ABSTRACT
Our knowledge of the cryopreservation of human embryos in liquid nitrogen has made great strides in recent times. The excess number of human embryos which are often present in programs of in vitro fertilization-embryo transfer (IVF-ET) have been a force in perfecting this process. Not all embryos developed can be transferred, owing to the risk of multiple pregnancies, thus necessitating 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/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 compromising the functioning of the ovaries (premature ovarian failure, endometriosis, cysts, and pelvic infections) could be assured of 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 because of career demands, the lack of a partner, or pathologies which temporarily prevent pregnancy another choice in family planning. And, as a last point, the utilization 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 biologic features of oocytes, and various questions have been raised about inducing aneuploidy after the gametes have been exposed to
cryoprotectants and the freezing-thawing process. 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, where the oocytes are extremely sensitive to changes in temperature and eventual depolymerization of the microtubules of the spindle caused by cryoprotectants or ice crystals formed during the freezingthawing process, the normal separation of the chromatids at the moment of fertilization could be impaired, thus inducing aneuploidy after extrusion of the second polar body. As cited in the literature, the low number of pregnancies after the cryopreservation of oocytes2-7 demonstrates the important technical difficulties that this procedure faces.
