Blastocyst: A blastocyst is a cluster of dividing cells made by a fertilized egg. It’s the early stage of an embryo. A blastocyst is one step among many that lead to a pregnancy. A blastocyst forms about five to six days after a sperm fertilizes an egg. Layers of cells in the blastocyst divide and separate. They eventually become the structures that protect and nourish the developing fetus. The blastocyst stage of a fertilized egg is especially important for in vitro fertilization (IVF). It is formed in a process called blastulation between 5-14 days after fertilization. The blastocyst is only found in mammals.

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.

Steps in Blastocyst Formation

• 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.

• 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.

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

• In the zygote fluid starts accumulating leading to the formation of the blastocyst. As the fluid starts accumulating, the blastocyst expands to form a blastocyst.

• The blastocyst which is 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.

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By Team Learning Mantras