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.)
• 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 villus
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 Hofbauer cell.
The placenta is the site of exchange of gases and metabolites between the maternal and fetal circulation
Fetal blood enters the placenta through a pair of umbilical arteries (Fig. 22.26). As they pass into the placenta, these arteries branch into several radially disposed vessels that give numerous branches in the chorionic plate. Branches from these vessels pass into the villi, forming extensive capillary networks in close association with the intervillous spaces. Gases and metabolic products are exchanged across the thin fetal layers that separate the two bloodstreams at this level. Antibodies can also cross this layer and enter the fetal circulation to provide passive immunity against a variety of infectious agents, e.g., those of diphtheria, smallpox, and measles. Fetal blood returns through a system of veins that parallel the arteries except that they converge on a single umbilical vein.
Maternal blood is supplied to the placenta through 80 to 100 spiral endometrial arteries that penetrate the basal plate. Blood from these spiral arteries flows into the base
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