Researchers said they have transformed stem cells isolated from women's ovaries into viable-looking eggs, a provocative experiment that might suggest new ways for treating infertility.

Biologists have long held that women are born with a finite supply of eggs that gets depleted with age. The latest experiment, published in Nature Medicine, describes how stem cells found in the ovary could potentially be coaxed into rejuvenating the natural egg supply.

Men produce sperm all their life. Now, women "are no longer faced with the idea that there's a fixed bank account of eggs at birth with only withdrawals and no deposits," said Jonathan Tilly, a reproductive biologist at Massachusetts General Hospital in Boston and lead author of the paper. The study was funded by the National Institutes of Health and other groups.

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The research is at an early stage, and the quest for practical applications could founder on many obstacles. The human egg is an unstable cell prone to genetic error. Creating eggs from stem cells could enhance those risks.

"When you amplify stem cells in culture they can become unstable," said David Albertini, a reproductive biologist at the University of Kansas Medical Center, who wasn't involved in the study. "There's a difference between Mother Nature doing this" and attempting it in a lab.

Underpinning the new approach is Tilly's discovery that the ovaries of reproductive-age women harbor tiny quantities of stem cells that can potentially be isolated and then cultured in the lab to become oocytes, or normal, immature egg cells.

Using existing procedures, a woman concerned about infertility can have her eggs extracted and frozen, then stored for use at a later date. The goal under the new procedure would be for doctors to extract and freeze a small piece of her ovarian tissue containing stem cells -- a potentially less-invasive and faster procedure.

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Stem cells, which are about one-tenth the size of eggs, also hold much less water and therefore could be less damaged by the freezing and thawing process.

An estimated 10 percent of all child-bearing-age women in the US have difficulty getting or staying pregnant, according to the US Centers for Disease Control and Prevention. About half seek treatment, few get it, and not all are successful. Some procedures can be lengthy, expensive and unpleasant.

The ovary of a female fetus at five months holds seven million eggs, but that number drops to one million at birth and 300,000 or fewer by puberty. The supply keeps falling and gets exhausted at menopause, typically when a woman is in her late 40s or early 50s. Biologists have believed there was no way to increase the supply of eggs.

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