Placental Functions
The placenta functions in metabolism, in the transport of substances and in endocrine secretion.
Metabolism:
During early pregnancy, the placenta synthesizes glycogen, cholesterol and fatty acids, which serve as sources of nutrients and energy for the embryo and fetus.
Transport:
The placenta has a very large surface area, which facilitates the transport of substances in both directions. The surface area at 28 weeks is 5 square metres, and at term it is almost 11 square metres. About 5 to 10% of this surface area is extremely thin, measuring only a few microns.
The bulk of the substances transferred from mother to fetus consists of oxygen and nutrients. The fetus eliminates carbon dioxide and waste materials (eg., urea and bilirubin) into the maternal circulation.
The exchange of gases occurs via diffusion. The placenta is also highly permeable to glucose, but much less permeable to fructose and other common disaccharides. Amino acids are transported through speciic receptors. Some proteins are transferred slowly through the placenta, mainly via pinocytosis. The transfer of maternal antibodies (mainly IgG) is important in providing passive immunity to the newborn. Another maternal protein, transferrin, carries iron to the placental surface, from there it is actively transported into the fetal tissues. Steroid hormones easily cross the placental barrier; protein hormones are much more poorly transported (but maternal thyroid hormone gains slow access to the fetus, and fetal insulin can reduce symptoms of maternal diabetes).
The placenta is also very permeable to alcohol and other drugs and to some viruses. These agents can cause birth defects.
Placental hormone synthesis:
The syncytiotrophoblast is an important endocrine organ for much of the pregnancy. It produces both protein and steroid hormones. The major placental hormones are listed below.
Human chorionic gonadotropin (hCG): Synthesis of hCG begins before implantation, and is responsible for maintaining the maternal corpus luteum that secretes progesterone and estrogens. It is the basis for early pregnancy tests. Production peaks at eight weeks and then gradually declines. Structurally, this glycoprotein resembles LH.
Estrogens and progesterone: The placenta can produce progesterone independently from cholesterol precursors, and estrogen in concert with the fetal adrenal gland, as it does not contain all the necessary enzymes itself. By the end of the first trimester, the placenta produces enough of these steroids to maintain the pregnancy and the corpus luteum is no longer needed.
Human placental lactogen (hPL) or human chorionic somatomammotropin (hCS): This hormone is similar to growth hormone and influences growth, maternal mammary duct proliferation, and lipid and carbohydrate metabolism.
Human placental growth hormone: This hormone differs from pituitary GH by 13 amino acids. From 15 weeks until the end of pregnancy, this hormone gradually replaces maternal pituitary GH. Its major function is the regulation of maternal blood glucose levels so that the fetus is ensured of an adequate nutrient supply. Its secretion is stimulated by low maternal blood glucose levels; in turn, it stimulates gluconeogenesis in the maternal liver.
Human chorionic thyrotropin (hCT): Small amounts produced, functions similar to pituitary hormone.
Human chorionic adrenocorticotropin (hACTH): Small amounts produced, functions similar to pituitary hormone.
Insulin-like growth factors: Stimulates proliferation and differentiation of the cytotrophoblast.
Endothelial growth factor: First produced by 4 to 5-week-old placenta; stimulates proliferation of the trophoblast.
Relaxin: Produced by decidua cells; softens the cervix and pelvic ligaments in preparation for childbirth.
In addition, the placenta produces dozens of proteins that have been identified immunologically but whose function is poorly understood.
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