What is the Difference Between Amnion and Allantois

The main difference between amnion and allantois is that amnion is the thin layer in direct contact with the developing embryo whereas allantois is an extra-embryonic layer present between amnion and chorion.

Amnion and allantois are two of the four extra-embryonic layers important in the prenatal development of the embryo, providing nourishment and protection for its growth in placental mammals. All the extra-embryonic layers have different functions in the protection and growth of the embryo.   

Key Areas Covered

1. What is Amnion
   • Definition, Structure, Function
2. What is Allantois
   • Definition, Structure, Function
3. Similarities Between Amnion and Allantois
   • Outline of Common Features
4. Difference Between Amnion and Allantois
   • Comparison with Key Differences

Key Terms

Allantois, Amnion, Extra-Embryonic Layer

What is Amnion

The amnion is the innermost layer that covers the embryo. It is filled with a clear liquid known as amniotic fluid. The expansion of the amniotic fluid eventually makes up the amniotic sac. the amnion is avascular, and hence, it does not contain blood vessels. Therefore, the nearby chorionic and amniotic fluid provides the necessary nutrients and oxygen. Cuboidal and columnar epithelial layers are the main characteristic feature of the amnion. The columnar cells occur in the vicinity of the placenta, whereas the cuboidal cells are found in the periphery. In early pregnancy, microvilli cover the amniotic epithelium, increasing the number of microvilli throughout the pregnancy. Here, the main function of microvilli is to associate a densely-packed glycocalyx with anionic binding sites that synthesize the intra-amniotic lipids.

Figure 2: Amnion

Furthermore, the main function of the amnion is to maintain the shape of the embryo with the help of the suspended amniotic fluid. The amniotic also fluid provides equal pressure throughout the amnion, forming a cushion. Inside the embryo sac, the embryo flows freely, contributing to the development of the skeletomuscular activity. At the time of birth, the amniotic fluid is released, signaling the child is ready to come out.

What is Allantois

The allantois is the layer present between the amnion and chorion. It is sac-like, filled with a clear liquid, and expands between the two layers serving as the temporary respiratory organ. Moreover, its cavity stores fetal excretions. Allantois contains blood vessels, allowing the exchange of gases from the developing embryo. However, these blood vessels eventually form the arteries and veins of the umbilical cord.

Figure 1: Allantois

Moreover, the allantois forms urachus: a canal-like, fiber-like structure that is a remnant of the allantois, serving as a site for drainage of the urinary bladder of the fetus. It eventually joins the umbilical cord.

Similarities Between Amnion and Allantois

• Amnion and allantois are two extra-embryonic layers while the other two are yolk sac and chorion.
• Other than the protection of the embryo, they have different functions.

Difference Between Amnion and Allantois

Definition

Amnion refers to the innermost membrane that encloses the embryo of a mammal, bird, or reptile while allantois refers to a hollow sac-like structure filled with clear fluid that forms part of a developing amniote’s conceptus.  

Growth

Usually, amnion expands throughout the pregnancy while allantois stops growing once fused with the chorion and umbilical cord.

Forms

Amnion forms an amniotic sac while allantois fuse to the umbilical cord.

Location

Generally, amnion occurs as the innermost layer that covers the embryo while allantois occurs in between amnion and chorion.  

Blood Vessels

In addition, amnion does not contain blood vessels while allantois contains blood vessels.

Function

Amnion equalizes the pressure inside the embryo, allowing free movement of the embryo while allantois acts as the temporary respiratory organ and stores excretory materials.

Conclusion

In brief, the amnion is the innermost layer that covers the embryo. It is a non-vascular structure, forming the amniotic sac. On the other hand, allantois occurs between amnion and chorion. Moreover, it is a vascular structure that stores fetal excretory materials. Therefore, the main difference between amnion and allantois is their structure and function.

References:

1. Encyclopædia Britannica, inc. (n.d.). Amnion. Encyclopædia Britannica. Retrieved August 9, 2022.
2. Encyclopædia Britannica, inc. (n.d.). Allantois. Encyclopædia Britannica. Retrieved August 9, 2022.

Image Courtesy:

1. “Fetal membrane” By GCForsyth- Own Work (CC BY-SA 4.0) via Commons Wikimedia
2. “Chicken egg diagram” By KDS4444 – Own Work (CC BY-SA 4.0) via Commons Wikimedia