The cryopreservation of oocytes, a window of hope for many women fertility

The cryo preservation of oocytes is the freezing of the female gamete for later use. These cryopreserved oocytes should retain their intact ability to generate viable embryos, capable of generating normal pregnancies and healthy newborns.

This technique brida a possibility of fertility to women who until now had it difficult for different reasons. It would be the case, for example, of women who, for social or economic reasons, to safeguard their fertility, decide to undergo an ovarian stimulation program and obtain a number of oocytes to freeze and use them in the future; women who are going to be subjected to treatments (oncological or radiotherapy) that could damage their oocytes and compromise their future fertility; women who have exhausted their ovarian reserve or who are of poor quality, with repeated failure in treatments, and who require the donation of an oocyte; infertile of unknown origin, among others.

The doctors who work in assisted reproduction, the freezing of oocytes and their subsequent thawing, fertilization and transfer with rates of reasonable success, has freed us from the problem of frozen embryos , their storage, donation and, as has already happened in some country, its destruction.

The vitrification

The vitrification has solved many of the problems of the cryo preservation of oocytes , being the procedure that is now used and obtaining results similar to those of fresh oocytes.

With this technique, an ultra-rapid cooling of the oocyte that has been previously exposed to a hyperosmolar solution for dehydration is performed. The cytoplasm solidifies acquiring a glassy consistency, hence the name. With this procedure, the formation of intracellular crystals, one of the main problems of freezing, and oocyte damage do not occur.

The oocyte must be dehydrated rapidly using a hyperosmolar freezing medium. The main problem of vitrification is derived from the cyto-toxic effects of the cryo-protectors themselves and from the oocyte's own exposure to extreme temperatures.

To try to solve this, several mixtures of cryopreservators have been designed and, in general, the principle of reducing the volume and increasing the speed of vitrification is followed as the most obvious way to avoid oocyte damage .

Risks and results

The risks in the cryo preservation of oocytes are related to the cross-contamination of samples from open systems -although there are very demanding protocols that guarantee that this does not occur as ultraviolet sterilization techniques-, or the damage of the meiotic spindle of the oocyte or the cytoplasmic structure, either by exposure to cryo-preservers or high temperatures.

The microstructural analyzes performed on post-thaw oocytes and the clinical results themselves have shown that this does not occur with vitrification and new cryopreservatives .

The vitrification does not exert any harmful effect that diminishes the capacity of implantation of the embryos generated by this procedure being an essential technique in any unit of reproduction that opens a window of hope for the fertility of many women. The data corroborates this, as it has a 97% survival rate, a 76% fertilization rate, a 65% pregnancy rate and an implantation of 41% similar to those obtained with fresh .