What is the Difference Between Epiblast and Hypoblast

The main difference between epiblast and hypoblast is that epiblast is one of the two layers of the embryonic disk, forming three primary germ layers, whereas hypoblast is the second layer of the embryonic disk, forming the yolk sac.

Epiblast and hypoblast are two layers arising from the inner cell mass. Generally, they occur in the mammalian blastocyst. They also occur in the blastula of reptiles and birds.

Key Areas Covered

1. What is Epiblast
     – Definition, Structure, Function
2. What is Hypoblast
     – Definition, Structure, Function
3. Similarities Between Epiblast and Hypoblast
     – Outline of Common Features
4. Difference Between Epiblast and Hypoblast
     – Comparison of Key Differences

Key Terms                                                                                                                  

Epiblast, Hypoblast

What is Epiblast

Epiblast, or the primitive ectoderm, is one of the two distinct cell layers arising from the inner cell mass of the blastocyst of mammals. It also occurs in the blastula of reptiles and birds. Mammalian embryogenesis is the differentiation and segregation of cells of the inner cell mass of the blastocyst, yielding two distinct layers that are the epiblast and hypoblast. Generally, the cells of the epiblast occur on day 8 of human embryonic development. It also makes up a columnar epithelium containing dense microvilli on the apical surface.

Figure 1: Epiblast and Hypoblast

The main function of the epiblast is to differentiate into three germ layers: ectoderm, endoderm, and mesoderm. In addition to the three germ layers, epiblast differentiates into the extraembryonic mesoderm of the visceral yolk sac, amnion, and allantois. Therefore, epiblast develops into the cells of the three germ layers of the fetus and extraembryonic structures. This differentiation occurs during gastrulation. Furthermore, Christian Heinrich Pander (1794-1865) was the first to discover the epiblast.

What is Hypoblast

Hypoblast is the other distinct cell layer arising from the inner cell mass of the blastocyst of mammals. It develops into the yolk sac that, in turn, gives rise to the chorion. Generally, the hypoblast contains small cuboidal cells. In fish, the hypoblast contains precursor cells for both endoderm and mesoderm. In birds and mammals, the hypoblast contains precursor cells for the extraembryonic endoderm of the yolk sac.  

Figure 2: Embryo

Moreover, the cells in the hypoblast do not contribute to the embryo. It also influences the orientation of the embryo. On the other hand, hypoblast inhibits the formation of the primitive streak. In chicken embryos, the absence of hypoblast results in the formation of multiple primitive streaks. Normally, a hypoblast is also called a primitive endoderm. It also develops into the yolk sac, ensuring proper organogenesis.  

Similarities Between Epiblast and Hypoblast

• Epiblast and hypoblast are two layers of the inner cell mass occurring in the embryo.
• They develop into different layers in the blastocyst.

Difference Between Epiblast and Hypoblast

Definition

Epiblast refers to the outermost layer of an embryo before it differentiates into endoderm, ectoderm, and mesoderm, while hypoblast refers to an inner (lower) layer of the embryonic disc located beneath the epiblast.

Also Known as

Epiblast is also known as primitive ectoderm, while hypoblast is also known as primitive endoderm.

Significance

Epiblast is one of the two cell layers of the inner cell mass, while hypoblast is the other cell layer of the inner cell mass.

Develop into

Epiblast develops into the three germ layers; endoderm, ectoderm, and mesoderm, while hypoblast develops into an embryonic disc.

Types of Cells

Epiblast contains columnar epithelium, while hypoblast contains small cuboidal cells.

Conclusion                                                      

In brief, epiblast and hypoblast are two types of cell layers that occur in the inner cell mass of the mammalian blastocyst. They occur in the blastula of reptiles and birds. Epiblast is one of the two layers of the inner cell mass of the blastocyst. It develops into the three germ layers: endoderm, ectoderm, and mesoderm. In comparison, the hypoblast is the other layer of the inner cell mass of the blastocyst. It develops into the embryonic disc that further develops into the chorion. Therefore, the main difference between epiblast and hypoblast is their development.                   

References:                

1. Epiblast. LifeMap Discovery®. (n.d.). Retrieved March 22, 2023
2. Blastocyst. Encyclopædia Britannica. Retrieved March 22, 2023

Image Courtesy:

1. “2907 Embroyonic Disc, Amniotic Cavity, Yolk Sac-02” By OpenStax College – Own work (CC BY 3.0) via Commons Wikimedia
2. “Human Embryo Day9” By Panzer VI-II – Own Work (CC BY-SA 3.0) via Commons Wikimedia