Human Anatomy
Fetal Membranes and Placenta
Fetal membranes and placenta for MBBS: chorion, amnion, yolk sac, allantois, placental structure and function, umbilical cord and twinning, mapped to NMC codes AN80.1 to AN80.7.
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Fetal Membranes and Placenta
Fetal membranes and placenta for MBBS: chorion, amnion, yolk sac, allantois, placental structure and function, umbilical cord and twinning, mapped to NMC codes AN80.1 to AN80.7.
This chapter describes the formation and fate of the fetal membranes, the structure and functions of the placenta and umbilical cord, and the embryological basis of twinning. It covers the placental barrier and the endocrine role of the placenta.
High-yield: Fetal Membranes and Placenta
- The fetal membranes are the chorion, amnion, yolk sac and allantois, which support the developing embryo.
- The chorion is formed by the trophoblast and extra-embryonic mesoderm and contributes the fetal part of the placenta.
- The amnion encloses the amniotic cavity and the amniotic fluid that cushions the fetus.
- Amniotic fluid protects the fetus, allows movement and is partly formed by fetal urine in later pregnancy.
- The yolk sac is an early site of blood cell formation and gives rise to the primordial germ cells.
- The allantois is involved in early blood formation and its remnant becomes the urachus and median umbilical ligament.
- The placenta has a fetal part from the chorion frondosum and a maternal part from the decidua basalis.
- The functional units of the placenta are the chorionic villi, across which exchange occurs.
- The placental barrier separates maternal and fetal blood and thins as pregnancy advances.
- The placenta transfers oxygen and nutrients to the fetus and removes carbon dioxide and waste.
- The placenta is an endocrine organ secreting human chorionic gonadotrophin, progesterone, oestrogen and placental lactogen.
- The umbilical cord normally contains two umbilical arteries and one umbilical vein in a jelly of Wharton.
- The umbilical vein carries oxygenated blood to the fetus and the umbilical arteries carry deoxygenated blood away.
- Monozygotic twins arise from one fertilised ovum, while dizygotic twins arise from two separately fertilised ova.
- The degree of shared membranes in monozygotic twins depends on the timing of the split of the inner cell mass.
Placenta and umbilical cord essentials
- **Placental parts:** Fetal part from the chorion frondosum, maternal part from the decidua basalis; chorionic villi are the exchange units.
- **Placental functions:** Gas, nutrient and waste exchange, plus secretion of hCG, progesterone, oestrogen and placental lactogen.
- **Umbilical cord:** Two arteries and one vein in Wharton's jelly; the vein carries oxygenated blood to the fetus.
- **Twinning:** Monozygotic from one ovum, dizygotic from two ova; shared membranes depend on the timing of the split.
NMC competencies in this chapter
- **AN80.1:** Formation, function and fate of the chorion, amnion, yolk sac, allantois and decidua
- **AN80.2:** Formation and structure of the umbilical cord
- **AN80.3:** Formation of the placenta, its functions, fetomaternal circulation and placental barrier
- **AN80.4:** Embryological basis of monozygotic and dizygotic twinning
- **AN80.5:** Role of placental hormones in uterine growth and parturition
- **AN80.6:** Embryological basis of the estimation of fetal age
- **AN80.7:** Types of umbilical cord attachment
Frequently Asked Questions
What are the fetal membranes?
They are the chorion, amnion, yolk sac and allantois. Together they protect and nourish the embryo, and the chorion contributes the fetal part of the placenta.
What does the umbilical cord contain?
Normally two umbilical arteries and one umbilical vein embedded in Wharton's jelly. The vein carries oxygenated blood to the fetus and the arteries carry deoxygenated blood back to the placenta.
What functions does the placenta perform?
It exchanges gases, nutrients and waste between mother and fetus and acts as an endocrine organ, secreting human chorionic gonadotrophin, progesterone, oestrogen and placental lactogen.
How do monozygotic and dizygotic twins differ?
Monozygotic twins arise from a single fertilised ovum that splits, so they are genetically identical. Dizygotic twins arise from two separately fertilised ova and are no more alike than ordinary siblings.
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