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Cryopreservation is the freezing of live cells or tissue specimens for later use. Today, both sperm and embryos are routinely frozen, stored in liquid nitrogen, and later thawed for use in assisted or advanced reproductive technology (ART) treatments.

Sperm Freezing

Sperm have been undergoing freezing and storage for many years. The procedures for doing this are relatively easy. The most common procedure for sperm freezing involves washing the sperm cells out of semen, and then mixing the purified cells with special fluids (media) and ice crystal preventing substances (cryoprotectants). This mixture is then submerged in liquid nitrogen which is very very cold and the cells are deep frozen. One of the most common sperm freezing media is made from egg yolks, so this is why many commercially available frozen sperm specimens are yellow colored after they are thawed. Cryopreserved sperm can stay frozen for many years; at least for decades.

Sperm are sometimes obtained directly from the male reproductive tract by various surgical retrieval procedures. This may be done because the man has no sperm in his semen or has an otherwise uncorrectable problem. These small amounts of "harvested" sperm can also then be frozen. Generally there are too few sperm from these collections to do normal insemination with tham after thawing. These sperm are generally used in combination with ART and are directly injected into eggs individually to produce embryos with the ICSI (intracytoplasmic sperm injection) procedure.

Embryo Freezing

The first baby born from a frozen embryo was back in 1984. Since then, the techniques for freezing embryos have become very well established and have become completely routine in ART labs. For couples undergoing IVF treatment, cryopreservation allows surplus embryos to be frozen, and then later thawed for additional opportunities for a pregnancy. Freezing of embryos not only gives you extra chances for a pregnancy, but it also allows fertility clinics to transfer fewer embryos without lowering their overall fertility success rate. The transfer of fewer embryos lowers the risk for multiple births when pregnancy is achieved with IVF. Worldwide, cryopreservation of human embryos has been shown to be a beneficial procedure as there are no reports of increased birth defects in pregnancies achieved through this process.

Cryopreservation Techniques

Cryoprotectants are used in the solution in which the embryos or sperm are frozen. There are two types in use - permeating and extracellular. These additives are useful because they lower the freezing point and prevent the formation of ice crystals as the temperature is lowered. They also protect cells from damaging their outer membranes as they change from a pliable to a rigid state during freezing.

To freeze an embryo, it is placed in the solution and the temperature is slowly lowered in a very controlled manner. Once frozen, the embryos are then placed in liquid nitrogen to maintain temperature until the embryos are thawed for use later.

Embryos can be frozen at the pronuclear stage (one cell), or at any stage after that up to and including the blastocyst stage (5-7 days after fertilization). The type of protectant used depends upon the stage of embryo development. Embryo survival rates after thawing in most IVF programs are highest for embryos that were frozen at the pronuclear stage, or at the blastocyst stage. The survival rates for good-quality embryos at all stages of development are high in good labs.

Pregnancy Success Rates

Success rates for frozen embryo transfer cycles vary considerably from clinic to clinic. Some programs have never had a pregnancy and live birth from their frozen embryo transfer cycles while the best programs have delivery rates of about 20-30% per transfer for frozen-thawed embryo cycles.

There is really no way to know the success rates of an IVF program that you are considering unless you investigate sources of public-certified success rates. To do this, you can first check with the Society for Assisted Reproductive Technology (SART) for programs reporting results to their national database. If the clinic you are considering is not reporting their data to SART, you will need to ask very specific questions of your physician to understand their success rates.

Two key questions to ask are:

1. What percentage of embryo thaw cycles initiated resulted in a transfer? Since some thaws may not result in a viable embryo for transfer, this question helps you understand how well the program can not only freeze embryos, but also ready them for a frozen transfer cycle.

2. What is the rate of pregnancy and live birth resulting from the frozen embryo transfers done by the clinic?

Egg Freezing

Egg freezing is a rapidly improving technology. Advancements are being made to offer commercial egg freezing services, but at this point in time it is a controversial practice that is not endorsed by any of the leading fertility organizations. This is because the egg, on its own, is extremely fragile and susceptible to damage in the freezing process. Thus far, success in freezing and thawing eggs remains unpredictable in most hands.

New techniques are showing promise, however. One procedure called "vitrification” allows for eggs to be frozen without the formation of any ice crystals, which is what typically damages an egg when it is frozen. Some fertility clinics are already offering egg freezing, but you should be aware that the technology is still considered “emerging” because there are not many studies showing it offers a better rate of successful thawing than the conventional freezing methods. Some groups continue to improve on their slow freezing methods and some groups from around the world are reporting competitive success rates. There is already a commercial frozen egg donor bank here in the US that has a business model very similar to those of sperm donor companies (although frozen eggs cost much more than frozen sperm samples). Stay tuned for updates! You can rest assured that advances in this field will be fast and furious.

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