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Advances in egg freezing

10 October 2005
By BioNews
Appeared in BioNews 329

A new technique for freezing human eggs increases the pregnancy rate per embryo transferred to 34 per cent, scientists report. A research team based in Japan, Denmark and the US, 'vitrified' the eggs on the surface of a fine plastic strip before thawing and fertilising them using ICSI. The results, published in the September issue of Reproductive Biomedicine Online, are a 'big step forward', according to UK IVF expert Simon Fishel. Meanwhile, a trial has begun in the Boston IVF clinic, US, to test the efficiency of a new technique that involves injecting eggs with a sugar before freezing.

Freezing eggs is notoriously difficult, since they are very large cells that are susceptible to damage by ice crystals that form during the freezing process. To try and overcome this problem, clinics usually store eggs in tiny plastic straws after using a solution to draw out the moisture - a process called vitrification. However, the success rate of fertility treatment using frozen eggs has remained low, at best about ten per cent. Only around 200 babies worldwide have been born using frozen eggs so far.

In the latest study, the scientists placed the egg in a drop of vitrifying solution on the surface of a specially constructed fine polypropylene strip attached to a plastic handle. They first showed that when freezing cow eggs, the technique worked better than using plastic straws. The researchers then tested the 'Cryotop' method on 64 human egg cells. After thawing, 58 (91 per cent) of the eggs appeared undamaged, and 52 were successfully fertilised using ICSI - 32 of which developed to the blastocyst stage of development (when the embryo consists of a few hundred cells).

The team then transferred a total of 29 embryos into patients, an average of 2.2 embryos per woman. Seven healthy babies have since been born, and three women are still pregnant. Embryologist Virginia Bolton, of King's College Hospital in London, said that 'if these figures can be borne out through other research it will be very promising'. Simon Fishel, of the Care centre in Nottingham, said that until now, doctors had been discouraging women from freezing their eggs for so-called 'social reasons' because of the low success rates. 'This research could change all that', he told the Sunday Telegraph newspaper. A trial at Boston IVF clinic has enrolled 20 patients and 20 egg donors to test another new technique for freezing eggs, which involves injecting them with a sugar called trehalose. Study leaders Tom Toth and Mehmet Toner told the Boston Globe that they drew inspiration from creatures like brine shrimp and arctic frogs, which use sugars naturally to protect their cells when they freeze or 'dry up'. They have developed a technique for coating the inside of an egg with trehalose, which is now licensed to  a US company called Gamete Technology. The trial should provide 'exactly the kind of data the field would welcome in an ongoing assessment of the status of the technology', said Marc Fritz of the American Society of Reproductive Medicine.

SOURCES & REFERENCES
A baby-step closer to freezing eggs
The Boston Globe |  3 October 2005
Egg freezing breakthrough will create generation of 'ice babies'
The Daily Telegraph |  9 October 2005
Highly efficient vitrification method for cryopreservation of human oocytes
RBM Online |  1 September 2005
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