Difference Between Blastocyst and Embryo

Main Difference – Blastocyst vs Embryo

Blastocyst and embryo are two stages of the embryonic life of animals. Embryogenesis starts with a rapid mitotic division of the zygote, immediately following fertilization. The cell mass produced at the end of the cleavage is referred to as the morula. The morula is developed into the blastula. The blastula in mammals is referred to as the blastocyst. After the implantation of it in the uterine wall, the blastocyst is referred to as the embryo. The embryo undergoes gastrulation where the three primary germ layers are developed. The main difference between blastocyst and embryo is that blastocyst is a thin-walled hollow structure from which the embryo arises whereas embryo is the early stages of the placental development from which the fetus arises.

Key Areas Covered

1. What is a Blastocyst 
      – Definition, Development Process, Features
2. What is an Embryo
      – Definition, Development Process, Features
3. What are the similarities between Blastocyst and Embryo
      – Common Features
4. What is the difference between Blastocyst and Embryo
      – Comparison of Key Differences

Key Terms: Blastocyst, Blastomeres, Blastocoel, Blastoderm, Blastulation, Blastula, Diploblastic, Embryoblast, Embryo, Embryonic stage, Epiblast, Gastrulation, Hypoblast, Morula, Neurulation, Organogenesis, Trophoblast, Triploblastic, Zona pellucida

What is Blastocyst

The blastocyst is a thin-walled hollow structure, which occurs in the early development of the embryo. The egg cell of animals is fertilized from a sperm cell at the Fallopian tubes of the mother. After undergoing fertilization, the zygote undergoes a series of rapid cell divisions by mitosis while reaching the uterus. This process is referred to as cleavage in which a ball of cells called morula is produced. The cells in the morula are known as blastomeres. From the morula, a fluid-filled cavity, which is known as blastocoel, is formed. When the blastocoel is formed, the morula is called the blastula. The blastoderm is the cell layer of blastomeres, which surrounds the blastocoel. The blastula in mammals is referred to as the blastocyst. The blastocyst comprises of an inner cell mass (ICM) and trophoblast. The ICM, which subsequently develops into the embryo, is also called the embryoblast. Trophoblast is the hollow sphere which forms the extra-embryonic tissues like placenta.

Morula is 8-cell or 16-cell embryo. The process of the formation of blastula from the morula is known as blastulation. The blastocyst formation begins 5 days after fertilization. The diameter of the blastocyst is about 0.1-0.2 mm and it contains about 200-300 cells. Once the blastocyst arrives at the uterus, it is embedded in the endometrium in the process called implantation. During implantation, the blastocyst hatches the zona pellucida, which is a thick, transparent membrane surrounding the mammalian ovum. The implantation is completed within 11-12 days after the fertilization. The blastocyst is used in the in-vitro fertilization (IVF), implanting a five day old fertilized egg in the uterus. The embryonic stem cells are also isolated from the ICM. Due to the plasticity and pluripotency of embryonic stem cells, they are used in regenerative medicine and tissue replacement after injury. A blastocyst, just before the implantation is shown in figure 1.

What is an Embryo

An embryo is a rudimentary stage of a living organism, which shows the potential for development. Human embryonic stage exists from the second to the eleventh week after fertilization. The ICM of the blastula develops into the embryo in animals. The next embryonic development stage is the gastrulation in which the blastocyst of animals is developed into its body plan. After the implantation, two main cell layers of the bilaminar embryonic disk called hypoblast and epiblast are formed at the beginning of the second week. Either embryoblast or trophoblast gives rise to the two cell layers. The upper layer of the bilaminar disk or the epiblast serves as the primitive ectoderm and the lower layer or the hypoblast serves as the primitive endoderm. The primitive streak establishes the bilateral symmetry of the embryo and determines the site of gastrulation. The epiblast forms the floor of the amniotic cavity and the hypoblast forms the roof of the exocoelomic cavity. The formation of the bilaminar embryonic disk commences the gastrulation in which the three germ layers and the notochord are formed. The three germ layers are endoderm, ectoderm, and mesoderm. The organisms with three germ layers are known as triploblastic organisms. In contrast, diploblastic organisms do not have a mesoderm. The germ layers in an embryo are shown in figure 2.

Figure 2: Germ Layers in an Embryo

Following gastrulation, neurulation (the process where epithelial and neural tissue are developed) and organogenesis (the process where the organs are developed) occur in the embryo. Due to the potency of cells in the embryo, some of the cells can be removed from a pre-implanted embryo by biopsy without disturbing its development. These cells are used in preimplantation genetic diagnosis in order to define genetic diseases.

In flowering plants, the seed is considered as the embryo. The seed contains hypothetical tissues, which can develop into roots, stem, and leaves. After the germination of plants, a plantlet grows out of the seed. Rice embryos which exhibit GUSPluss expression are shown in figure 3.

Figure 3: Rice Embryos (blue)

Similarities Between Blastocyst and Embryo

• Blastocyst and embryo are two structures that form after the fertilization of an egg cell and a sperm cell in animals.

• Both blastocyst and embryo are diploid structures.

• Both occur inside the mother.

• The cells in both structures are potent; hence both cell types can be used in diagnostics.

Difference Between Blastocyst and Embryo

Definition

Blastocyst: The blastocyst is a thin-walled, hollow structure, which forms in the early development of the embryo.

Embryo: An embryo is a rudimentary stage of a living organism, which shows the potential for development.

Correlation

Blastocyst: The blastocyst occurs from the cleavage of the zygote.

Embryo: The embryo occurs from the ICM of the blastocyst in animals.

Events

Blastocyst: Blastocyst is formed in a process called blastulation.

Embryo: Embryo undergoes gastrulation, neurulation, and organogenesis.

Time

Blastocyst: Blastocyst forms from 5 days to two weeks after fertilization.

Embryo: Embryo forms from two weeks to eleven weeks after fertilization.

Occurrence

Blastocyst: Blastocyst is only found in mammals.

Embryo: Embryo is only found in both plants and animals.

In IVF

Blastocyst: The transfer of blastocyst in IVF shows high pregnancy rate.

Embryo: The transfer of embryo in IVF shows low pregnancy rate when compared to the blastocyst transfer.

Applications

Blastocyst: The embryonic stem cells, which are isolated from ICM are used in regenerative medicine and tissue replacement after injury.

Embryo: The cells in the embryo cells are used in preimplantation genetic diagnosis in order to define genetic diseases.

Conclusion

Blastocyst and embryo are two sequential stages of the embryonic development of mammals. In animals, the blastocyst is referred to as blastula, and it arises from the cleavage of the zygote in rapid mitotic divisions. The formation of the blastocyst from the morula is called blastulation. The blastocyst comprises of two cell types: ICM and the trophoblast. The ICM develops into the embryo while the trophoblast develops into the placenta. The three germ layers are developed from the embryo in the process called gastrulation. Following gastrulation, neurulation and organogenesis also occur in the later stages of the embryonic development. The main difference between blastocyst and embryo is in the formation of each structure in a sequential manner after fertilization.  

Reference:

1. “Blastocyst Formation – Boundless Open Textbook.” Boundless. N.p., 31 Oct. 2016. Web. Available here. 08 June 2017. 
2. Bowen, R. “Cleavage and Blastocyst Formation.” N.p., n.d. Web. Available here. 08 June 2017. 
3. “Cleavage, the Blastula Stage, and Gastrulation – Boundless Open Textbook.” Boundless. N.p., 08 Aug. 2016. Web. Available here. 08 June 2017. 
4. “Bilaminar Embryonic Disc Development – Boundless Open Textbook.” Boundless. N.p., 12 Dec. 2016. Web. Available here. 08 June 2017. 

Image Courtesy:

1. “Blastocyst English” By Seans Potato Business (derivative of the source cited above) – Blastocyst.png (CC BY-SA 3.0) via Commons Wikimedia
2. “2908 Germ Layers-02” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
3. “Rice embryo” By Richard Jefferson’s Center for the Application of Molecular Biology to International Agriculture (CAMBIA) – BioForge.net (CC BY-SA 2.5) via Commons Wikimedia

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