Oocyte cryopreservation

Human Oocyte cryopreservation (egg freezing) is a rapidly advancing, breakthrough technology in which a woman’s eggs (oocytes) are extracted, frozen and stored. Later, when she is ready to become pregnant, the eggs can be thawed, fertilized, and transferred to the uterus as embryos.

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History

Cryopreservation itself has always played a central role in assisted reproductive technology. With the first cryopreservation of sperm in 1953 and of embryos thirty years later, these techniques have become routine. Although Australia reported the world’s first pregnancy using frozen oocytes in 1984,^[1] until recently egg freezing has not produced success rates seen with traditional in vitro fertilization (IVF) using either fresh or frozen embryos. Now, according to an article published in Fertility and Sterility, "recent reports indicate pregnancy rates comparable to those for cryopreserved embryos by either slow-freeze or vitrification methods." ^[2]

Indications

Oocyte cryopreservation is aimed at three particular groups of women: those diagnosed with cancer who have not yet begun chemotherapy or radiotherapy; those undergoing treatment with assisted reproductive technologies who do not consider embryo freezing an option; and those who would like to preserve their future ability to have children, either because they do not yet have a partner, or for other personal or medical reasons.

Over 50,000 reproductive-age women are diagnosed with cancer each year in the United States. ^[3] Chemotherapy and radiotherapy are toxic for oocytes, leaving few, if any, viable eggs. Egg freezing offers women with cancer the chance to preserve their eggs so that they can have children in the future.

Oocyte cryopreservation is an important option for individuals undergoing IVF who object, either for religious or ethical reasons, to the practice of freezing embryos. Having the option to fertilize only as many eggs as will be utilized in the IVF process, and then freeze any remaining unfertilized eggs can be a positive solution. In this way, there are no excess embryos created, and there need be no disposition of unused frozen embryos, a practice which can create complex choices for certain individuals.

Egg freezing can also be beneficial for women who, for the purpose of education, career or other reasons, desire to postpone childbearing. Freezing eggs at an early age may ensure a chance for a future pregnancy.

Additionally, women with a family history of early menopause have an interest in fertility preservation. With egg freezing, they will have a frozen store of eggs, in the likelihood that their eggs are depleted at an early age.

Method

The egg-retrieval process for oocyte cryopreservation is the same as that for in vitro fertilization. This includes several weeks of hormone injections and hormonal contraception in order to stop ovulation, followed by more hormone injections to stimulate ovaries and ripen multiple eggs. When the eggs have matured, additional hormone is given and the eggs are removed with an ultrasound-guided needle through the vagina. The procedure is conducted under sedation. The eggs are immediately frozen.

The egg is the largest cell in the human body and contains a great amount of water. When the egg is frozen, the ice crystals that are formed can destroy the integrity of the cell. To prevent this, the egg must be dehydrated prior to freezing. The water is then replaced by a special cryoprotectant to inhibit the formation of ice crystals.

Eggs are frozen using either a slow-freeze method or a flash-freezing process known as vitrification. The slow-freeze method is the most studied and is most similar to current embryo freezing techniques. Vitrification is a rapid freezing process in which a high concentration of cryoprotectant is used. The result is a solid glass-like cell, free of ice crystals. There are differing schools of thought on which freezing method is superior, though both methods have availed acceptable pregnancy rates. ^[4]

Additionally, once frozen, the zona pellucida, or shell, of the egg hardens, requiring embryologists to inject the sperm into the egg with a needle in order for fertilization to occur. This technique is called ICSI (Intracytoplasmic Sperm Injection) and is also used in IVF.

Success Rates

Approximately 200 births resulting from frozen eggs have been documented worldwide. Among these births, the rate of birth defects and chromosomal defects has been consistent with that of the general population. ^[5] Additionally, genetic screening of the eggs, and the resultant embryos is possible.

Recent modifications in protocol regarding cryoprotectant composition, temperature and storage methods have had a large impact on the technology, and while it is still considered an experimental procedure, it is rapidly becoming a real option for women. Slow freezing is the most commonly used method to cryopreserve oocytes, and is the method that has resulted in the vast majority of babies born from frozen oocytes worldwide. Ultra-rapid freezing, or Vitrification, achieving a glass-like state, represents a potential, though less frozen, alternative to freezing.

In the fall of 2004, The American Society for Reproductive Medicine (ASRM) issued an opinion on oocyte cryopreservation concluding that the science was "promising" because recent laboratory modifications have resulted in improved oocyte survival, fertilization, and pregnancy rates from frozen-thawed oocytes in IVF. ^[6] The ASRM noted that from the limited research performed to date, there does not appear to be an increase in chromosomal abnormalities, birth defects, or developmental deficits in the children born from cryopreserved oocyes. The ASRM recommends that, pending further research, oocyte cryopreservation should be introduced into clinical practice only on an investigational basis and under the guidance of an Institutional Review Board (IRB). As with any new technology, safety and efficacy must be evaluated and demonstrated through future research.

Cost

The cost of egg freezing, (including the embryo transfer) is comparable to that of IVF and ranges from $12,000 to $20,000. Egg storage can be several hundred dollars or more per year.

See also

In vitro fertilization

References