Blastocyst | Zoology Optional Notes for UPSC PDF Download

What is a Blastocyst?

• The hollow cellular mass formed during the early development consisting of cells forming the trophectoderm, the inner cell mass, and the fluid-filled cavity or the blastocoel is known as the blastocyst.
• It is formed in a process called blastulation between 5-14 days after fertilization.
• The outermost layer forms the placenta and other supportive tissues which are necessary for the development of the foetus. The inner cell mass gives rise to the body tissues.
• The embryonic stem cells isolated from the inner cell mass are used in tissue replacement after injury and in regenerative medicine.
• The blastocyst is only found in mammals.

Blastocyst Formation

After fertilization, a single-celled structure called the zygote is formed. The zygote undergoes a series of cell divisions leading to the formation of a hollow cell mass known as the blastocyst.

Following are the stages of the formation of the blastocyst:

• The zygote encased in the zona pellucida divides in such a way that the entire process progresses through 2-celled, 4-celled, 8-celled and 16-celled stages. The cells during the cleavage stage are known as blastomeres. The blastomeres in the 2-celled stage undergo mitosis and cytokinesis simultaneously.
• After the development of the 8-celled or 16-celled embryo, the blastomeres form tight junctions with one another. This leads to the deformation of the round shape and the formation of a mulberry shaped cluster of cells called the morula.
• The junction between the blastomeres forms an outer cell mass and an inner cell mass of the embryo.
• Fluid starts accumulating inside the zygote leading to the formation of the blastocyst. As the fluid starts accumulating, the blastocyst expands to form a blastocyst.
• The blastocyst so formed consists of two distinctive tissues, the outer trophoblast, and the inner cell mass.
• Finally, the stretched zona pellucida breaks and releases the blastocyst. This is known as hatching. The blastocyst then undergoes implantation in the uterus.

The blastocyst adheres to the endometrial wall of the uterus. This process of attachment of blastocyst to the uterine wall is known as implantation.

Blastocyst

• Definition: A blastocyst is a hollow, thin-walled structure that forms during the early stages of embryo development.
• Nature: It is not a rudimentary stage but rather a specific structure within the developmental timeline.
• Formation: The process of its formation is known as blastulation.
• Origin: It is formed by the cleavage of the zygote.
• Timing: Typically, blastocysts are formed 5-14 days after fertilization.
• Transfer in IVF: Transfer of a blastocyst in in-vitro fertilization often yields a high pregnancy rate.

Embryo:

• Definition: An embryo is a rudimentary stage of a living being with the potential for further development.
• Nature: It represents a more advanced stage in development, encompassing processes such as gastrulation, neurulation, and organogenesis.
• Formation: The processes of formation include blastulation, gastrulation, neurulation, and organogenesis.
• Origin: It is formed from the inner cell mass of the blastocyst.
• Timing: Embryos are formed 2-11 weeks after fertilization.
• Transfer in IVF: In in-vitro fertilization, the transfer of an embryo shows a relatively lower pregnancy rate compared to blastocyst transfer.
• Widespread Occurrence: Embryos are found not only in mammals but also in plants and animals.

Similarities Between Blastocyst and Embryo

• Embryo and blastocyst are diploid structures.
• Both are formed after the fertilization of gametes in animals.
• Both the structures possess potent cells and hence can be used in diagnostics.
• Both are formed inside the mother.