What is the Difference Between Blastula and Blastocyst

The main difference between blastula and blastocyst is that blastula is the early form of the embryonic development of animals whereas blastocyst is the blastula of mammals. Furthermore, blastula is characterized by the presence of a spherical layer of cells filled with fluid while blastocyst is lined by a layer of trophoblast cells and it contains an inner cell mass at one end and the fluid-filled cavity is known as blastocoel.

Blastula and blastocyst are two types of early stages of embryonic development of animals. Blastula undergoes gastrulation, developing into the gastrula.

Key Areas Covered

1. What is Blastula
     – Definition, Structure, Development
2. What is Blastocyst
     – Definition, Structure, Development
3. What are the Similarities Between Blastula and Blastocyst
     – Outline of Common Features
4. What is the Difference Between Blastula and Blastocyst
     – Comparison of Key Differences

Key Terms

Blastocyst, Blastula, Blastulation, Early Embryonic Development, Gastrulation

What is Blastula

Blastula is a stage of early embryonic development of animals. It develops from the morula, which is a solid mass of cell developed by the cleavage of the zygote. Generally, blastula consists of an outer layer of cells known as blastomeres. The inside of the blastula is a fluid-filled cavity known as blastocoel. The blastula and blastocoel develop into the gastrula, which contains the three germ layers in a process. The process is known as gastrulation.

Figure 1: Blastula

Furthermore, one of the main characteristic features of the blastula of vertebrates is the presence of a layer of blastomeres known as the blastoderm. Moreover, several activities occur in the blastula stage of the embryo including the establishment of cell polarity, regulation of gene expression, cell specification, axis formation, etc. Some animals including Drosophila and Xenopus undergo an event known as midblastula transition in which the degradation of maternal mRNA occurs in order to control the over development.

What is Blastocyst

The blastocyst is an early stage of embryonic development in mammals. The formation of the human blastocyst starts after 5 days of fertilization with the opening up of the fluid-filled cavity in the morula. The mammalian blastocyst consists of an outer layer of cell known as the trophoblast and the inner cell mass (ICM) or the embryoblast at one end inside the cavity. Also, the fluid-filled cavity here is known as the blastocoel.

Figure 2: Blastocyst

Generally, the diameter of the blastocyst is 0.1-0.2 mm. It comprises of around 200-300, rapidly dividing cells. About after 5-6 days after fertilization, the blastocyst reaches the uterus and it begins to embed into the endometrium. We call this process implantation. Implantation completes after 11-12 days of fertilization. Moreover, blastocyst undergoes its further developing process known as gastrulation in which the single-layered blastula reorganizes into a multilayered structure. More, importantly, since blastocyst is the implanting stage of the embryo, it is used in in vitro fertilization techniques to implant the fertilized egg in the uterus.

Similarities Between Blastula and Blastocyst

• Blastula and blastocyst are two types of early embryonic development stages in animals.
• Both stages consist of a spherical cell layer, which surrounds a fluid-filled cavity known as blastocoele or blastocoel.
• Also, both stages develop from the morula.
• Moreover, they further develop into the gastrula, containing multiple layers in a process known as gastrulation.
• However, the study of the blastula stage is important in stem cell techniques and fertility treatments.

Difference Between Blastula and Blastocyst

Definition

Blastula refers to an animal embryo at the early stage of development when it is a hollow ball of cells whereas blastocyst refers to mammalian blastula in which some differentiation of cells has occurred. Thus, this is the main difference between blastula and blastocyst.

Occurrence

Also, another difference between blastula and blastocyst is that bastula occurs in animals while blastocyst occurs in mammals.

Outer Cell Layer

The outer cell layer of the blastula is known as blastomeres while the outer cell layer of the blastocyst is known as the trophoblast.

Inner Cell Mass

Furthermore, inner cell mass is another difference between blastula and blastocyst. Blastula does not contain an inner cell mass while blastocyst contains an inner cell mass.

Pluripotency

Moreover, blastomere or blastoderm is pluripotent in the blastula while the inner cell mass is pluripotent in the blastocyst. Hence, this is another difference between blastula and blastocyst.

Conclusion

Blastula is an early stage of embryonic development in animals. It consists of an outer cell layer called blastomeres, which surrounds a fluid-filled cavity called the blastocoel. In comparison, the blastocyst is the early stages of development of mammals, consisting of an outer cell layer called the trophoblast and the inner cell mass at one side of the blastocyst. Blastocoel is the cavity of the blastocyst as well. Both blastula and blastocysts develop from the morula and give rise to the gastrula in a process called gastrulation. However, the main difference between blastula and blastocyst is their occurrence.  

References:

1. “Fertilization and Early Embryonic Development|Boundless Biology.” Lumen Learning, Lumen, Available Here.

Image Courtesy:

1. “Blastula (PSF)” By Pearson Scott Foresman – Archives of Pearson Scott Foresman, donated to the Wikimedia Foundation (Public Domain) via Commons Wikimedia  
2. “Blastocyst English” By Seans Potato Business (derivative of the source cited above) – Blastocyst.png (CC BY-SA 3.0) via Commons Wikimedia