Maternal Circulation

decidua basalis stump of main stem villi main stem villus umbilical arteries

decidua basalis placental septa cotyledons basal plate endometrial spiral artery amnion placental septa cotyledons basal plate endometrial spiral artery endometrial vein myometrium 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 cany the blood back through a system of veins that parallels the fetal arteries.

of the intervillous spaces, which contain about 150 mL of maternal blood that is exchanged 3 to 4 times per minute. The blood pressure in the spiral arteries is much higher than that in the intervillous spaces. As blood is injected into these spaces at each pulse, it is directed deep into the spaces. As the pressure decreases, the blood flows back over the surfaces of the villi and eventually enters endometrial veins also located in the base of the spaces.

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 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.

The following peptide hormones are secreted by the placenta:

• hCG, the synthesis of which begins around day 6, even before syncytiotrophoblast formation. hCG exhibits marked homology to pituitary thyroid-stimulating hormone (TSH) and stimulates the maternal thyroid gland to increase secretion of tetraiodothyronine (T4). It also maintains the corpus luteum during early pregnancy. Measurement of hCG is used to detect pregnancy and assess early embryonic development.

• Human chorionic somatomammotropin (hCS), also known as human placental lactogen (hPL), is closely related to human growth hormone. Synthesized in the syncytiotrophoblast, it promotes general growth, regulates glucose metabolism, and stimulates mammary duct proliferation in the maternal breast. hCS effects on maternal metabolism are significant, but the role of this hormone in fetal development remains unknown.

The mature placenta measures about 15 to 20 cm in diameter and 2 to 3 cm in thickness, covers 25 to 30% of the uterine surface, and weighs 500 to 600 g at term. The surface area of the villi in the human placenta is estimated to be about 10 m2. The microvilli on the syncytiotrophoblast increase the effective area for metabolic exchange to more than 90 m2. After birth, the uterus continues to contract, reducing the luminal surface and inducing placental separation from the uterine wall. The entire fetal portion of the placenta, fetal membranes, and the intervening projections of decidual tissue are released. During uncomplicated labor, the placenta is delivered approximately 30 minutes after birth.

After delivery of the placenta, the endometrial glands and stroma of the decidua basalis regenerates. Endometrial regeneration is completed by the end of the third week postpartum except at the placental site, where regeneration usually extends through the next 3 weeks. During the first week after delivery, remnants of the decidua are shed and constitute the blood-tinged uterine discharge known as the lochia rubra.

• IGF-I and IGF-II are produced by and stimulate proliferation and differentiation of the cytotrophoblast.

• Endothelial growth factor (EGF) exhibits an age-dependent dual action on the early placenta. In the 4-to 5-week-old placenta, EGF is synthesized by the cytotrophoblast and stimulates proliferation of the tro-phoblast. In the 6- to 12-week-old placenta, synthesis of EGF is shifted to the syncytiotrophoblast; it then stimulates and maintains the function of the differentiated trophoblast.

• Relaxin is synthesized by decidual cells and is involved in the "softening" of the cervix and the pelvic ligaments in preparation for parturition.

Leptin is synthesized by syncytiotrophoblast, particularly during the last month of gestation. Leptin appears to regulate maternal nutrient storage to the nutrient requirements of the fetus. It is also involved in transporting nutrients across the placental barrier from mother to the fetus.

• Other growth factors stimulate cytotrophoblastic growth (e.g., fibroblast growth factor, colony-stimulat-ing factor [CSF-1], platelet-derived growth factor, and interleukins [IL-1 and IL-3]) or inhibit trophoblast growth and proliferation (e.g., tumor necrosis factor).

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