Which of the inner layer of the foetal sac?
A. Chorian
B. Amnion
C. Yolk sac
D. Decidua E
Explanation
Inner layer of the foetal sac: 👉 Amnion
Explanation
The foetal sac is formed by two main membranes:
- Amnion (inner layer)
- Chorion (outer layer)
🔹 Amnion (Correct answer)
- It is the innermost membrane of the foetal sac.
- Directly surrounds the fetus.
- Forms the amniotic cavity, which contains amniotic fluid.
- Functions:
- Protects the fetus from mechanical injury
- Maintains constant temperature
- Allows free movement of the fetus
- It is thin, transparent, and avascular.
🔹 Chorion (Outer layer – not correct for this question)
- Lies outside the amnion
- Contributes to placenta formation
- Has chorionic villi involved in maternal–fetal exchange
✅ Conclusion
The inner layer of the foetal sac is the Amnion.
Other Options Details
Here is a clear description of each option related to fetal membranes and pregnancy:
A. Chorion
- The chorion is the outermost fetal membrane.
- It develops from the trophoblast and extra-embryonic mesoderm.
- It forms chorionic villi, which take part in placenta formation.
- It lies outside the amnion and is in contact with the decidua (maternal tissue).
- Function: Helps in gas exchange, nutrition, and waste removal via the placenta.
B. Amnion
- The amnion is the innermost fetal membrane.
- It encloses the amniotic cavity, which is filled with amniotic fluid.
- The fetus floats in this fluid.
- Functions:
- Protects the fetus from mechanical injury
- Maintains constant temperature
- Allows free fetal movements
- Prevents adhesion of the fetus to membranes
C. Yolk Sac
- The yolk sac is an early embryonic membrane.
- It is important in early pregnancy, especially before placental circulation is established.
- Functions:
- Early nutrition to the embryo
- First site of blood cell formation (hematopoiesis)
- Gives rise to primordial germ cells
- It usually degenerates later in pregnancy.
D. Decidua
- The decidua is the modified endometrium of the uterus during pregnancy.
- It is maternal tissue, not a fetal membrane.
- Types:
- Decidua basalis – forms the maternal part of placenta
- Decidua capsularis – covers the embryo
- Decidua parietalis – lines the rest of the uterus
- Function: Provides nutrition and immune protection to the fetus.
Key Summary
Structure Origin Position Chorion Fetal Outermost membrane Amnion Fetal Innermost membrane Yolk sac Fetal Early embryonic structure Decidua Maternal Modified uterine lining
Correct Answer: B. Amnion
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Writer: Vandita Singh, Lucknow (GS India Nursing Group