Inner Cell Mass (ICM)

Definition - What does Inner Cell Mass (ICM) mean?

The inner cell mass (ICM) is a group of cells in a fertilized egg that develops into an embryo. It is the central part of a structure called the blastocyst, which is formed around 5 days after fertilization. During the process of in vitro fertilization (IVF), blastocyst grading is used to determine which embryos are most likely to implant within the uterine lining. The inner cell mass is one element evaluated to determine which blastocysts are the most ideal for use in IVF.

FertilitySmarts explains Inner Cell Mass (ICM)

After an egg is fertilized, it divides rapidly into many cells. After about 5 days, the mass of cells is called a blastocyst. It is very small, 0.1-0.2 mm in diameter, and is made of about 200-300 cells. There are two layers in this structure: the outer trophoblast, which develops into the placenta, and the inner cell mass, which eventually forms the embryo. If you imagine blowing up a balloon with a ball inside, the balloon is the trophoblast around the outside, while the ball is the inner cell mass inside.

When fertilized eggs are grown to the blastocyst stage as part of an IVF procedure, embryologists assign a grade to the inner cell mass. There is a direct relationship between the size and shape of the inner cell mass and how likely the blastocyst is to implant into the uterine wall. Inner cell masses that are larger and more oval shaped are more likely to implant. There is also a link between the compactness of the inner cell mass and the likelihood of developing into twins. Inner cell masses that are loosely packed in shape are more likely to develop into monochorionic, diamniotic twins (identical twins that share a placenta but have two amniotic sacs).


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