By Merritt McKinney

NEW YORK, Mar 02 (Reuters Health) - Sperm is routinely frozen for later use in assisted reproduction, but freezing human eggs has proved more difficult, with many eggs not surviving the thawing process. But by tinkering with the way eggs are frozen, fertility doctors in Italy say they have greatly improved the survival rate of frozen eggs.

The researchers were able to increase egg survival to greater than 80%, according to a report in the March issue of the journal Human Reproduction.

Besides helping couples conceive, the advance in the freezing process, or cryopreservation, should reduce the need to freeze embryos, according to the study's lead author. An embryo is the product of sperm and egg, and is much more amenable to freezing and thawing.

"The most important benefit coming from oocyte (egg) cryopreservation is to solve ethical problems coming from embryo storage," Dr. Rafaella Fabbri, of the University of Bologna, told Reuters Health.

When couples do not use a frozen embryo, the embryo may end up lingering in cold storage indefinitely or be thrown away, a practice that troubles many people, she said. Disposing of extra frozen eggs should not raise any ethical problems, since they are unfertilized cells, according to Fabbri.

"If oocyte cryopreservation becomes more common in other centers, we will be able to avoid embryo cryopreservation," she said. The process may also benefit female cancer patients at risk for treatment-related infertility. Such patients may want to store their eggs so that if they want to have children later in life, the possibility exists.

Fabbri and her colleagues found that two changes in the freezing process improved the likelihood that eggs would survive.

Before eggs are frozen, they are exposed to a solution that contains a chemical protectant and sugar. Doubling the amount of sugar in this solution increased the survival rate from 34% to 60%. The survival rate reached 82% when the investigators tripled the amount of sugar.

The researchers suspect that adding more sugar to the solution encouraged more complete dehydration of the eggs before freezing. Eggs must be dehydrated before freezing to prevent the formation of ice crystals, which can kill an egg by puncturing its membrane.

Fabbri's team also found that prolonging the time eggs are exposed to the protective solution increased the survival odds. Seventy percent of eggs exposed to the solution for 10.5 to 15 minutes survived, compared to 55% to 56% of eggs that spent less time in the solution.

Surprisingly, the presence of the cumulus oophorus, a cluster of nourishing cells that surrounds an egg, did not have an effect on an egg's survival odds. The cells have been thought to improve the odds of an egg surviving the thawing process, but Fabbri and her colleagues found that eggs without a cumulus oophorus were no less likely to survive.

Of course, the proof is in the pudding, which in this case is the rate of successful fertilizations and pregnancies. In a sample of 96 women who had their eggs frozen, more than half of all eggs survived the thawing process. And 91% of embryos formed from these eggs reached the so-called cleavage stage the day after fertilization, which is a good indicator of future survival. Seventy-four percent of the embryos were of "good or fairly good quality," according to the researchers.

Fabbri told Reuters Health that 26 pregnancies have resulted, including several healthy births.

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