What Is the Yolk Sac Structure and Function in Embryos
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.
(Image will be uploaded soon)
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.
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.
Secondary Stage
The formation and separation of the extraembryonic mesoderm lead to this phase. Here, the extraembryonic coelom is formed.
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.
FAQs on Yolk Sac in Human Embryonic Development
1. What is the yolk sac?
The yolk sac is an extraembryonic membrane that provides early nourishment and support to the developing embryo. It forms from the hypoblast during early embryonic development and is connected to the embryo by the vitelline duct.
- It is the first structure visible inside the gestational sac on ultrasound.
- It plays a key role before the placenta becomes fully functional.
- It is present in all vertebrates, though its size and function vary.
2. What is the function of the yolk sac in humans?
In humans, the yolk sac mainly supports early embryonic development by providing nutrition and producing blood cells. Its key functions include:
- Nutrient transfer to the embryo before placental circulation is established.
- Formation of the first blood cells (primitive hematopoiesis).
- Origin of primordial germ cells, which later form sperm or ova.
3. How does the yolk sac form during embryonic development?
The yolk sac forms from cells of the hypoblast during the second week of embryonic development. The formation occurs in stages:
- Hypoblast cells migrate and line the inner surface of the blastocyst cavity.
- This forms the primary yolk sac.
- It is later remodeled into the smaller secondary (definitive) yolk sac.
4. Is the yolk sac present in all vertebrates?
Yes, the yolk sac is present in all vertebrates, but its size and function differ among groups.
- In birds and reptiles, it contains a large yolk that nourishes the embryo inside the egg.
- In fish, it supports development until the larva can feed independently.
- In mammals, especially humans, it is small and mainly supports early development before the placenta forms.
5. What is the difference between the yolk sac and the placenta?
The yolk sac provides early embryonic support, while the placenta becomes the main organ for nutrient and gas exchange later in pregnancy. Key differences include:
- Timing: Yolk sac functions early; placenta functions throughout most of gestation.
- Structure: Yolk sac is a simple membrane; placenta is a complex organ formed from maternal and fetal tissues.
- Role: Yolk sac initiates blood formation; placenta handles long-term nourishment and waste removal.
6. What is the vitelline duct and how is it related to the yolk sac?
The vitelline duct is a narrow tube that connects the yolk sac to the embryonic midgut. It allows communication between the developing gut and the yolk sac during early development.
- It normally disappears by the 7th week of gestation.
- If it persists, it can lead to conditions such as Meckel’s diverticulum.
7. What role does the yolk sac play in blood formation?
The yolk sac is the first site of blood cell formation in the embryo. This process, called primitive hematopoiesis, begins in the third week of development.
- Clusters of cells called blood islands form in the yolk sac wall.
- These give rise to early red blood cells and blood vessels.
- Later, blood formation shifts to the liver, spleen, and bone marrow.
8. When does the yolk sac disappear in human development?
The yolk sac typically regresses by the end of the first trimester of pregnancy. As development progresses:
- The placenta becomes fully functional.
- The yolk sac shrinks and is incorporated into the umbilical cord remnant.
- It is usually no longer visible after about 12 weeks of gestation.
9. What are primordial germ cells and how are they related to the yolk sac?
The primordial germ cells are the precursors of sperm and ova, and they originate in the wall of the yolk sac. During early development:
- These cells form near the base of the yolk sac.
- They migrate to the developing gonads (testes or ovaries).
- They later differentiate into mature gametes.
10. Why is the yolk sac important in early pregnancy ultrasound?
The yolk sac is an important marker of early pregnancy viability on ultrasound. It is usually visible by the 5th week of gestation.
- Its presence confirms an intrauterine pregnancy.
- Its size and shape can indicate normal or abnormal development.
- An absent or enlarged yolk sac may suggest early pregnancy complications.
