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. (Based on Moore KL, Persaud TVN. The Developing Human, Clinically Oriented Embryology. Philadelphia: WB Saunders, 1993.)
blood vessels that have developed in their connective tissue cores (Fig. 22.24b)
As the tertiary villi are forming, cytotrophoblastic cells in the villi continue to grow out through the syncytiotrophoblast. When they meet the maternal endometrium, they grow laterally and meet similar processes growing from neighboring villi. Thus, a thin layer of cytotrophoblastic cells, called the trophoblastic shell, is formed around the syncytiotrophoblast. The trophoblastic shell is interrupted only at sites where maternal vessels communicate with the intervillous spaces. Future growth of the placenta is accomplished by interstitial growth of the trophoblastic shell.
Two types of cells are recognized in the connective tissue stroma of the villi: mesenchymal cells and Hofbauer cells (Fig. 22.25). Hofbauer cells are more common in the early placenta. They appear to be macrophages. The vacuoles in these cells contain lipids, glycosaminoglycans, and glycoproteins. Recent studies of HIV-infected placentas indicate that HIV is primarily localized within Hofbauer cells as well as in the syncytiotrophoblast.
Early in development, the blood vessels of the villi become connected with vessels from the embryo
Blood begins to circulate through the embryonic cardiovascular system and the villi at about 21 days. The intervillous spaces provide the site of exchange of nutrients, metabolic products and intermediates, and wastes between the maternal and fetal circulatory systems.
During the first 8 weeks, villi cover the entire chorionic surface, but as growth continues, villi on the decidua cap-sularis begin to degenerate, producing a smooth, relatively avascular surface called the chorion laeve. The villi adjacent to the decidua basalis rapidly increase in size and number and become highly branched. This region of the chorion, which is the fetal component of the placenta, is called the chorion frondosum or villous chorion. The layer of the placenta from which the villi project is called the chorionic plate.
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, which is the maternal component of the placenta. 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. 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 due in part to the degeneration of the inner cytotrophoblast layer.
At its thinnest, the placental barrier consists of the
• Discontinuous inner cytotrophoblast layer
cytotrophoblastic shell tertiary villus syncytiotrophoblast intervillous space maternal blood maternal sinusoid
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