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What’s the difference between slow freezing and vitrification?

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What’s the difference between slow freezing and vitrification?

November 21, 2016   |   Ask the Doctor, The Real Deal

Let’s say you’re a woman freezing your eggs. You’ve finished your medication, you did your trigger shot, and your retrieval was a painless success. Hooray! Now what comes next for your eggs?

There are two types of methods that can be used by embryologists to freeze and preserve eggs: vitrification, a “flash freezing” technique, and slow freezing, an older technique that is, well, exactly what it sounds like.

Here’s how slow freezing works: the cell is cooled very slowly—at a decrease of .3º–2º Celsius per minute—until it reaches the final storing temperature of -196º Celsius (or about -320º Fahrenheit). That’s the temperature required for the frozen eggs to be safely preserved—it ensure all biological processes cease inside the cell. In total, the entire process takes a couple of hours, which is why this technique is termed “slow freezing.” (It’s also sometimes called “controlled-rate” or “slow programmable” freezing.)

In contrast, vitrification is a “flash freezing” method that cools cells so quickly to -196ºC that they become “glass-like” or “vitrified.” While the slow freezing technique takes hours, vitrification is completed in minutes.

As it turns out, speed is really important for the success of egg freezing. Studies have demonstrated that the longer the freezing process takes, the more likely it is that ice crystals will form in the cell. Ice crystals are the bane of the cryopreservation world—they can cause damage to cell structures, reducing the chance that the cell will be functional when it’s thawed. Vitrification, alternatively, requires a higher concentration of cryoprotectants to freeze cells at a faster rate, decreasing the risk that ice crystals will form inside the cell. This is especially important for egg freezing, since eggs (as opposed to other things we might freeze, like sperm) are large size cells with a high content of water.

Slow freezing, first used in 1986, was the only reliable way to freeze eggs for almost 20 years, before about a decade ago, when biologist Dr. Masashige Kuwayama and others developed breakthrough vitrification protocols that brought the vitrification process to a safe and consistent stage that could be used at the clinical settings. We say “breakthrough” because it was truly a game-changer for cryopreservation: studies have found that vitrified eggs have an over 83% freeze-thaw survival rate (compared to the approximately 66% survival rate of eggs during slow freezing).

Time and time again, studies have demonstrated the superiority of vitrification for egg freezing. One study that appeared in Fertility and Sterility found that “vitrification…is an efficient, fast, and economical method for oocyte cryopreservation that offers high rates of survival, fertilization, embryo development, and ongoing normal pregnancies, providing a new alternative for the management of female infertility.” Another study determined that the oocyte (egg) survival rate was significantly lower in the slow freezing group (61%) than in the vitrification group (91%) and that slow freezing was associated with higher rates of egg abnormalities. And another report, A critical appraisal of cryopreservation (slow cooling versus vitrification) of human oocytes and embryos, concludes that “available evidence suggests that vitrification is the current method of choice when cryopreserving MII [mature] oocytes.”

In fact, it was partially due to the success of vitrification that, in 2013, the ASRM decided to remove the label of “experimental” for egg freezing.

Both types of cryopreservation are still in use by embryologists today. But here at Extend Fertility, we listen to the science—the many studies that have clearly demonstrated that vitrification is the superior freezing technique with much higher success rates for egg preservation. That’s why vitrification has become the cornerstone of our lab.

In the video below, see our embryologist Dr. Leslie Ramirez talking about what happens inside our lab:

Learn more about cryopreservation and vitrification!


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