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Placenta
Medical expert of the article
Last reviewed: 04.07.2025
[ Functions of the placenta
The placenta provides nutrition to the fetus, supplies it with oxygen, and removes metabolic waste from the fetus. The placenta protects the fetus from harmful substances (protective, barrier function). The blood of the mother and fetus does not mix in the placenta due to the presence of the so-called hematoplacental barrier. This barrier is formed by the walls of the uterine and fetal vessels and adjacent tissues located close to each other 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 membrane of the trophoblast and syncytiotrophoblast. Nutrients, vitamins, and some hormones enter the fetus’s blood through this barrier by passive and active transport. Certain substances circulating in the mother’s blood are absorbed by the syncytiotrophoblast and do not enter the fetus’s blood due to the barrier function of the placenta.
Structure of the placenta
The placenta is shaped like a disk, about 20 cm in diameter and about 5 cm thick in the center. The umbilical cord extends from the placenta to the fetus, containing the umbilical vessels (two arteries and a vein). By the end of pregnancy, the placenta occupies an area of about half the inner surface of the uterus. The placenta is formed after the implantation of the embryo due to the growing trophoblast (embryo membrane) and the decidual (rejected) part of the mucous membrane of the uterus, with the help of which the placenta is attached to its wall. Numerous villi are formed from the growing trophoblast, and the cells covering them lose their boundaries and turn into the so-called trophoblastic syncytium (syncytiotrophoblast). This syncytium ensures the growth of villi into the mucous membrane, which facilitates the introduction of the embryo into the wall of the uterus. The fetal part of the placenta is formed by the trophoblast, which transforms into a villous membrane - the chorion with blood vessels (capillaries) of the fetus that have grown into the villi. The maternal part of the placenta is formed from the mucous membrane underlying the embryo that has implanted itself in the wall of the uterus. This part of the mucous membrane is called the basal decidua. In it, which is a layer of the endometrium, are located the uterine glands, and spiral arteries and veins pass. These blood vessels open into a narrow space (intervillous), limited by the surface of the decidua and the villi of the chorion, covered with a layer of syncytiotrophoblast.
The villous part of the chorion (the fetal part of the placenta) contains about 200 so-called main villi, branching many times to terminal villi. The total surface area of all the villi, washed by the mother's blood entering the intervillous space, reaches 7 m.
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