Women who become infertile after being treated for cancer could one day be able to have children.
Thousands of women become infertile after cancer treatment
Doctors in the United States say they have successfully transplanted ovarian tissue into a 36-year-old woman.
The tissue was removed from the woman six years previously, before she had chemotherapy for breast cancer.
Writing in The Lancet, they said the woman now had fully functional ovaries. They said the technique could help other women to become fertile again.
Hundreds of thousands of woman around the world become infertile after undergoing treatment for cancer. Chemotherapy, radiotherapy and radical surgery can all trigger early menopause.
The woman involved in this case had her ovarian tissue frozen before she had such treatment. The tissue was put back six years later.
Doctors at New York Presbyterian Hospital who carried out the procedure said her ovaries started working again after three months.
The woman has since undergone IVF to try to get pregnant. Doctors have managed to remove 20 eggs from her ovaries.
One of these eggs was fertilised and implanted in the woman. However, she did not become pregnant.
Nevertheless, doctors say the transplant could help women to have children even after cancer treatment.
"This research represents a potentially significant reproductive advancement in two respects," said Dr Kutluk Oktay, one of those involved in the work.
"First, women can preserve their fertility by freezing their ovarian tissue, and, second, pregnancy may be possible even after the tissue remains frozen for a long time."
In an editorial in The Lancet, a leading European expert said the treatment was still experimental.
"The whole procedure should still be presented as experimental to patients," said Johan Smitz of the Centre for Reproductive Medicine at Vrije Universiteit.
"Safety concerns exist at different levels when applying this technique in patients."
Professor Bill Ledger, head of obstetrics and gynaecology at the Royal Hallamshire Hospital in Sheffield, welcomed the study.
"This takes them over another big hurdle," he told the BBC.
"It is the first time that a human embryo has been formed from an egg that was produced from frozen ovarian tissue transplanted back into woman."
But he warned the further research would be needed before it could be offered to patients.
"The problem is that it raises hopes in women with cancer desperate to want to do this, but we are years away from this being available to young women as a way of storing tissue."
Dr Virginia Bolton, a clinical embryologist at King's College Hospital in
London, said the research results were "very encouraging".
"This technique is a big step forward but we should not get overexcited as
there is a long way to go before this becomes a routine treatment," she said.