Embryos may be frozen at different stages of development, using various slow-freezing or vitrification (ultra-rapid freezing) techniques.
In slow-freezing, embryos are placed in a cryogenic medium containing substances which protect the embryo against the cold. These substances and the slow decrease in temperature protect the embryos against the formation of ice crystals, which is important for their survival. The embryos are then transferred from the medium into thin straws for storage and frozen using cryogenic equipment whereby the rate of temperature decrease is carefully controlled. Frozen embryos are then placed in liquid nitrogen (-196 °C) for long-term storage.
In vitrification, the cryogenic medium contains a larger concentration of substances protecting the embryo against the cold than the media used in the slow-freezing process, and the embryos are frozen rapidly by placing them directly in liquid nitrogen (-196 °C). The cryogenic medium forms a protective layer around the embryo and no ice crystals are formed. Vitrification is particularly suitable for the freezing of eggs and it has considerably improved egg survival. At the moment our clinics only use the vitrification technique on an investigational basis.