Blastocyst: what is it


A blastocyst is a 5/6 day embryo with a complex cell structure consisting of about 200 cells. The blastocyst stage is the stage of development, following the morula stage, and prior to the implantation of the embryo in the mother’s uterus.

This cellular structure is characterized by two areas that clearly define the quality of the embryo and its options for implanting and achieving pregnancy:

  • The first is the internal cell mass (ICM). which is a group of cells that are located within the blastocyst or cavity that will give rise to the fetus.
  • the second is a layer of epithelial cells that covers the blastocele, called trophoectoderm (TE), which will give rise to extra-embryonic tissues (placenta and amniotic membranes).

How blastocyst is formed in natural fertilization

The fertilization of the oocyte present in the ovaries with the sperm leads to the initial formation of the so-called zygote, a cell characterized by a new genetic heritage that is the result of the fusion of that of the two parents.

From the zygote an embryo is obtained first, composed of recognizable and accounting cells, and then, with increasing cell divisions, a larger embryo, called blastocyst. In fact, there is an intermediate phase, of passage, between zygote and blasticuste called morula.

In humans, blastocyst formation begins about 5 days after fertilization, when a cavity filled with fluid opens in the morula. The human blastocyst consists of 70-100 cells

How blastocyst is formed in IVF

In an IVF cycle a blastocyst is formed in a culture system in a laboratory. Eggs are retrieved from a woman’s ovaries, fertilized with sperm, and an embryo is created.

The embryo divides and multiplies its cells in 5-6 days to become a blastocyst. Embryos that survive this stage of development have a high implantation potential once transferred into the uterine cavity.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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