15.2 The Mammalian Placenta

15.2 The Mammalian Placenta

Two tissues build one organ

A bird embryo can live on yolk and on gases that cross the shell. A placental mammal soon needs a different organ. The placenta delivers oxygen and nutrients, carries away carbon dioxide and urea, and secretes hormones that keep the pregnancy going. It is built from two partners. The fetal partner is the chorion, whose chorionic villi invade the uterine lining. The maternal partner is the endometrium, the lining of the uterus. Both are required. A placenta described as maternal tissue alone, or as the yolk sac under a new name, is not this organ.

Formation starts with the trophoblast, the outer cell layer of the blastocyst. After the embryo attaches to the uterus, trophoblast cells grow into the endometrium. As development continues, the chorion extends villi into that same lining. Each villus is a fetal structure. Fetal blood vessels run inside it. Maternal blood occupies spaces beside the villi. In the human placenta those spaces form the intervillous space, so maternal blood surrounds the villi while fetal blood remains inside fetal capillaries.

The two bloodstreams come very close, often separated by a thin trophoblast layer. They do not normally mix. Whole blood does not cross, and maternal blood does not flow inside fetal vessels. That separation is the normal design. Exchange happens by diffusion and by membrane transport across the barrier. A stem that sends maternal red cells up the umbilical cord has removed the barrier the placenta is built to keep. Closeness makes exchange fast. It does not make the two circulations one circulation.

Cargo and direction

What moves toward the fetus

Oxygen moves toward the fetus, from maternal blood into fetal blood. Nutrients move the same direction. Glucose, amino acids, and ions leave the maternal side and enter fetal capillaries in the villi. Some of these substances diffuse. Others, including glucose, cross on transport proteins in the trophoblast. The fetus is growing quickly, and this inbound traffic is its food and its oxygen supply. The placenta is doing the work that yolk did in the bird egg, but the supply is continuous instead of a one-time deposit of yolk.

What moves toward the mother

Carbon dioxide and urea move toward the mother. The fetus produces both as its cells metabolize. Carbon dioxide diffuses into maternal blood and leaves when the mother exhales. Urea diffuses into maternal blood and leaves in the mother's urine. The placenta does not store that waste the way an allantois stores uric acid in a shell. It hands the waste to the mother's circulation, and her lungs and kidneys finish the disposal. Direction matters on exam items: oxygen and nutrients inbound, carbon dioxide and urea outbound.

Hormones that hold the lining

Human chorionic gonadotropin

The early trophoblast, and later the placenta, secretes human chorionic gonadotropin (hCG). Pregnancy tests detect this hormone in maternal urine or blood. Its essential early target is the corpus luteum, the temporary endocrine structure left on the ovary after ovulation. hCG keeps the corpus luteum producing progesterone. Progesterone maintains the endometrium so the lining is not shed. Estrogen from the early corpus luteum supports that lining as well. If hCG were missing in the first weeks, the corpus luteum would regress, progesterone would fall, and the pregnancy would lose the lining that holds it.

The trophoblast is the source of the signal. The corpus luteum is the ovary's responder. Mixing those two roles is a common miss: the trophoblast does not itself replace progesterone during those first weeks, and the corpus luteum does not secrete hCG.

Steroids from the placenta itself

The corpus luteum does not have to carry the pregnancy alone until birth. Later the placenta produces progesterone and estrogen in amounts that maintain the uterine lining. The handoff happens during the first trimester, after the placenta's own steroid production is underway. The placenta is therefore an endocrine organ as well as an exchange organ. Early on, hCG saves the corpus luteum. After placental steroid output is established, progesterone and estrogen from the placenta hold the endometrium. Removing the ovary would threaten the lining early. It would not, by itself, remove the later placental source of those steroids.

The umbilical cord

The umbilical cord connects the fetus to the placenta. The vessels inside it are fetal vessels. Two umbilical arteries carry oxygen-poor blood from the fetus to the placental villi. There the blood can give up carbon dioxide and urea and take up oxygen and nutrients. One umbilical vein carries oxygen-rich blood from the placenta back toward the fetus. The pattern surprises anyone who has just memorized adult systemic arteries, because those adult arteries leave the heart rich in oxygen. In the cord, the arteries are on their way out for a refill, so they are the oxygen-poor pair. The vein is the oxygen-rich return.

The shunts that route this inbound blood around the fetal lungs, and past much of the liver, belong to the next section. For the placenta, the cord facts stop at direction and ownership. Fetal blood travels in the cord. Maternal blood stays on the maternal side of the villus barrier. The cord is a pipeline between fetus and placenta, not a hose that lets the mother's bloodstream enter the fetal heart.