ABSTRACT
Our knowledge of the cryopreservation of human embryos in liquid nitrogen has made great steps forward in recent times. The excess number of human embryos that are often present in the programs of in vitro fertilization/embryo transfer (IVF/ET) were a force in perfecting this process. Not all the embryos developed can be transferred, owing to the risk of multiple pregnancies, thus necessitating the storage of the surplus embryos in liquid nitrogen. However, doctors, patients, legislators, and, above all, the public have legal, moral and religious problems with the cryopreservation of human embryos. The use of this technique has been restricted or even forbidden in some countries, such as Germany, Austria, Switzerland, Denmark and Sweden.1 One solution to these problems could be the cryopreservation of female gametes. The condition of iatrogenic sterility after chemo-or radiotherapy in neoplastic pathologies would be avoided by the preservation of oocytes, as in the cryostorage of sperm. In addition, even women who suffer from pathologies of the reproductive system such as the functioning of the ovaries (premature ovarian failure, endometriosis, cysts, and pelvic infections) could insure potential fertility using this technique, which was unheard of until recently. The use of frozen oocytes in a program of assisted fertilization would be able to guarantee the maintenance of fertility in patients with these pathologies. The cryopreservation of oocytes could also allow women who delay maternity due to career demands, the lack of a partner, or to pathologies that momentarily prevent pregnancy, another choice in family planning. And, as a last point, the use of frozen oocytes could be included in a program of oocyte donation. The storage of male gametes or human embryos has faced fewer problems than the cryopreservation of oocytes. This is due to the biological features of the oocytes, and various questions have been raised about inducing aneuploidy after the gametes have been exposed to cryoprotectants and the freezing and thawing process. The oocytes are, in fact, blocked at ovulation at the metaphase of the second meiotic division, where 23 dichromatidic chromosomes are bound to the microtubules of the meiotic spindle. In this phase, when the oocytes are extremely sensitive to changes in temperature and the eventual depolymerization of the microtubules of the spindle caused by cryoprotectants or ice crystals formed during the freezing and thawing process, the normal separation of the chromatids at the moment of fertilization could be impaired, thus inducing aneuploidy after the
extrusion of the second polar body. As cited in the literature, the low number of pregnancies after the cryopreservation of oocytes2-7 shows the important technical difficulties this procedure faces.
