Two teams of researchers have independently shown it is possible to take immature egg cells from women's ovaries and grow them to full maturity inside a mouse. They believe the development offers a way of maturing the eggs of young women or girls who have had their ovaries removed before undergoing cancer treatment that would have left them permanently sterile.
The research will be greeted with dismay by organisations opposed to developments in human reproductive medicine or to animal experiments. But the breakthrough could benefit scores of women undergoing cancer therapy who have had their ovary tissue frozen in the hope that it might one day be used to help them have babies.
This week the world learned that a British scientist, Roger Gosden, professor of reproductive biology at Leeds University, had helped to pioneer the first ovary transplant, which promises to revolutionise female fertility by allowing the use of stored ovaries. Professor Gosden has also managed to take immature human eggs cells from an ovary and "ripen" them inside the body of an immune-deficient mouse, which does not reject "foreign" tissue.
The research was repeated by a team of reproductive scientists from the Toronto Hospital in Canada, who hope to use the technique to restore the fertility of cancer patients.
Professor Gosden said in his recent book, Designer Babies, that a trans- species "xenograft" should be used only in extreme circumstances. "If we did use them the animal would not, of course, become pregnant, as the egg would have to be removed and fertilised with a human sperm in vitro and then transferred to a human womb to stand any chance of pregnancy," he wrote. "I would not recommend growing eggs in animals until a great deal of research has been done to reassure us that there are no risks of viruses or prions [the proteins that cause BSE] jumping species and infecting the egg."
The Leeds team inserted the human ovarian follicles under the kidney membrane of the laboratory mouse where they grew to full maturity and were ready for transplantation into a human womb. The Toronto team grew the follicles under a mouse's skin. Half of the eggs matured.
Adam Balen, consultant obstetrician and gynaecologist at Leeds University, who worked with Professor Gosden, said the ethical and practical difficulties of using animals as living incubators makes the research less attractive than chemical cultivation of human eggs in the laboratory.
"It's not something we are actively pursuing," Mr Balen said. "Reproductive medicine is on the interface between all sort of ethical and moral questions and we have to handle these things very carefully.
"So whatever we do must be done properly, and within ethical constraints."