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Blastocyst Definition

The hollow cellular mass formed during the first development consisting of cells forming the trophectoderm, the inner cell mass, and therefore the fluid-filled cavity or the blastocoel is understood as the blastocyst.

The process in which blastocyst forms is called blastulation and it takes place during 5-14 days after fertilization.

Placenta is the outermost layer furthermore other supportive tissues necessary for the development of the foetus are found. The body tissues are formed from the inner cell mass.

The embryonic stem cells isolated from the inner cell mass are utilized in tissue replacement after injury and in regenerative medicine.

The blastocyst is only found in mammals.

Blastocyst Formation

Zygote a single-celled structure is formed after fertilization. The zygote then undergoes rapid cell division leading to the formation of blastocyst which is a hollow cell mass.

The stages which lead to the formation of blastocyst are mentioned below:

  1. The zygote encased in the zona pellucida segregates itself 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 referred to as blastomeres. The blastomeres within the 2-celled stage undergo mitosis and cytokinesis simultaneously.

  2. The blastomeres form tight junctions with one another once the embryo reaches the 8-celled or 16-celled stage. This ultimately leads to the deformation of the round shape and in turn leads to the formation of a morula; a mulberry shaped cluster of cells.

  3. The blastomeres form the outer and inner cell mass of the embryo in between the junctions.

  4. In the zygote fluid starts to get accumulated which leads to blastocyst formation. The accumulation of fluid expands the blastocoel and forms blastocyst.

  5. The blastocyst which is formed is composed of two very distinct tissues, namely the outer trophoblast and the inner cell mass.

  6. Finally, the stretched zone breaks and releases the blastocyst. This is known as hatching. Implantation of the blastocyst takes place in the uterus.

The blastocyst cling or get attached onto the endometrial wall of the uterus. This process is known as implantation.

Blastocyst Vs Embryo



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

It is an undeveloped stage of a living being with the capability for development.

The process of formation is called blastulation

Embryo undergoes the following processes of formation; gastrulation, neurulation  and last but not the least organogenesis.

Formed by the cleavage of zygote.

Embryo is 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, High pregnancy rate is seen if blastocyst is transferred.

In-vitro fertilization shows a low pregnancy rate if transfer of embryos takes place.

Found only in mammals.

Found in both plants and animals.

Similarities Between Blastocyst and Embryo

  • Embryo and blastocyst are diploid structures.

  • Both Blastocyst and embryo are formed after the fertilization of gametes in animals.

  • Both the structures possess potent cells and hence are often utilized in diagnostics.

  • Both are formed inside the mother.

What is an Embryo?

Embryo development refers to the various stages within the development of an embryo. Embryonic development of plants and animals vary. In animals, Different stages are observed during embryonic development where it differs from species to species.

After fertilization, the zygote is formed. The zygote divides mitotically to make 2, 4, 8, 16 celled stages. These cells are known as blastomeres.

Morula- Embryo having 8 to 16 blastomeres.

The morula divides mitotically and transforms into blastocyst. The outer layer of blastocyst known as the trophoblast gets attached to the wall of uterine known as the endometrium. The implantation starts within the first week but gets completed by 2nd week.

The inner cell mass of the blastocyst forms an embryo. The embryonic and extraembryonic tissues are formed due to differentiation of blastocyst. The implantation completes in the 2nd week.

The interdigitated chorionic villi of trophoblast and uterine cells form the placenta, which is the connection between the mother and the growing foetus.

The embryo gets nourishment and oxygen from the placenta moreover the placenta helps in removing waste produced and carbon dioxide by the embryo. It also acts as an endocrine and secretes various hormones like hCG (Human Chorionic Gonadotropin), estrogen, progestogens, etc. for maintenance of pregnancy.

Gastrulation starts within the 3rd week, the inner cell or embryo starts differentiating into three germinal layers, i.e. ectoderm, endoderm and mesoderm. These cells transform and obtain differentiation to all or any tissues and organs, like nerve, blood, muscle, bone, alimentary canal, etc.

Ectoderm- it forms brain, hair, spinal cord, nails, nervous system, epidermis etc.

Mesoderm- animal tissue, muscles, cardiovascular system, notochord, bone, kidney, gonads.

Endoderm- it forms the stomach, gut lining, liver, bladder, internal organs,  lung, pancreas etc.

FAQ (Frequently Asked Questions)

1. What is a Blastocyst?

Ans: A blastocyst in simple words is a structure which is composed of the inner cell mass which forms an embryo. Trophoblast are the outer layer of cells.

2. What is the Duration of the Blastocyst Stage?

Ans: The zygote becomes a blastocyst within 5-6 days after fertilization.