During the period of rapid growth of the chorion frondosum, at about the fourth to fifth month of gestation, the fetal part of the placenta is divided by the placental (decidual) septa into 15 to 25 areas called cotyledons. Wedge-like placental septa form the boundaries of the cotyledons, and because they do not fuse with the chorionic plate, maternal blood can circulate easily between them. Cotyledons are visible as the bulging areas on the maternal side of the basal plate.
The decidua basalis forms a compact layer known as the basal plate that is the maternal component of the placenta.
maternal sinusoid syncytiotrophoblast cytotrophoblast cytotrophoblastic shell tertiary villus developing vessels in wall of chorionic sac intervillus space
FIGURE 23.23 • Schematic diagrams of sections through a developing human embryo. a. This drawing shows the chorionic sac and placenta at 16 days of development. b. The same embryo at 21 days of development. The diagrams illustrate the separation of the fetal and maternal blood vessels by the placental membrane, which is composed of the endothelium of the capillaries, mesenchyme, cytotrophoblast, and syncytiotrophoblast.
Vessels within this part of the endometrium supply blood to the intervillous spaces. Except for relatively rare rupturing of capillary walls, which is more common at delivery, fetal blood and maternal blood do not mix.
Fetal and maternal blood are separated by the placental barrier.
Separation of the fetal and maternal blood, referred to as the placental barrier, is maintained primarily by the layers of fetal tissue (Fig. 23.25). Starting at the fourth month, these layers become very thin to facilitate the exchange of products across the placental barrier. The thinning of the wall of the villus is caused in part by the degeneration of the inner cytotrophoblast layer.
At its thinnest, the placental barrier consists of the:  Syncytiotrophoblast Discontinuous inner cytotrophoblast layer  Basal lamina of the trophoblast Connective (mesenchymal) tissue of the villus Basal lamina of the endothelium  Endothelium of the fetal placental capillary in the tertiary
This barrier bears a strong resemblance to the air–blood barrier of the lung, with which it has an important parallel function, namely, the exchange of oxygen and carbon dioxide—in this case between the maternal blood and the fetal blood. It also resembles the air–blood barrier by having a particular type of macrophage in its connective tissue— in this instance, the fetal placental antigen–presenting cells (Hofbauer cell).
FIGURE 23.24 • Photomicrographs of a human placenta. a.