Often at the end of an in vitro fertilization (IVF) cycle, there are extra embryos that can be frozen (cryopreserved) for later use. There are several advantages to embryo freezing. If the patient conceives with the fresh embryos, then she can have another baby when she is ready. If the fresh cycles is unsuccessful, then the patient can have another chance at pregnancy without having to go through another stimulation and egg retrieval. Using a frozen embryo in an IVF cycle is relatively cheap and simple when compared with a fresh IVF cycle.
Historically, fertility clinics have used slow-freezing techniques involving computerized freezing machines. For the past few years, these machines have been replaced by an ultra-rapid cooling technique called vitrification. At InVia Fertility Specialists, we generally freeze embryos 5-6 days after egg retrieval when they are at the blastocyst stage.
What is vitrification?
It is an ultra rapid cooling technique that produces a glasslike solidification of living cells by elevating the viscosity during the cooling period. It is a cryopreservation method for preserving the structure, viability, and function in living tissues for an extended period of time. It preserves the integrity of the cells because it prevents ice crystal formation
How is it performed?
Vitrification is a relatively simple procedure where the cells are placed directly into cryoprotectants and then plunged into liquid nitrogen. First, the embryos are placed in an equilibrium solution for about 8 minutes to begin the dehydration process. The first solution contains about 15% cryoprotectants. Through osmosis the cyoprotectants will draw out the cell’s water and cross the cell membrane, filling the cell to reach equilibrium. After that, the embryos are moved into a solution containing 30% cryoprotectants for 60 seconds and loaded onto a straw. Immediately following, the straw is plunged into liquid nitrogen.
Below is a diagram of the process. A: An embryo (clearly visible is the inner cell mass and the trophectoderm). B: The cell begins to shrink down because water is being drawn out. C: Cryprotectants enter the cell and allow it to re-expand. D: The embryo re-expanded and survived the vitrification process.
Collapsing the Embryo
A majority of the embryos vitrified at are at the blastocyst stage. At the blastocyst stage there is a large fluid filled cavity. If the embryo is not collapsed before being placed in the equilibration solution for cryopreservation then the cryoprotectants have a hard time entering the cell and the fluid leaving the cell are slowed. In order to bypass this issue, a small hole is made between two outer cells using a laser allowing the embryo to collapse.
One downfall from the use of cryoprotectants is it can cause hardening of the zona pellucida. This causes an issue because the embryo needs to escape from the zona and attach to the uterus wall. With the use of a laser, a small hole is made in the zona to overcome hardening and allow the embryo to escape.
Kelly Schorsch is one of the members of the Embryology team at InVia Fertility. She works in both the Andrology and Embryology sections. She completed her graduate studies at Eastern Virginia Medical School in Clinical Embryology and Andrology and her undergraduate studies at Roosevelt University with a major in Biology, minor in Chemistry, and certificate in Biotechnology. She loves to be behind the scenes in the laboratory to help couples achieve their dreams of one day having a baby.