Embryo Freezing

Embryo Cyropreservation (Freezing)

Embryo cryopreservation techniques and risk associated with embryo freezing

There are several techniques of embryo freezing and they are continuously improved. Currently, these techniques include the so-called slow freezing – gradual freezing of embryos supported by special devices; and rapid freezing, or vitrification.

The most harmful factor in the process of embryo freezing are ice crystals forming inside the embryos. They can damage the fragile inner structures of cells and in the result disturb or even prevent further embryo development.

Slow freezing controlled slow freezing

In order to reduce the formation of crystals, various procedures using special chemical compounds called cryoprotectants are applied. These are usually polyalcohols and monosaccharides: polyethylene glycol, glycerol, glucose, sucrose, and others. Cryoprotectants are intended to protect cells by displacing intracellular water from which crystals form. However, the level of these compounds in a cell cannot be too high as, in such a case, they become toxic for cells.


Vitrification is a process of transition from liquid to solid state without the formation of ice crystals unfavourable for cells. This effect is achieved by applying high concentration of cryoprotectants and rapid cooling (at the speed of 1,000-30,000°C/minute). A structure forming during the process of vitrification resembles glass.

How are the eggs 'frozen'?

The donor eggs are technically not frozen – rather, they are vitrified. Vitrification is an advanced technique that allows eggs to be stored at -196 degrees Celsius (-320 Fahrenheit) with little or no effect on their function upon warming. At this temperature all metabolic activity ceases and the eggs are essentially in ‘suspended animation’. Although most eggs are used within months or years of being frozen, they are viable for decades at this temperature.


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