Untitled Quiz

The Placenta

• The placenta is the primary site of nutrient and gas exchange between the mother and fetus.
• The placenta consists of two components: the fetal part, which develops from the chorionic sac, and the maternal part, which is derived from the endometrium.
• The fetal surface of the placenta is smooth, covered by the amnion, and provides attachment for the umbilical cord.
• The maternal surface is rough and irregular, with 15-30 cotyledons that are separated by fissures.
• At full term, the placenta measures 15-25 cm in diameter, 3 cm thick at the center, weighs 500 gm, and has a surface area of 14 square meters.
• The placenta typically attaches near the fundus of the uterus.

Functions of the Placenta

• The placenta, along with the umbilical cord, acts as a transport system for substances passing between the mother and fetus.
• Nutrients and oxygen pass from the maternal blood to the fetal blood through the placenta.
• Waste materials and carbon dioxide pass from the fetal blood to the maternal blood through the placenta.
• The placenta and fetal membranes perform several functions, including:
  • Protection
  • Nutrition
  • Respiration
  • Excretion
  • Hormone production
• The placenta facilitates the exchange of metabolic and gaseous products between the maternal and fetal bloodstreams.
• It allows the exchange of gases, including the delivery of oxygen (25 ml/minute) and removal of carbon dioxide from the fetal blood.
• The placenta plays a role in the exchange of nutrients, including carbohydrates, lipids, polypeptides, amino acids, and vitamins, as well as electrolytes.
• It transmits maternal antibodies (IgG, immunoglobulins).
• It produces hormones such as progesterone, estrogen (mainly estriol), relaxin, placental lactogen, and HCG.
• It removes waste products including urea, carbon dioxide, and others.
• The placental membrane acts as a protective barrier, preventing maternal and fetal blood from mixing, which helps to prevent antigenic reactions. These activities are vital for sustaining pregnancy and supporting normal fetal development.

Placental Development

• Early placental development is characterized by rapid trophoblast proliferation and the formation of the chorionic sac and chorionic villi.
• By the end of the third week, the anatomical arrangements necessary for maternal-embryonic exchange are established.
• A complex vascular network forms in the placenta by the end of the fourth week, enabling the exchange of gases, nutrients, and waste products between the mother and embryo.
• Chorionic villi initially cover the entire chorionic sac until the eighth week.
• The chorion laeve (smooth chorion) is an avascular area where villi degenerate due to compression and reduced blood supply.
• The villous chorion (chorion frondosum) is a bushy area with rapidly growing and branching villi connected to the decidua basalis.
• The villous chorion forms the fetal part of the placenta, with villi projecting into the intervillous space containing maternal blood.
• The maternal part of the placenta is formed by the decidua basalis.
• By the end of the fourth month, the decidua basalis is mostly replaced by the fetal component of the placenta.
• The fetal part of the placenta attaches to the maternal part via the cytotrophoblastic shell.
• Endometrial arteries and veins pass through the cytotrophoblastic shell and enter the intervillous space.
• The placenta's shape is determined by the persistent area of chorionic villi, typically forming a discoid shape.
• Decidual tissue erodes, expanding the intervillous space and forming placental septa.
• Placental septa divide the fetal part of the placenta into cotyledons, each containing stem villi and their branches.
• By the end of the fourth month, the decidua basalis is mostly replaced by the cotyledons.
• The decidua capsularis fuses with the decidua parietalis, obliterating the uterine cavity.

Placental Circulation

• The intervillous spaces of a mature placenta contain approximately 150 ml of maternal blood, exchanged about 3-4 times per minute.
• Fetal blood enters the placenta through two umbilical arteries that branch into capillaries within the villi.
• Gas exchange occurs between fetal blood in the villi capillaries and maternal blood in the intervillous space.
• Oxygenated fetal blood returns to the fetus through the umbilical vein.
• The intervillous space develops from lacunar networks in the syncytiotrophoblast.
• Placental septa divide the intervillous space into compartments, but there is free communication between them.
• Maternal blood enters the intervillous space from spiral endometrial arteries.
• Maternal blood is drained by endometrial veins.
• Branch villi are continuously bathed in maternal blood, allowing the exchange of oxygen, nutrients, and waste products.

Amniotic Fluid

• The amniotic sac enlarges faster than the chorionic sac, resulting in the fusion of the amnion and smooth chorion to form the amniochorionic membrane.
• Amniotic fluid plays a crucial role in fetal development, contributing to:
  • Symmetric external growth
  • Protection against infection
  • Normal lung development
  • Prevention of amnion adherence
  • Cushioning against injuries
  • Temperature regulation
  • Free movement for muscular development
  • Fluid and electrolyte homeostasis
• Most amniotic fluid derives from maternal tissue and interstitial fluid.
• Fluid is also secreted by the fetal respiratory and gastrointestinal tracts.
• Beginning in the 11th week, the fetus contributes urine to the amniotic fluid.
• Amniotic fluid volume increases with gestation, reaching 30 ml at 10 weeks, 350 ml at 20 weeks, and 700-1000 ml by 37 weeks.
• The water content of amniotic fluid is constantly refreshed, with water moving into maternal tissue and capillaries.
• The fetus swallows amniotic fluid, which is absorbed by the respiratory and digestive tracts.
• Excess water in the fetal blood is excreted and returned to the amniotic sac.

Twin Placenta

• Twins can be dizygotic (DZ) or monozygotic (MZ).
• DZ twins result from the fertilization of two oocytes and develop from two zygotes.
• DZ twins may be of the same sex or different sexes and are no more genetically alike than siblings born at different times.
• DZ twins always have two amnions and two chorions, but the chorions and placentas may be fused.
• MZ twins result from the fertilization of one oocyte and develop from one zygote.
• MZ twins are of the same sex and are genetically identical.
• MZ twinning usually begins in the blastocyst stage and results in two embryos, each in its own amniotic sac, sharing a common placenta (monochorionic-diamniotic).
• A less common variation in MZ twins involves the early separation of embryonic blastomeres, leading to two amnions, two chorions, and two placentas that may or may not be fused.

Bottom Line

• Fetal membranes are extraembryonic structures that develop from the zygote but are not part of the embryo itself.
• Fetal membranes include the chorion, amnion, yolk sac, allantois, and umbilical cord.
• Functions of fetal membranes include protection, nutrition, waste excretion, and respiration.
• After delivery, the fetal membranes are expelled from the uterus along with the placenta and umbilical cord.
• The placenta acts as a temporary endocrine gland that connects the fetus to the uterus via the umbilical cord.
• Placental development involves both fetal and maternal components, including the chorion and decidua basalis.
• Functions of the placenta include hormone production, gas exchange, transmission of antibodies, protection against infection, and temporary storage.
• The placenta is not considered to be a part of the fetal membranes.