Difference between Amnion and Allantois

An embryo requires nourishment and protection for its growth, called prenatal development. There are four extra-embryonic layers found in birds, reptiles and placental mammals that help in the overall development of the embryo. The four layers are amnion, allantois, yolk sac and chorion. Each layer functions differently to provide for nourishment and growth of the embryo.

Let us look at the differences between amnion and allantois in the table below.

What is Amnion?

Amnion is the innermost layer that covers the foetus. It is filled with a clear liquid called the amniotic fluid. The amniotic fluid expands and the amnion eventually forms an amniotic sac.

The shape of the embryo is maintained because it is suspended in the amniotic fluid. The amniotic fluid provides an equal pressure throughout the sac, and protects the embryo from outer hindrances by forming a cushion. It also allows the embryo to float freely in the sac, which contributes to skeletomuscular development.

The amniotic fluid is released at the time of childbirth. The rupturing of the sac is a signal that the child is ready to come out of the womb.

What is Allantois?

Allantois is the layer present between amnion and chorion. It is a sac-like structure that literally translates to sausage (according to the Latin dictionary). It expands greatly between the two layers to serve as a temporary respiratory organ for the foetus, and its cavity stores all the foetal excretions. It is filled with a clear liquid.

The allantois is composed of blood vessels that allow exchange of gases from the developing embryo. The blood vessels eventually become the arteries and veins of the umbilical cord.

The allantois form a unique structure called urachus. It is a canal-like structure that is a remnant of allantois and is fibre-like. It serves as a site for the drainage of the urinary bladder of the foetus and also joins with the umbilical cord.

Amnion vs Allantois

Amnion

Allantois

Description

Amnion is a thin layer that is in direct contact with the developing embryo.

Allantois is an extra-embryonic layer that is present between the amnion and chorion.

Location

It is the innermost layer that covers the foetus.

It is present between the amnion and chorion.

Presence of Blood Vessels

It is a non-vascular structure.

In birds, reptiles and mammals (except some marsupials) the allantois is a vascular structure.

Function

  • It provides equal pressure throughout the embryo and protects it from outside trauma by acting like a cushion.
  • It allows free movement of the embryo in the sac, which contributes to skeletomuscular development.
  • It acts as a temporary respiratory organ for the foetus by allowing exchange of gases between the foetus and mother.
  • The layer also has a cavity that stores the foetal excretory materials.

Growth

It continues to expand till the end of pregnancy.

It ceases to grow, once it is fused with the chorion and umbilical cord.

Eventually Forms

It eventually expands to form the amniotic sac.

It fuses with the umbilical cord and becomes a part of it.

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Frequently Asked Questions

What is the difference between amnion and chorion?

Amnion is the innermost layer that surrounds the developing embryo, whereas chorion is the outermost layer around the developing embryo.

Where do the amnion, chorion, yolk sac and allantois come from?

The amnion and chorion arise from the somatopleure which is an extraembryonic layer that comprises the ectoderm and mesoderm. The allantois arises from the posterior wall of the yolk sac. The yolk sac is the first layer that is formed by the hypoblast cells of the blastocyst.

Is amnion a part of the placenta?

Yes, the placenta is composed of three layers, the innermost amnion, the middle allantois and the outermost chorion

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