Japanese scientists have presented research at the annual conference of the European Society of Human Reproduction and Embryology (ESHRE) in Prague, Czech Republic, today, suggesting that the inequality between men and women when it comes to freezing gametes - particularly following treatment for cancer, which might render them infertile - may be able to be balanced out. The research team 'vitrified' the eggs on the surface of a fine plastic strip before thawing and fertilising them using intracytoplasmic sperm injection (ICSI).

Sperm cells freeze easily, and can be thawed and used in treatments such as IVF with high levels of success. Egg freezing has proved to be successful in a small number of cases, but is less effective than freezing sperm as eggs cells are much larger and contain more water - so when they freeze, ice crystals can form that can damage the cell. Because of the low success rate, egg freezing has so far been used mainly for women undergoing treatments that can affect their fertility. In the UK, the first baby to be born from a previously frozen egg was as late as 2002 - having only been made legal in 2000 - and only just over 200 such babies have been born so far worldwide.

Dr Masashige Kuwayama, from the Kato Ladies' Clinic in Tokyo, told the conference that he has developed a new method of freezing human eggs, which he first used when he was involved in animal research. 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 used the 'ultra-rapid' Cryotop' method on 111 human egg cells. After thawing, 94.5 per cent of the eggs appeared undamaged, and 90.5 were successfully fertilised using ICSI. Fifty per cent of these were then successfully developed to a stage at which they could be transferred to the womb. The team then transferred a total of 29 embryos into patients aged between 25 and 37, an average of 2.3 embryos per woman. Twelve pregnancies resulted from these transfers (making a pregnancy rate of 41.9 per cent) and 11 healthy babies were born, to nine women.

Dr Arne Sunde, former chairman of ESHRE, said that biological differences between egg cells, sperm cells and embryos was the reason for the different success rates in freezing techniques - while sperm and embryo freezing has been a 'routine procedure' since the 1980s, freezing mature egg cells had proved 'very difficult' - Kuwayama's team may have made a 'major improvement' in egg cryopreservation, he said. He added that 'for the first time, cryopreservation of oocytes represents a realistic option for the preservation of fertility in women who are in need of aggressive treatment for malignant diseases'.

Reproduced from BioNews with permission, a web- and email-based source of news, information and comment on assisted reproduction and human genetics, published by Progress Educational Trust.