Reproductive endocrinologists use two methods to freeze embryos: vitrification (flash freezing) and slow programmable freezing. Although the processes differ greatly, each works by cooling embryonic cells with various cryoprotectants (“antifreeze” fluids). Cells, including those forming embryos, are primarily made of water. The concern with freezing embryos is that ice will form and damage cells when they are eventually thawed. Cryoprotectants are used to safeguard biological tissue from damage by preventing water from crystallizing during the freezing process.
In the slow-freezing method, lab embryologists place embryos in a special machine that slowly cools them in stages. They then add cryoprotectants and once the embryos are frozen, they are stored in liquid nitrogen at -321 degrees Fahrenheit. The entire process takes about two hours. Vitrification is a newer, more successful technique that places embryos in a solution containing a much higher concentration of cryoprotectants. Embryos are then immersed in liquid nitrogen, instantly freezing them into a glass-like substance. Using this form of freezing, the embryo is stored before ice crystals have a chance to form, thereby increasing the embryo’s chance of survival and viability after thawing. The laboratory team at Women and Infants Fertility Center exclusively uses vitrification for all new embryo cryopreservation.
No matter the method used, frozen embryos are effectively suspended in time. All biological activity within the embryo is stopped, including cell growth or death. When needed, embryos are slowly thawed and soaked in fluids to remove cryoprotectants and to restore water within cells. Frozen embryos can be stored for an indefinite period of time. However, the longest duration an embryo stayed frozen and was successfully used resulting in a healthy pregnancy was 19 years.
