Extraembryonic Layers: Definition, Meaning, Types, Description, & Facts

During the development of the embryo, four extraembryonic membranes are formed, which are then discarded by the organism after birth. The four membranes are the chorion, amnion, allantois, and the umbilical sac. The chorion arises from the trophoblast and contains the mesodermal germ layer. It encloses the embryo, along with the other three extraembryonic membranes.

This Story also Contains

1. What are Extraembryonic Layers?
2. Extraembryonic Layers
3. Functions of the Extraembryonic Layers
4. Timeline of the Development of Extraembryonic Layers
5. Clinical Relevance
6. MCQs on Extraembryonic Layers
7. Recommended video on Extraembryonic Layers

The amnion is a layer that has a fluid-filled amniotic sac, which helps to absorb mechanical shock and also keeps the embryo from drying out. The amnion has the ectoderm and mesoderm germ layers. The allantois and umbilical sac become a part of the umbilical cord. The extraembryonic layers are an important part of the biology subject.

What are Extraembryonic Layers?

Extraembryonic layers refer to membranes that support the developing embryo during human reproduction but do not become part of the embryo. These tissues play a role in the protection, nourishment, and excretion of the developing embryo during its early development.

Extraembryonic Layers

Extraembryonic layers are tissues formed together with the embryo. They comprise tissues such as the amnion, chorion, yolk sac, and allantois. These membranes help in keeping the embryo alive and thriving in the uterus of the female reproductive system. The embryo surrounded by the extraembryonic layers is present in the uterus and supplied with nutrients by the placenta. The layers are:

Amnion

The amniotic envelope surrounds the embryo, forming an amniotic cavity that is filled with amniotic fluid. This fluid is what cushions the embryo and surrounds it in a constant environment. Amniocentesis is carried out on the amniotic fluid to detect any chromosomal abnormalities or genetic aberrations in the developing embryo.

Chorion

The chorion is the outermost layer. It forms the essential component that provides for gas and nutrient exchange between the mother and the developing embryo. It also develops the placenta.

Yolk Sac

This is the sac that provides early nourishment to the embryo before the complete development of the placenta. It also forms blood cells throughout early development.

Allantois

The allantois acts as a storage sac where waste products are stored and transported. Apart from the umbilical cord, it also forms from this layer and forms part of the chorion, which participates in gas exchange.

Functions of the Extraembryonic Layers

The extraembryonic membranes, such as the chorion, amnion, allantois, and umbilical cord, perform several important functions. The functions are given below:

Protection

The amnion helps cushion the embryo against mechanical shocks and temperature fluctuations.

Nourishment

The yolk sac and subsequently the placenta provide nutrients for the developing embryo.

Waste

The allantois contributes to storing and removing waste products. These waste products are discarded from the embryo through the placenta.

Timeline of the Development of Extraembryonic Layers

The development of the extraembryonic membranes is distributed throughout the pregnancy. The details are given below:

Early Development

The amnion and yolk sacs develop shortly after fertilisation and implantation.

Mid-Pregnancy

The chorion becomes more prominent as the development of the placenta takes place.

Late Pregnancy

The allantois mainly contributes to waste and gas exchange as the placenta takes over supplying nutrients.

Clinical Relevance

Understanding extraembryonic layers is vital in understanding pregnancy and fetal development. Any aberration in these layers can result in miscarriages and developmental disorders.

MCQs on Extraembryonic Layers

Q1. Match the following columns and select the correct option.

Option 1:

Option 2:

Option 3:

Option 4:

Correct answer: (4)

Explanation:

The placenta produces human chorionic gonadotropin (hCG), a hormone essential for maintaining pregnancy. The zona pellucida is the glycoprotein layer surrounding the ovum, which helps regulate fertilization by allowing only one sperm to penetrate. The bulbourethral glands secrete a lubricating fluid that helps to neutralize any acidic residue in the urethra and facilitates smoother sperm passage during ejaculation. Leydig cells, located in the testes, are responsible for producing androgens, including testosterone, which regulate the development of male secondary sexual characteristics.

Hence, the correct answer is option (4).

Q2. Which extra - embryonic membrane in humans prevents desiccation of the embryo inside the uterus?

Option 1: Chorion

Option 2: Amnion

Option 3: Allantois

Option 4: Yolk Sac

Correct answer: (2) Amnion

Explanation:

Extraembryonic Layers -

• These membranes are formed outside the embryo from the trophoblast and embryonic layers only in amniotes (reptiles, birds and mammals) and perform specific functions.

• There are four extraembryonic membranes:

  • Amnion

  • Yolk sac

  • Allantois

  • Chorion

Amnion is an extra-embryonic layer of an embryo which helps in protection from any kind of mechanical shock to the baby during pregnancy. It also prevents desiccation of the developing foetus
Hence, the correct answer is option (2) Amnion

Q3. Assertion: Chorion is an outer layer of developing foetus.

Reason: Embryonic membrane formed from the somatopleuric extra embryonic mesoderm inside and trophoblast outside is chorion.

Option 1: If both Assertion & Reason are true and the reason is the correct explanation of the assertion

Option 2: If both Assertion & Reason are true but the reason is not the correct explanation of the assertion

Option 3: If Assertion is true statement but Reason is false

Option 4: If both Assertion and Reason are false statements

Correct answer: (1) If both Assertion & Reason are true and the reason is the correct explanation of the assertion

Explanation:

There are four main extra embryonic membranes: amnion, chorion, allantois and yolk sac.

• Amnion- composed of trophoblast inside and somatopleuric extra embryonic mesoderm outside.

• Chorion- Embryonic membrane formed from the somatopleuric extra embryonic mesoderm inside and trophoblast outside.

• Allantois- composed of endoderm inside and splanchnopleuric extra embryonic mesoderm outside. Small and non-functional in humans except for furnishing blood vessels to the placenta.

Hence, the correct answer is Option (1) If both Assertion & Reason are true and the reason is the correct explanation of the assertion

Recommended video on Extraembryonic Layers