Until recently, the only method for oocyte cryopreservation was a slow-freezing. Unlike sperm, which has been successfully frozen and used for years, oocytes contain a great deal of water, which makes freezing much more difficult. It is namely associated with a risk that ice crystals can be formed when the eggs are frozen, which may cause destruction of the cell’s structure.
Vitrification is a novel device for safe storage of oocytes while using a specialized technique, which freezes the egg so quickly that ice crystals don’t have time to appear. By this reason, vitrification is now widely applied for cryopreservation in reproductive medicine.
In an ordinary freezing procedure, scientists remove some of the water as the egg is slowly freezing, but it’s impossible to remove all the water, and thus to prevent formation of some ice crystals. Consequently, fertilization and pregnancy rates for these slow-frozen eggs, once thawed, is relatively low.
Vitrification of eggs involves using high concentrations of an anti-freeze substance. Firstly, the oocyte is placed in the bath with a lower concentration anti-freeze, along with some sucrose, or sugar, which helps to draw some of the water out of the egg. Then, the egg is put in high-concentrated bath of anti-freeze for less than one minute, while being instantaneously frozen. Because anti-freeze is potentially toxic to the egg, the technique requires special care.
When it’s time to thaw the egg, the oocyte must be quickly warmed up and removed from the anti-freeze solution right away. Once thawed, the egg can be fertilized using Assisted Reproductive Technology, including IVF or ICSI.
Scientists highly appreciate the method of oocyte vitrification which is important for women with various indications for assisted reproduction technology. It is especially beneficial for young women with cancer who run the risk of having no viable oocytes following cancer treatment and couples who have religious or moral issues with embryo freezing. Recent studies have demonstrated the safety of oocyte vitrification in cancer patients and have not shown earlier recurrence or worsening of survival rates.
Modern method of oocyte vitrification has recently been one of the major topics of reproductive medicine discussions, in particular, on ESHRE and ASRM meetings. It appears that this method of egg freezing has clearly won out over the older slow-freezing. In the framework of International reproductive medicine symposium “From Science to Practice” that was held in Kyiv on 5-7 April 2012, a special presentation containing information and results on studies involving oocyte vitrification was given by Dr. Zsolt Peter Nagy, of Reproductive Biology Associates, Atlanta, USA. He emphasized that one of the novels pertaining to the use of oocyte vitrification also lies in the field of oocyte donation. Currently, nearly all oocyte donation cycles are fresh donations from an egg donor undergoing ovarian stimulation. However, it is realistic that in the next few years that egg donors may be able to undergo ovarian stimulation and have oocytes frozen well in advance of the time that they are going to be used. It is likely that egg banks will be established by fertility clinics that would be a viable option for couples.
As for Ukrainian legislation, vitrification method for cryopreservation of sperm, oocytes, embryos and ovarian tissue is expressly stipulated by the „Instruction on Procedures for Assisted Reproductive Technologies”, adopted by the Order of the Ministry of Health of Ukraine No. 771 of 12/23/2008, and successfully applied by Ukrainian fertility clinics.