Egg freezing, also known as human oocyte cryopreservation, is a method to preserve a woman's eggs for fertilization at a later time. This is usually undertaken in patients with cancer, as a means to safeguard the egg from the harmful effects of chemotherapy and radiation treatments.
The first successful pregnancy resulting from the use of frozen eggs occurred in 1986. Since then, the technology required for oocyte cryopreservation has improved, and the pregnancy rate has gradually increased. Specifically, the discovery of effective cryoprotectants and improved regulation of the freezing/thawing rates have made egg freezing a viable option for women.
Cryoprotectants are compounds that prevent the formation of ice crystals within the egg. Ice crystal formation within the egg can damage and rupture the cell membrane during freezing. To prevent this, the egg is immersed in cryoprotectants before freezing. The most effective are 1,2-propanediol, sucrose and DMSO. The duration and concentration of the exposure must be controlled to prevent damage to the egg. During the thawing process, the cryoprotectant must be removed through progressive dilution. Otherwise, the egg may swell and burst as water flows in. An advantage of using sucrose as the cryoprotectant is that it also acts as a buffer between the egg and the water in the thawing medium. This reduces the risk of the egg swelling.
The safest method to freeze the egg is through a gradual process. The cryoprotectant is added sequentially every 10 to 20 minutes. Afterward, the temperature is reduced between 0.3 and 2 degrees each minute, until it reaches -196 degrees Celcius. This takes about two hours. The slow freezing allows the cryoprotectant to enter the egg and displace water, which eliminates the risk of ice crystals.
A quicker method for freezing eggs is called vitrification. It involves immersing the eggs directly into liquid nitrogen and using a much higher concentration of cryoprotectants. The increased concentration of cryoprotectant is toxic to the eggs at room temperature, so rapid cooling is required immediately. There is a risk of infectious contamination when eggs are in direct contact with liquid nitrogen. The goal of vitrification is to avoid a phase change from liquid to solid and preserve the egg in its existing state.
When eggs are ready for in vitro fertilization, they are thawed by air and water bath. The process is quicker than freezing and is done in a step method. The cryoprotectants are slowly diluted as the temperature is raised to room temperature and then to body temperature.
Changes in the Egg
Regardless of the way the egg is frozen, there are several factors which hinder the success of fertilization. The main obstacle is a shell around the egg called the zona pellucida. It undergoes changes when cortical granules are released. Normally, they are released at fertilization, to prevent multiple sperm from entering the egg. Upon freezing or thawing, the release is premature, blocking any sperm from entering.
Improvements in Fertilization
A technique called intracytoplasmic sperm injection (ICSI) greatly improved the success of in vitro fertilization of frozen eggs. ICSI uses a micropipette to insert a sperm into an egg by piercing the membrane. This method overcomes the problem of premature cortical granule release.