For oocyte cryopreservation, there are several methods available to achieve the process. As the demands for embryo freezing have increased, so have the number of IVF clinics adopting vitrification. At many clinics, this is the freezing method of choice.
Conventional freezing methods have several problems. Numerous studies have determined that ice crystals cause damage to embryos. Although survival rates after thawing them varied between 50% and 60%, many embryos did not survive slow freezing.
Vitrification is an advanced cell-freezing technique that allows a greater survival rate after thawing. High concentrations of cryoprotectant are used in vitrification, along with reduced volumes and a generally more rapid process.
Vitrification can be used to freeze oocytes or embryos. The survival rate is rated over 80%. For oocytes, this technique enables vitrified oocytes to have similar attributes to their fresh, unfrozen counterparts.
IVF treatment involves the hormonal stimulation of a woman’s ovaries to produce eggs for collection and subsequent fertilization in a laboratory setting. After fertilization, one or two embryos are transplanted into the womb two days later. The remaining embryos can undergo slow freezing or vitrification, then long-term storage or applied for use soon if the initial treatment cycle fails.
Cryoprotectants have characteristics similar to antifreeze: they have a higher viscosity than water, which prevents the formation of sharp crystalline structures that invariably cause cell damage. Instead of crystallizing, the syrupy solution becomes an amorphous substance free of ice crystals.
Vitrification occurs by increasing the viscosity of the solution and decreasing the freezing temperature to sub-zero levels rapidly. Compared to slow freezing, vitrification has an increased potential to successfully bank human eggs.
In terms of embryos, the following are some advantages of vitrification over slow freezing:
- The process is so fast that it prevents the formation of intracellular ice, avoiding trauma to the embryo.
- Vitrified embryos have a freeze-thaw survival rate above 95%. The potential of these embryos to result in pregnancy is comparable to that of fresh embryos.
- This technique is efficient. Ten embryos can be frozen within the span of a half hour.
The disadvantages of slow freezing include the following issues:
- It causes ice crystal formation.
- The survival rate for embryos and oocytes is only around 50%.
- For the embryos that survive the freeze-thaw procedure, they have less than half the likelihood of resulting in pregnancy than do fresh embryos.
- Very time consuming. Embryos are cooled using a programmable freezer before they reach the appropriate temperature. It can take several hours to freeze embryos using this technique.
