Exchange of gases and metabolic products occurs as the blood passes over the villi. Normally, water, carbon dioxide, metabolic waste products, and hormones are transferred from the fetal blood to the maternal blood; water, oxygen, metabolites, electrolytes, vitamins, hormones, and some antibodies pass in the opposite direction. The placental barrier does not exclude many potentially dangerous agents such as alcohol, nicotine, viruses, drugs, exogenous hormones, and heavy metals. Therefore, during pregnancy, exposure to or ingestion of such agents should be avoided to reduce the risk of injury to the embryo or fetus.
Before the establishment of blood flow through the placenta, the growth of the embryo is supported in part by metabolic products that are synthesized by or transported through the trophoblast. The syncytiotrophoblast synthesizes glycogen, cholesterol, and fatty acids, as well as other nutrients used by the embryo.
The placenta is a major endocrine organ producing steroid and protein hormones.
The placenta also functions as an endocrine organ, producing steroid and peptide hormones as well as prostaglandins that play an important role in the onset of labor. Immunocytochemical studies indicate that the syncytiotrophoblast is the site of synthesis of these hormones.
The steroid hormones progesterone and estrogen have essential roles in the maintenance of pregnancy. As pregnancy proceeds, the placenta takes over the major role in the secretion of these steroids from the corpus luteum. The placenta produces enough progesterone by the end of the eighth week to maintain pregnancy if the corpus luteum is surgically
FIGURE 23.26 • Schematic diagram of mature human placenta. The sagittal section of the uterus (left) with the developing embryo shows the most common location of the placenta. The mature placenta (right) is divided into cotyledons by placental septa that are formed by outgrowths of the decidua basalis. Maternal blood enters the placenta through numerous endometrial spiral arteries that penetrate the basal plate. As the blood enters the cotyledon, it is directed deep into the intervillous spaces (red arrows). It then passes over the surface of the villi, where exchange of gases and metabolic products occurs. The maternal blood finally leaves the intervillous space (black arrows) through endometrial veins. The fetal blood enters the placenta through the umbilical arteries that divide into a series of radially disposed arteries within the chorionic plate. Branches from the vessels pass into the main stem villi and there form extensive capillary networks. The veins within the villi then carry the blood back through a system of veins that parallels the fetal arteries.
removed or fails to function. In the production of placental estrogen, the fetal adrenal cortex plays an essential role, providing the precursors needed for estrogen synthesis. Because the placenta lacks the enzymes needed for the production of estrogen precursors, a cooperative fetoplacental (endocrine) unit is established. Clinically, the monitoring of estrogen production during pregnancy can be used as an index of fetal development.