Definition - What does Blastocoel mean?
The blastocoel is fluid filled spaced that forms within the fertilized egg on the 5th day after conception. A fertilized egg at this stage of development is called a blastocyst.
The condition of the cells that make up the blastocyst, and the size of the blastocoel within it, indicate the blastocyst's quality and the likelihood that it will result in a successful pregnancy and is a factor in embryo grading for in vitro fertilization (IVF).
FertilitySmarts explains Blastocoel
After an egg is fertilized by sperm, the cells begin to divide and move through various stages of development. The first stage is called a zygote and results when a fertilized egg begins to divide.
The second stage occurs 4 days after fertilization. At this point, a solid ball of 10-30 cells, known as a morula, is created.
The third stage occurs on the 5th day after fertilization. The dividing cells within the morula begin to differentiate. At the same time a cavity begins to form as cells become specialized and move around. This space is known as the blastocoel. At this stage, there is 70-100 cells present and the structure is called a blastocyst.
At one end of the blastocyst, there is a group of cells called the inner cell mass (ICM). These cells are stem cells which will later produce all the cells of the fetus. There is also a layer of cells forming the outer rim of the blastocyst, known as the trophoblast. These cells provide food to the developing embryo and eventually contact the uterine wall. The blastocoel fills with fluid and protects the ICM from surrounding tissues.
As the blastocyst develops, the blastocoel expands and the zona pellucida (a membrane surrounding the blastocyst which is still present from the egg) begins to thin. The blastocyst needs to break free of the zona pellucida in order to successfully implant in the uterine wall. This process is called "hatching" of the blastocyst. The larger the blastocoel is, the closer the blastocyst is to hatching. For this reason, a blastocyst with a well expanded blastocoel is given a higher grade by embryologists than those with smaller blastocoels. Blastocysts with the highest grades will be selected for transfer as they are most likely to result in a successful pregnancy.