What's New in the IVF Laboratory?
There are two methods that can be used to freeze embryos. The first, and most common method is known as slow freezing . Recently, however, the use of vitrification (from vitrum, the Latin word for glass) has been on the rise due in part to the development of improved methods and the availability of commercial vitrification kits. To freeze embryos successfully, regardless of the method used, the water in the cells must be removed. As the temperature is lowered, water forms ice crystals and ice causes cellular damage. The water in the cells is replaced with cryoprotectants which act like antifreeze and protects cells from damage during freezing.
Slow freezing, as the name suggests, is done slowly. The concentration of cryoprotectant used is relatively low. The embryo is cooled at 0.3 centigrade degrees per minute to -30oC, plunged into liquid nitrogen, and stored in liquid nitrogen at -196oC. Because the concentration of cryoprotectant is low, ice crystal formation may still occur and damage part or all of the embryo. When an embryo is vitrified, it is exposed to much higher concentrations of cryoprotectants and an extremely high rate of cooling (up to 23,000 centigrade degrees per minute). Ice crystal formation is totally eliminated as the insides of the cells form a glass-like solution.
The trend in IVF today is to replace our slow freezing methods with vitrification. With the elimination of ice crystal formation, vitrification has proven to be superior to more traditional methods. However, just as with slow freezing, embryo quality is important. Regardless of which method is used, the best quality embryos have the highest chance of surviving freezing and thawing and resulting in a viable pregnancy.
For years it has been relatively easy to freeze sperm and embryos. However, methods to reliably freeze eggs (oocytes) have been elusive with only a few research groups worldwide achieving success using slow freezing methods. Over the past few years the shift in interest towards applying methods of vitrification to eggs has led to remarkable success. Simplified methods have been developed and tested. Commercial kits are now available making it feasible for most IVF laboratories to add oocyte freezing to their repertoire of techniques.
The success of oocyte vitrification has opened up the field of fertility preservation to women. Women suffering from illnesses who are at risk of losing their fertility due to the disease or its treatment may wish to freeze and store their eggs until they are ready to start a family. The ability to freeze eggs successfully may also be of interest to healthy young women wishing to delay their child rearing years for social reasons. Oocyte vitrification could potentially stop the biological clock and eliminate the age-related limit on female reproduction.
We have all heard of donor sperm banks. They have been around for a long time.. Now, as the methods for oocyte freezing have become simplified and more reliable, egg banks are being established. It is now possible to select an egg donor and have her frozen eggs thawed, inseminated, and the embryos transferred without the inconveniences of synchronizing schedules and cycles inherent in fresh egg donation.
Vitrification promises to be the next major advance in the IVF laboratory after intracytoplasmic sperm injection (ICSI), preimplantation genetic diagnosis (PGD), and the development of culture media to grow human embryos to the blastocyst stage. The next few years will bring continued improvements and excitement to those of us who spend our days in IVF labs.
An important note about egg freezing:
The Practice Committee of the American Society for Reproductive Medicine (ASRM) has issued an opinion stating that egg freezing is an experimental procedure. At the present time, there is not enough research data available to show that oocyte freezing in effective or safe enough to be considered established medical practice. The committee recommends that women who are considering having their eggs frozen should be counseled extensively.