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Placenta
Last reviewed: 23.04.2024
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[Functions of the placenta
Through the placenta, the fetus is fed, supplied with oxygen, removed from the fetal body metabolic products. The placenta protects the fetus from harmful substances (protective, barrier function). The blood of the mother and fetus in the placenta is not mixed due to the presence of the so-called hematoplacental barrier. This barrier is formed by closely located walls of the vessels of the uterus and fetus and adjacent tissues in the placenta. The hematoplacental barrier consists of the endothelium of the fetal capillaries, a layer of loose connective tissue surrounding the capillaries, the basal membranes of the trophoblast and syncytiotrophoblast. Through this barrier, through passive and active transport, nutrients, vitamins, and certain hormones enter the fetal blood. Certain substances that circulate in the maternal blood are absorbed by the syncytiotrophoblast and do not enter the fetal blood due to the barrier function of the placenta.
Structure of the placenta
The placenta has a disk shape with a diameter of about 20 cm and a thickness of about 5 cm in the center. From the placenta, the umbilical cord, containing umbilical vessels (two arteries and a vein) leaves the fetus. By the end of pregnancy, the placenta occupies an area of about half of the inner surface of the uterus. The placenta is formed after implantation of the embryo due to the expanding trophoblast (embryo envelope) and the decidual (tearing off) part of the uterine mucosa, with which the placenta is attached to its wall. From the expanding trophoblast numerous villi are formed, and the cells covering them lose their borders and become a so-called trophoblastic syncytium (syncytiotrophoblast). This syncytium ensures the germination of the villi in the mucous membrane, which facilitates the introduction of the embryo into the wall of the uterus. Due to the trophoblast, which turns into a villous shell - a chorion with ingrown into the villi blood vessels (capillaries) of the fetus, a fetal part of the placenta is formed. The maternal part of the placenta is formed from the mucous membrane underlying the embryo implanted in the uterine wall. This part of the mucosa was called the basal decidual membrane. In it, which is a layer of the endometrium, the uterine glands are located, spiral arteries and veins pass. These blood vessels open into a narrow space (intervillaceous), bounded by the surface of the decidua and the villi of the chorion, covered with a layer of syncytiotrophoblast.
The nasal part of the chorion (the fetal part of the placenta) contains about 200 so-called main villi, repeatedly branched to the final villi. The total surface area of all the villi, washed by the blood of the mother, entering the intervillaceous space, reaches 7 m.