Difference Between Amnion and Allantois

The key difference between amnion and allantois is that amnion is the innermost thin membrane that surrounds the developing embryo or fetus, forming a hydrostatic cushion, while allantois is an extension of the posterior wall of the yolk sac, and lies between the amnion and chorion.

A developing embryo is surrounded by several fetal membranes. These membranes protect the embryo from external pressures and damages. Amnion and allantois are two membranes out of the four types. In fact, they are fluid-filled sacs. Moreover, the presence of amnion and allantois is a characteristic feature of mammals, birds and reptiles.

CONTENTS

1. Overview and Key Difference
2. What is Amnion
3. What is Allantois
4. Similarities Between Amnion and Allantois
5. Side by Side Comparison – Amnion vs Allantois in Tabular Form
6. Summary

What is an Amnion?

Amnion is a feature of mammals, reptiles and birds. Structurally, the amnion is a membrane that covers the embryo. It is a thin membrane that surrounds the dorsal aspect of developing embryo. Moreover, it forms a hydrostatic cushion around the embryo. It is filled with the amniotic fluid. Amniotic fluid mainly consists of water that originates from maternal blood and fetal urine. The amniotic fluid causes the expansion of amnion. Therefore, it is converted into the amniotic sac, which provides a protective environment for the developing embryo.

Generally, amnion starts to form during the second week of embryo development. It fuses with other membranes with time and forms a fluid-filled sac in which the fetus develops safely. At birth, amnion membrane ruptures and its fluids come out through the birth canal. The break of the amniotic membrane and the coming out of water signal the imminent birth of the baby.

What is an Allantois?

The allantois is another membrane that surrounds the developing embryo. It is an extension of the posterior wall of the yolk sac and lies between the amnion and the chorion. It appears as a sausage.

Moreover, it gives rise to a structure called urachus. The distal part of the urachus becomes a fibrotic cord after birth. It helps in the exchange of gases and handles liquid wastes from the developing organism. The blood vessels of allantois become the blood vessels of the umbilical cord. Moreover, allantois eventually forms into the umbilical cord of placental animals.

What are the Similarities Between Amnion and Allantois?

• Amnion and allantois are two of the four main membranes that surround an embryo.
• They form a protective sac around the embryo.
• Humans, other mammals, birds and reptiles possess amnion and allantois.
• Both amnion and allantois are fluid-filled sacs.

What is the Difference Between Amnion and Allantois?

Amnion is a membrane forming a fluid-filled cavity that encloses the embryo. Allantois is another membrane surrounding the embryo, and it helps gas exchange and taking up of nitrogenous waste of fetus. So, this is the key difference between amnion and allantois. Besides, a further difference between amnion and allantois is that the amnion is the innermost layer, while allantois is the layer located between amnion and chorion.

The following table summarizes the difference between amnion and allantois.

Summary – Amnion vs Allantois

Amnion and allantois are two membranes that surround and protect the developing embryo. Amnion is the innermost membrane that provides a protective environment for the embryo. Allantois is another membrane that lies between the amnion and chorion and helps in taking up nitrogenous wastes of embryo and gas exchange. So, this is the key difference between amnion and allantois.

Reference:

1. “Allantois.” Biology Dictionary, 28 Apr. 2017, Available here.
2. “Amnion.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 16 Nov. 2018, Available here.

Image Courtesy:

1. “Chicken egg diagram” By Dennis M DePace, PhD – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “FetalMembranes1L” By KDS4444 – Own work (CC BY-SA 4.0) via Commons Wikimedia