thick muscular walls. These are arranged as two layers, an inner longitudinal layer and an outer circular layer. Elastic lamellae are poorly developed in these vessels and, indeed, may be absent. The umbilical vein is similar to the arteries, also having a thick muscular wall arranged as an inner longitudinal and an outer circular layer.
Placenta, human, H&E ×16.
A section extending from the amniotic surface into the substance of the placenta is shown here. This includes the amnion (A), the chorionic plate (CP), and the chorionic villi (CV). The amnion consists of a layer of simple cuboidal epithelium and an underlying layer of connective tissue. The connective tissue of the amnion is continuous with the connective tissue of the chorionic plate as a result of their fusion at an earlier time. The plane of  fusion, however, is not evident in H&E sections; the separation (asterisks) in parts of this figure in the vicinity of the fusion is an artifact.
The chorionic plate is a thick connective tissue mass that contains the ramifications of the umbilical arteries and vein. These vessels (BVp) do not have the distinct organizational features characteristic of arteries and veins; rather, they resemble the vessels of the umbilical cord. Although their identification as blood vessels is relatively simple, it is difficult to distinguish which vessels are branches of an umbilical artery and which are tributaries of the vein.
The main substance of the placenta consists of chorionic villi of different sizes (see Plate 100). These emerge from the chorionic plate as large stem villi that branch into increasingly smaller villi. Branches of the umbilical arteries and vein (BVv, in figure below) enter the stem villi and ramify through the branching villous network. Some villi extend from the chorionic plate to the maternal side of the placenta and make contact with the maternal tissue; these are called anchoring villi. Other villi, the free villi, simply arborize within the substance of the placenta without anchoring onto the maternal side.
Placenta, human, H&E ×70; inset ×370.
The maternal side of the placenta is shown in this figure. The basal plate, or stratum basale (SB), is on the right side of the illustration. This is the part of the uterus to which the chorionic villi anchor. Along with the usual connective tissue elements, the basal plate contains specialized cells called decidual cells (DC). The same cells are shown at higher magnification in the inset.
Decidual cells are usually found in clusters and have an epithelial appearance. Because of these features, they are easily identified.
Septa from the basal plate extend into the portion of the placenta that contains the chorionic villi. The septa do not contain the branches of the umbilical vessels and, on this basis, can frequently be distinguished from stem villi or their branches.
PLATE 99 A, amnion BVp, blood vessels in chorionic plate BVv, blood vessels in chorionic villi CP, chorionic plate CV, chorionic villi DC, decidual cells SB, stratum basale asterisk, separation that is actually an artifact KEY