A blastocyst is a cluster of dividing cells created by a fertilized egg, representing the early stage of an embryo. It forms about five to six days after a sperm fertilizes an egg, and the layers of cells in the blastocyst divide and separate to become the structures that protect and nourish the developing fetus. The blastocyst stage is particularly important for in vitro fertilization (IVF), a process aimed at creating an embryo outside of the birthing parent’s body to assist with pregnancy.

What are the steps in fertilization and embryonic development?

After a person begins their menstrual period, their ovaries release one egg (ovum) about 14 days later. The egg travels into one of their fallopian tubes, where it awaits fertilization by a sperm. If fertilization doesn’t occur, the egg moves into the uterus and is released from the body during the next menstrual period. However, if fertilization does occur, the fertilized egg becomes a zygote, which is a single cell with genetic material from both parents. The zygote then travels through the fallopian tube to the uterus, a process that takes about three to five days. The zygote’s cell continuously divides, eventually forming a hollow ball of cells known as the blastocyst. The blastocyst remains in the uterus for several days before implanting in the inner lining of the uterine wall (endometrium). It continues to develop, forming new cells that separate into layers. About 10 to 12 days after fertilization, the blastocyst develops into an embryo, which remains an embryo until about nine weeks after implantation, at which point it becomes a fetus.

What is the purpose of the blastocyst?

The blastocyst stage is a crucial part of embryonic and fetal development. If the blastocyst fails to implant in the person’s endometrium, pregnancy will not occur. For implantation to take place, hormones trigger a process known as hatching. The blastocyst sheds its clear outer membrane, a step that occurs one to three days after it enters the uterus. Following this, cells on the outer layer of the blastocyst attach to the outer layer of the endometrium. They release a sticky protein called L-selectin, which binds with substances in the endometrium. These outer cells eventually develop into the fetus. Meanwhile, cells in the inner layer of the blastocyst embed more deeply into the endometrium. These cells ultimately form the placenta, which provides oxygen and nutrients to the developing fetus. Some of the placental cells also contribute to the formation of the

Why is the blastocyst so important for IVF?

In IVF, healthcare providers assess and grade blastocysts in a lab to identify the embryos most likely to result in a pregnancy. Blastocysts are typically transferred to the uterus five or six days after fertilization. Healthcare providers assess blastocysts based on factors such as maturity, shape, cell number, and density.