Human eggs don't freeze well -- or so IVF specialists have always thought. But biologists in New Jersey now say they have overcome the problem, by abandoning the idea that eggs should be frozen in a solution that resembles body fluids.
The researchers have obtained high survival rates after cryopreserving and then thawing mouse eggs. If their technique also works with human eggs, then women whose eggs are frozen -- before they undergo chemotherapy that damages their ovaries, for example -- will have a better chance of becoming mothers.
Fertility centres routinely freeze sperm and embryos, but only a few births have been reported using frozen eggs. James Stachecki of the Institute for Reproductive Medicine and Science of St Barnabas Medical Center in West Orange, New Jersey, wondered if the saline solution used to freeze eggs was to blame.
Cells must be surrounded by a more concentrated solution during freezing. This pulls water out of the cells by osmosis and reduces the chances of ice crystals forming, which can damage cell structures. Cryobiologists usually use a saline designed to mimic body fluids.
But Stachecki suspected that sodium ions from such solutions were getting into the eggs and poisoning them. Instead, he decided to try a solution containing choline ions, which do not readily cross cell membranes. Choline is an organic molecule found in many plant and animal tissues, and is a constituent of B-complex vitamins. Cryopreservation experiments on other types of cells using choline solutions had produced promising results.
Using hundreds of mouse eggs, Stachecki and his colleagues found that the choline solution allowed 90 per cent to survive freezing and thawing. And when the surviving eggs were fertilised, 60 per cent developed into the ball of cells called a blastocyst (Cryobiology, vol 37, p 346).
"The results are very encouraging," says Roger Gosden, a reproductive biologist at the University of Leeds. With conventional saline, only 50 per cent of eggs survived freezing and just 10 per cent of those fertilised went on to form a blastocyst.
"The plan now is to do more work in humans to test how well it works," says Stachecki. Several women have already volunteered to donate their eggs for the research. If the new technique works for humans, it may allow easier egg banking at fertility clinics.
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