For women at risk of losing their fertility through medical treatments or premature menopause, preserving some of their eggs ahead of time by freezing them can leave open the possibility of having children. But such cryopreservation of human eggs has proved an extremely difficult technique to master. Indeed, in the 15 years since researchers first attempted the feat, only some 30 babies have resulted. Results reported today in the journal Human Reproduction, however, may offer new hope. According to new research, tweaking the freezing technique enabled scientists to more than double egg survival.
Eggs generally deteriorate during the freezing process because they are not sufficiently dehydrated. Excess water forms ice crystals, which can pierce the egg's membrane and kill the cell. Rafaella Fabbri of the University of Bologna and her colleagues found that increasing the amount of time the eggs were exposed to chemical protectants and boosting the amount of sugar in the freezing solution improved dehydration. In fact, they were able to increase egg survival from 34 percent to as much as 82 percent. Subsequent fertilization by injection of a single sperm into an egg (ICSI) resulted in a 57 percent success ratecomparable to that for fresh oocytes.
"Our next step will be to improve even further the oocyte survival rate after thawing," Fabbri notes. "But our study has established that it is possible to cryopreserve human oocytes and that ICSI could be an efficient method of achieving satisfactory fertilization."