What Is Embryo Vitrification and How Does It Work?

Embryo vitrification is a cryopreservation method used in assisted reproductive technologies (ART) to store embryos for later use. It allows for the preservation of embryos from an in vitro fertilization (IVF) cycle, which can then be transferred in a future cycle. This approach offers flexibility for individuals and couples building their families.

The Vitrification Procedure Explained

Vitrification is founded on the principle of ultra-rapid cooling, which transforms an embryo and its surrounding medium into a glass-like state without forming ice crystals. The primary challenge in freezing biological materials is that water inside cells can form sharp ice crystals, causing severe damage to cellular structures. Vitrification avoids this by using high concentrations of cryoprotectants and an extremely fast cooling rate.

The procedure begins by exposing embryos to solutions with increasing concentrations of cryoprotectants, which replace the water in the cells to prevent ice formation. The embryos are then loaded onto small, specialized devices and plunged directly into liquid nitrogen at -196°C. This ultra-rapid cooling solidifies the embryo into a vitrified state in minutes.

The warming process must be as rapid and controlled as the cooling. The device holding the embryo is quickly warmed, and the cryoprotectants are washed away and replaced with a culture medium. This precise warming prevents ice crystals from forming as the embryo returns to a liquid state, ensuring its cellular structures remain intact.

Clinical Uses of Embryo Vitrification

Its most common application is preserving surplus embryos from an IVF cycle. When a patient has more healthy embryos than needed for a fresh transfer, vitrification allows them to be stored. This provides the opportunity for future transfers without another cycle of ovarian stimulation and egg retrieval, saving time, money, and reducing physical stress.

Vitrification is also a method for fertility preservation. Individuals about to undergo medical treatments that can harm their fertility, such as chemotherapy or radiation, can freeze embryos for later use. It also provides an option for those who wish to delay starting a family for personal or social reasons, allowing them to preserve their fertility at their current age.

The technique is also used with Preimplantation Genetic Testing (PGT), which screens embryos for genetic or chromosomal abnormalities. Vitrification allows embryos to be safely stored while awaiting test results, after which a healthy embryo can be selected for transfer. This process increases the chances of a successful pregnancy and reduces the risk of genetic disorders. Additionally, vitrification is used in embryo donation programs to store and transfer donated embryos to recipients.

Success Rates and Embryo Viability

A key measure of vitrification’s effectiveness is the embryo survival rate after thawing, which is often above 95% with modern techniques. This means most frozen embryos remain viable for transfer. The quality of the embryo at the time of freezing is a major factor influencing this success.

Clinical outcomes for vitrified-warmed embryos are often comparable to, and sometimes better than, fresh embryo transfers. Implantation rates can range from 40-60%, depending on factors like maternal age and embryo quality. Some studies suggest frozen transfers may lead to higher live birth rates because the transfer can be timed for a cycle when the uterine lining is more receptive, free from the effects of ovarian stimulation hormones.

Other factors impacting success include the embryo’s developmental stage at the time of vitrification and the laboratory’s proficiency. Studies have shown that the health outcomes of children born from vitrified embryos are comparable to those born from fresh transfers and natural conception.

Vitrification Versus Slow Freezing

Vitrification has largely replaced the older method of embryo cryopreservation, known as slow freezing, in fertility clinics. The slow-freezing technique involved a gradual and controlled reduction in temperature. A major drawback of this method was the risk of ice crystal formation, which could cause irreversible damage and lead to lower survival rates after thawing. The process was also much longer, sometimes taking up to three hours.

The primary advantage of vitrification over slow freezing is its prevention of damaging ice crystals. This results in significantly higher embryo survival rates and better preservation of the embryo’s cellular integrity. The speed of the vitrification process, which takes about 10 minutes compared to several hours for slow freezing, is another benefit.

Due to these advantages, vitrification has become the standard of care in most advanced IVF laboratories. The shift to vitrification has led to an increase in the number of frozen embryo transfers performed annually. The improved outcomes and reliability have changed IVF treatment strategies, making “freeze-all” cycles a more common and effective approach.