Right after fertilization occurs , a series of cell divisions begin to take place to form a hollow ball of cells called a blastula . For the blastocyst to exist , the first step that must be taken is fertilization, which occurs when a male’s sperm fuses with a female’s egg during conception .

What is a blastocyst?

A blastocyst is an embryo that develops from the fifth day after the fertilization of the ovum has occurred and that will then travel to be able to lodge or implant within the uterine cavity of the woman.

  • Definition
  • Characteristics of the blastocyst
  • Parties
  • Formation and development
  • Classification
  • What percentage of embryos reach a blastocyst
  • When a blastocyst is implanted in a transfer
  • Implantation rate
  • Importance


The blastocyst is an embryonic structure that forms during the early stages of development in the pregnancy of mammals , a process known by the name of embryogenesis , which occurs about 4 or 5 days after it has given the process fertilization and before implantation occurs to the endometrium.

Characteristics of the blastocyst

The most important characteristics of blasts are:

  • Together, the blasts form a unit known as a blastocele .
  • Its cells are pluripotent , this means that they have an internal cell mass that can become any type of tissue, with the exception of the placenta.
  • Its synonym is the word blastula .
  • It is an embryo that is between five and six days old .
  • Its cellular structure is made up of about 200 cells that have been created from the division of the zygote .


The blastocyst has two fundamental parts, the part or internal cell mass that will be responsible for giving rise to the fetus and on the other hand, the trophectoderm , which will be responsible for forming the placenta . We can also find a protective layer known as the zona pellucida , which is responsible for providing protection until the embryo breaks the area in order to implant.

Formation and development

Fertilization results in the formation of a single-celled embryo called a zygote , which contains a set of DNA from each parent. This zygote is the earliest stage that an organism can have. Although we all start out as a small cell or zygote, that cell forms a bunch of copies of itself, resulting in a ball of multiple cells that produces cell growth in a round structure called a morula . After the morula forms, a liquid appears causing the morula to develop as a fluid-filled cavity, transforming into a blastocyst . TheCell division that occurs after fertilization is called excision and this process forms the blastula . The blastula will continue to change during a process called gastrulation , which forms the three main types of tissues in a developing organism.

Approximately 2 days after the blastocyst reaches the uterus, approximately 6 to 7 days after fertilization, the blastocyst is positioned and adheres to the endometrial epithelium of the uterus . After attaching itself to the uterus, the trophoblast rapidly begins to proliferate, gradually forming in two layers: the cytotrophoblast inward and the syncytiotrophoblast on the external side.


The classification of blasts is as follows:

  • Grade 1 or early blastocyst : this is when the blastocele begins to be seen.
  • Grade 2 or cavitated blastocyst : This classification is given when it is already possible to distinguish the trophectoderm and the ICM.
  • Grade 3 or expanded blastocyst : the blastocyst has increased in size and the layer that is covering it known as the zona pellucida, is thinner.
  • Grade 4 or blastocyst starting to hatch : it occurs when the blastocyst begins to leave the zona pellucida.
  • Grade 5 or hatched blastocyst : this is when the blastocyst has already left the zona pellucida completely.

What percentage of embryos reach a blastocyst

Generally, between 25-60% of the ovules that have been fertilized will become blastocysts . This number may be lower if the eggs are of lower quality for various reasons, for example in advanced maternal age or if the semen is of poor quality. This scarce formation of blasts may be due to the stress suffered by the embryos when they are not in their environment , to the quality of the ovum, since the embryo depends on genetics and the nutrients that the ovum provides or the quality of the sperm . 

When a blastocyst is implanted in a transfer

The transfer of embryos in the blastocyst stage is a step of In Vitro Fertilization and is carried out in the laboratory . When fertilization occurs, the development of the embryo begins that will give rise to the future baby. The moment in which a blastocyst is transferred to the uterus depends on each case, and it is the doctor together with the embryology expert who indicates when it is advisable to opt for a transfer in the blastocyst state. 

Implantation rate

At present, thanks to the advances that have been made in the field of assisted reproduction, better success rates are achieved in the transfer of vitrified blastocysts . The technique of vitrification in other words freeze the embryos flash way manages to achieve survival rates (90-95%) very high because the damage suffered by embryos in the freezing process are minimized.


The blastocyst is of utmost importance in the process of human reproduction , since it is through it that implantation in the woman’s uterus is successfully carried out , as it allows it to adhere correctly. In addition, the internal part of the blastocyst will be responsible for protecting the embryo and providing it with adequate nutrition throughout the entire period of pregnancy .

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