Reproductive Health Center, Tucson, Arizona

The technology behind freezing eggs, sperm, and embryos

The technology behind freezing eggs, sperm, and embryos – fertility clinic near Scottsdale

Although it was once thought to be impossible, cryopreservation (freezing) of eggs, sperm, and embryos has now become common. These types of tissues can now be placed into cold storage, and will still be viable (capable of life and development) when they are thawed. How has this become possible? What obstacles have scientists overcome to help couples have families?

Why do people freeze eggs, sperm, and embryos?

People who choose to freeze their eggs or sperm are usually aiming to preserve their fertility for the future. They may be about to undergo a medical treatment that might destroy their fertility, such as chemotherapy, and they don’t want to lose their chance at parenthood forever. For others, they’re simply not ready to have children yet, but want to ensure that healthy eggs or sperm are preserved while they’re young to give their future children the best possible chance. Aging affects the health of both eggs and sperm.

Embryos are frozen by couples who are undergoing IVF treatment. In some cases, this happens because more viable embryos are created than can be transferred in a single cycle, and the parents want to save these embryos for future use. For many patients at our fertility clinic near Scottsdale, the uterus is not in an ideal condition for transferring any fresh embryos (created right after the egg retrieval cycle). When this happens, all of the embryos are frozen, and will be thawed for transfer in future cycles when the uterus is ready.

What are the obstacles when freezing tissues?

If you’ve ever frozen a piece of fruit, vegetable, or meat, and then noticed that it was mushy when you thawed it out and had lost a lot of its taste, you’ve encountered the damage that can be done when tissues are frozen.

The reason that freezing causes tissue damage is that a living cell contains a high proportion of water. When it freezes, water expands. It also forms ice crystals, which tend to be sharp and cause serious damage to cells. Often, the formation of ice inside of a cell will create holes in the cell’s outer membrane. Such a tissue will not be viable when it is thawed.

This is why fertility specialists can’t simply put human tissues into a freezer to preserve them. It took years of research to figure out how to freeze cells so that they would still be viable when thawed.

How are cells protected during freezing?


In order to allow cells to be frozen without destroying them, scientists use a solution called a cryoprotectant. This is a mixture of compounds that acts very much like antifreeze. The cells are soaked in the cryoprotectant, which replaces much of the water inside of the cell. This helps to prevent ice crystals from forming as cells freeze, or from being damaged by the expansion of water as it becomes ice. Cryoprotectant also helps to pull water out of the cell, so it will be slightly dehydrated.

Because sperm are very small cells (the smallest in the body), they contain only a small amount of water. This makes them easier to protect. That’s why sperm were the first tissues to be frozen, and still remain viable after thawing. This occurred in the 1950s.

By contrast, the egg is the largest cell in the human body, and an embryo is made up of multiple cells. The larger size of embryos and eggs makes them more challenging to freeze. It took more research to find a cryoprotectant that would be capable of removing enough water from these tissues to protect them while freezing. Embryos were successfully frozen before eggs were. The first successful live birth of a baby who developed from a previously frozen embryo occurred in 1986, while the first live birth of a baby conceived from a previously frozen egg occurred in 1999.


In addition to protecting the cells with cryoprotectant, the speed of cooling is also important. The traditional technique for freezing live tissues was slow cooling. This worked relatively well for sperm, and there was some success with this technique for embryos and eggs as well.

However, a newer technique called vitrification has shown much promise for improving the survival rates of embryos and eggs. This involves cooling the tissues very quickly. The rapid cooling prevents the formation of crystals. Vitrification requires a higher concentration of cryoprotectant to protect the cells. Studies have shown that vitrification leads a higher survival rate for frozen embryos than does slow cooling, and that the embryos may be more likely to implant successfully and result in a live birth.

How long can tissues stay frozen?

Because there is very little biological activity at very cold temperatures, frozen tissues can remain viable for surprisingly long periods of time. Healthy babies born after being frozen as embryos for more than a decade are routine; the longest so far is 13 years. A healthy baby has been reported who was conceived with sperm that had been frozen for 28 years. Animal sperm have remained viable after being frozen for more than 50 years.

Although egg freezing has not been performed long enough to know for sure how long eggs will remain viable, scientists and doctors expect that eggs will respond to freezing similarly to embryos. This means that eggs are expected to retain viability for at least 10 to 15 years, and perhaps longer.

Fertility clinic near Scottsdale

At our fertility clinic near Scottsdale, we help people at many stages of the fertility journey. Young adults who aren’t yet ready to start a family, or those undergoing chemotherapy or other fertility-threatening medical care, may choose to freeze sperm or eggs. Those who are currently trying to achieve a pregnancy and are undergoing IVF may want to freeze their embryos for future use.

If you’d like to learn more about freezing sperm, eggs, or embryos, or using donated frozen tissues to help you start your family, please contact us to schedule a consultation. We’ve helped to create thousands of families throughout Arizona and beyond. Wherever you are in your fertility journey, you can trust us to provide you with the most up-to-date information about the latest in fertility technology, as well as our personal, caring touch.

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