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What is Yolk Sac?

Last updated date: 19th Mar 2023
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As mentioned earlier, the yolk sac is the first anatomical and structural development outside an embryo and adjacent to it. This sac is made of hypoblast cells and remains attached to the embryonic disc. It is also known as the umbilical vesicle but the former term is majorly used in the world of biology.

This sac holds the highest importance during the primary phases of embryonic development in animals. Its prime functions are to provide nutrition and blood to the embryo for cellular division and respiration. Within a month, this sac gets incorporated into the earliest form of the gut in a fetus.

The progressive development of this sac like structure can be seen in between the 5th and 10th week of gestation among humans. After the 14th week, it starts to decrease in size as the embryo becomes capable enough to seek nutrition and blood from the placental connection. Within the 20th week, this sac disappears and becomes undetectable in the imaging scans.


Yolk Sac and its Anatomical Features 

The development of an embryo inside the womb of an animal is a fascinating phenomenon where a life nurtures another life. The steps of fertilization resulting in the formation of an embryo and the controlled cellular division are so well-coordinated. It is during these stages, a yolk sac is formed to provide nutrients directly to the embryo connected to it. In this article, we will study the features and morphology of this sac, the first significant anatomical development.


Structural Features of Yolk Sac

Now that we know what is yolk sac, let us summarize the features of this temporary organ that develops alongside an embryo.

  • It is considered as the first complex organ-like element developing in the preliminary gestational period and remains attached to the embryo. In fact, it starts to develop within the 3rd day of gestation in humans.

  • It is located in the ventral portion of the embryo and is in-lined by the extra-embryonic endoderm tissue. Over this tissue covering, a single layer of embryonic mesenchyme can be seen. This mesenchyme tissue is derived directly originates from the epiblast tissue.

  • The blood supply is provided to the yolk sac from the primitive aorta. This aorta breaks down in fine capillaries for better circulation and transmission of nutrients and respiratory gases to the sac cells. The capillaries then conjugate to form the vitelline veins and return to the heart of the embryo.

  • The yolk sac size varies less than 6 mm during the 5-10 weeks of gestation. If the size increases more than 6-7 mm, it can be considered a bad obstetric symptom resulting in fetal abnormalities or miscarriages. If the size increases more than 25 mm then this yolk sac no fetal pole is considered as a failed pregnancy.

  • Before a placental connection is developed, it is this sac that provides the necessities to the embryo and contributes to its growth. It performs gaseous exchange and nutrient delivery to the embryo.

  • During the end of the 1st month of gestation, it appears like a pear and remains incorporated into the primitive digestive tube. This long narrow tube is called the vitelline duct.

  • Complete obliteration of the duct takes place after the 20th week of gestation. It is found that the sac like structure transforms to form the gastrointestinal system of a developing fetus.


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Formation of the Yolk Sac

As mentioned earlier, the yolk sac starts to form and take a prominent shape within the second week of the gestation period. It is during this time the amniotic sac also starts taking a prominent shape. The proliferation of the hypoblast results in the formation of this sac. Eventually, this tissue descends and increases its size laterally to form this sac. There are three stages in the development of the yolk sac.

  1. Primary Stage

It is during this stage, a vesicular structure is formed. The floor of this structure is called Heuser’s membrane and the ceiling is constituted by the cells of hypoblast.

  1. Secondary Stage

The formation and separation of the extraembryonic mesoderm lead to this phase. Here, the extraembryonic coelom is formed.

  1. Final Stage

During the completion of the 1st month of gestation, this yolk sac becomes surrounded by endoderm while organogenesis.


Functions of Yolk Sac

The prime function of the yolk sac is the exchange of gases and the absorption of nutrients. These gases and nutrients are then delivered to the growing embryo. It means it serves as the first external organ before a placenta is formed for the nourishment and survival of the embryo. It remains outside the embryo sac structure and provides nutrition to the embryo. It also directs the origin of blood cells for the first time in the embryo.

The visualization of this sac also aids in the detection of an intrauterine pregnancy, a decidual cast cyst, or a pseudo-gestational sac. It is done using advanced imaging techniques such as ultrasonography to check the health and condition of an embryo.

This is all you need to know about the yolk sac, its morphological features, and its functions. Study the features of this sac at different gestational periods to understand the anomalies and symptoms of fetal abnormalities.

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FAQs on Yolk Sac

1.Does the placenta develop with a yolk sac?

If we follow the functions of the yolk sac, it plays the role of a placenta. During the preliminary gestational weeks till the 20th week, this sac provides the nutrition and performs the exchange of gases for the embryo. It is considered the external organ for embryonic development. After the 20th week, the yolk sac placenta starts developing.

2.When does the yolk sac disappear?

After serving for 20 weeks during the gestational period, the yolk sac disappears. It happens due to the formation of the placenta, the prime connection between the mother and the fetus. The placenta then performs the yolk sac function entirely. The presence of this sac cannot be noticed in a healthy pregnancy term after the 5th month of gestation.


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