Estat actual de les tècniques de criopreservació en la optimització de la fertilitat femenina

Cryopreservation consists of freezing cells or tissues at very low temperatures using liquid nitrogen in order to avoid cellular death. Substances called cryoprotectants are used to protect the cell against thermal shock, ethylene glycol and dimethyl sulfoxide are used among others. However, as a result of cooling, deleterious effects occur on cells that could compromise their survival and viability. Cryopreservation is a useful process for assisted reproductive technologies because this method allows preserving the two types of sex cells, oocytes and spermatozoa, as well as embryos. Embryo and oocyte cryopreservation becomes important when female fertility must be preserved, for instance in cases of cancer or autoimmune diseases and, for women with reproductive dysfunction due to other reasons. Currently, there are two methods used for embryo and oocyte cryopreservation in the laboratories of clinical embryology: slow freezing and vitrification. In the first one, cells are exposed to moderate concentrations of cryoprotectants, both permeating and non-permeating and, a gradual decrease on temperature is produced up to liquid nitrogen boiling temperature (-196oC). In the vitrification procedure, cells are exposed to high concentrations of cryoprotectants and, by a direct contact with liquid nitrogen, ultrafast freezing is achieved. Embryo cryopreservation is widely used nowadays, while oocyte cryopreservation is still considered an experimental technique. This is because the advantages of embryo features in comparison to oocyte; for example, embryos have a better permeability to cryoprotectants and they are more resistant to cell damage after cryopreservation. Nevertheless, oocyte cryopreservation has some advantages in terms of medical handling and financial costs. In addition, the last one does not imply so many moral, ethical and religious controversies compared to embryo cryopreservation. The most commonly used cryopreservation method is mainly vitrification, which provides better success rates than slow freezing. Despite progress made in assisted reproductive technologies over last years, cryopreservation is still a subject of extensive research; because there is the purpose to improve the protocols in order to achieve the best possible results with the maximum security