Placenta, Extraembryonic Membranes, and Multiple Pregnancies Part 3

Placental Circulation • The placental circulation facilitates the exchange of materials between maternal and fetal blood, vital for the growth and development of the fetus. • Both the fetus and the mother contribute to this circulation, with distinct roles and structures involved. Fetal Circulation • Umbilical Arteries and Vein: Poorly oxygenated blood from the fetus travels to the placenta via two umbilical arteries, which branch extensively within the chorionic plate before entering the chorionic villi. These branches form capillary networks in the terminal villi, where the exchange of gases and nutrients occurs. The oxygen-rich blood then returns to the fetus through the umbilical vein. • Chorionic Villi: The chorionic villi are essential structures where this exchange happens. These villi have a large surface area, and the fetal blood is brought extremely close to the maternal blood, separated only by the thin placental membrane. The placental membrane initially consists of four layers (endothelial lining of fetal vessels, connective tissue, cytotrophoblastic layer, and syncytium) but thins out as the pregnancy progresses, increasing the efficiency of the exchange. Maternal Circulation • Maternal blood enters the placenta through 80 to 100 spiral arteries, which discharge blood into the intervillous spaces. Unlike the fetal blood, maternal blood is not contained within vessels but flows freely in these intervillous spaces, forming a “lake” that bathes the chorionic villi. • The blood enters the intervillous space under high pressure, which forces it deep into the space and allows it to flow slowly over the branch villi. This slow flow maximizes the exchange of oxygen and nutrients with the fetal blood. The blood eventually returns to the maternal circulation via endometrial veins. • The intervillous space contains about 150 mL of maternal blood, which is replenished three to four times per minute to ensure continuous and adequate exchange. Placental Membrane • The placental membrane is a structure composed of extrafetal tissues that separates the maternal and fetal blood. Initially, up to about 20 weeks of pregnancy, the placental membrane consists of four layers: 1. 2. 3. 4. Syncytiotrophoblast Cytotrophoblast Connective tissue of the villi Endothelium of fetal capillaries • After 20 weeks, the cytotrophoblast in many villi becomes thinner and eventually disappears in large areas, reducing the placental membrane to three layers. • In some regions, the membrane becomes so thin that the syncytiotrophoblast directly contacts the fetal capillary endothelium, forming a vasculosyncytial placental membrane. • The syncytiotrophoblast has microvilli that increase the surface area for material exchange. • As pregnancy progresses, the membrane becomes thinner, bringing fetal capillary blood very close to maternal blood in the intervillous space. • During the third trimester, syncytiotrophoblast nuclei aggregate to form syncytial knots, which can break off and enter the maternal circulation, where they are eventually destroyed. • The chorionic villi, with their extensive branching and microvilli on the syncytium, significantly increase the surface area for exchange. • Only villi with fetal vessels in close contact with the syncytial membrane participate in this exchange. • The placental membrane (also called as a barrier) allows many substances to pass freely between maternal and fetal blood. • The effectiveness of this exchange is critical for fetal health. • If the maternal blood supply to the placenta is compromised, it can lead to fetal hypoxia, intrauterine growth restriction (IUGR), or even fetal death in severe cases.