Scientists working with sheep have for the first time developed embryos from whole ovaries which were transplanted after being frozen and then thawed.

The journal Human Production reported that eggs obtained from two such ovaries produced early sheep embryos.

And researchers at Israel's Institute of Animal Science believe the procedure could one day work with humans.

Scientists have been seeking ways to preserve the fertility of women undergoing aggressive cancer treatment.

One option is to harvest, fertilise and then freeze a woman's eggs, but the rate of successful pregnancies following this method is low.

Another option is to freeze and transplant thawed strips of ovarian tissue. There have been reports of two babies born following this technique.

Again, the success rate is limited because the transplanted tissue may become scarred or may not develop the new blood supply it needs to survive.

Researchers have questioned whether it might be better to transplant the whole ovary in such circumstances.

Human whole ovary transplants have already been attempted twice in women, moving the patient's own ovary up in to the arm, but in neither case was the ovary frozen and thawed first.

Freeze-thaw method

Dr Amir Arav and his team at the Institute, in Bet Dagan, Israel, tested whether ovaries from eight sheep, together with their blood vessels, could survive the freezing (cryopreservation) and thawing process.

They chose to study sheep because their ovaries are similar to those of humans.

Five of the eight frozen and thawed ovaries were successfully transplanted and set up a normal blood flow in the sheep.

Two yielded eggs. One produced more than four eggs four months later. From these the researchers were able to make early sheep embryos.

Three years on, researchers went back and examined one of the sheep and found the transplanted ovary still appeared to be working and producing eggs.

Dr Arav said: "We have been able to demonstrate long-term intact cryopreservation with restored functioning following thawing and transplantation, in a large animal for around 36 months post-transplantation."

Co-investigator Yehudit Nathan, from Core Dynamics biotech company that funded the project, said: "There is a lot of research still to be done, but we hope that it will not take more than a few years for this to become a practicable option for women."

Dr Allan Pacey, senior lecturer at the University of Sheffield and Secretary of the British Fertility Society, said: "Research work is proceeding on a number of fronts to give women more fertility preservation options - freezing eggs or slices of ovarian cortex - but it is still hard to tell which technique will finally enter mainstream clinical practice.

"This work shows it might be possible to freeze a whole ovary prior to chemotherapy and transplant it after the treatment had finished.

"But we need to see if it works in humans and we need to make sure it is safe.