Researchers removed and froze ovaries from eight sheep, then thawed and re-implanted them. Eggs obtained from two of the animals produced six embryos.

Although fragments of ovarian tissue have been frozen and transplanted before, doctors agree it would be better to use whole ovaries.

An important advantage is that removing the whole ovary preserves vital blood vessels which otherwise have to re-grow.

'Could Revolutionize the Field of Cryopreservation'

The Israeli team led by Dr. Amir Arav, from the Agricultural Research Organization in Bet Dagan, removed and froze ovaries from eight sheep.

Attempts to thaw and re-implant the ovaries back into the animals resulted in five successful transplantations.

Eggs obtained from two of these sheep subsequently produced six embryos.

Dr. Arav said, "This approach could revolutionize the field of cryopreservation for diverse human applications, such as organ transplants, as well as helping women who face the loss of their fertility."

Normal Conception

Two babies have already been born after their mothers were given transplants of frozen and thawed ovarian tissue.

The first birth, which made headlines around the world, was announced by Belgian doctors in September last year.

Quarda Touirat had tissue from one of her ovaries removed, cut up, and frozen in liquid nitrogen seven years earlier before undergoing chemotherapy that made her infertile.

Doctors led by Professor Jacques Donnez, from the Universite Catholique de Louvain, implanted a piece of the tissue into Ms. Touirat's pelvis.

It began producing eggs, allowing normal conception to take place. At a press conference the day after baby Tamara's birth, Ms. Touirat, then 22, said, "I'm very happy. It's what I've always wanted. It was a dream."

More Convincing Evidence

Last June a second birth arising from a transplant of preserved ovarian tissue was reported in the New England Journal of Medicine.

This time the mother was an unnamed 28-year-old woman who underwent the procedure in Israel. Her doctors, led by Dr. Dror Meirow, from the Sheba Medical Center in Tel Aviv, said they had more convincing evidence than the Belgian team that the woman really had been infertile.

Dr. Arav's new research was published today in the journal Human Reproduction.