What Is a Blastocyst?
A blastocyst is a cellular mass which forms early in the process of embryo development in mammals. In humans, this stage of embryogenesis occurs five days after fertilization, when there are less than 100 cells in the mass. At this stage of development, the structure has not yet implanted in the uterus.
Blastocysts develop from precursors called zygotes, which are fertilized eggs. Once fertilized, the egg undergoes a rapid series of cell divisions, followed by differentiation of the cells. This takes several days, and at the five day mark, the zygote develops into the blastocyst.
Within the blastocyst there are two types of cells. In the interior of the blastocyst is the inner cell mass. A portion of the cells in this mass will soon begin to divide at a very rapid pace and become the developing fetus. The inner cell mass is surrounded by a fluid-filled cavity called the blastocoele. Surrounding this is a single layer of cells called the trophoblast, which will combine with the endometrial cells of the uterus to become the placenta.
Blastocyst transfer is an in vitro fertilization method which many people opt for as an alternative to three-day embryo transfer. Transferring blastocysts, which are five days old, is a more effective option because they tend to be more stable than three-day embryos, and more likely to result in a pregnancy. This is because approximately 50% of embryos do not reach day five of development, therefore by transferring five-day embryos there is a greater chance that the embryo will be viable after transfer.
Another advantage of using five-day blastocysts for implantation is that because there is a greater chance of viability, there is less need to implant multiple embryos. Many people prefer this option because there is a much lower chance of multiple pregnancies. This can be dangerous for the pregnant woman and for the developing babies, with an increased risk of underweight babies, miscarriage, and still birth.
Blastocysts are important in a field of scientific research called embryonic stem cell research. Embryonic stem cells are believed to have special properties that other types of stem cells do not have, perhaps due to the fact that in embryonic cells, gene expression has barely begun. Blastocysts used in this type of research are obtained from in vitro fertilization clinics with the express permission of the biological parents of the embryos. Generally, people allow their embryos to be used in this way if they are no longer needed for implantation, and if they are comfortable with the blastocysts being used in research.
Written by Emma Lloyd
