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.

Also read: Stem Cells

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:

  1. 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.

  2. 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.

  3. The junction between the blastomeres forms an outer cell mass and an inner cell mass of the embryo.

  4. 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.

  5. The blastocyst so formed consists of two distinctive tissues, the outer trophoblast, and the inner cell mass.

  6. 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 Vs Embryo



It is a hollow, thin-walled structure formed during the early development of the embryo.

It is a rudimentary stage of a living being with a potential of development.

The process of formation is called blastulation.

The processes of formation are called gastrulation, neurulation, and organogenesis.

Formed by the cleavage of zygote.

Formed from the inner cell mass of the blastocyst.

Formed after 5-14 days after fertilization.

Formed after 2-11 weeks after fertilization.

In in-vitro fertilization, transfer of blastocyst shows a high pregnancy rate.

In in-vitro fertilization, transfer of embryo shows a low pregnancy rate.

Found only in mammals.

Found in both plants and animals.

Also read: Embryo Development

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.

For more information on Blastocyst and its formation, keep visiting BYJU’S website or download the BYJU’S app for further reference.

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Frequently Asked Questions


What is a blastocyst?

A blastocyst is a structure consisting of an inner cell mass which forms an embryo. The outer layer consists of cells known as trophoblast.


What is the duration of the blastocyst stage?

The zygote divides its cells within 5 to 6 days to become a blastocyst.

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