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If confirmed, harnessing such cells may lead to better treatments for women left infertile by disease or age.

The Associated Press

WASHINGTON - For 60 years, doctors have believed that women were born with all the eggs they'll ever have. Now Harvard scientists say they've found that the ovaries of young women harbor rare stem cells capable of producing new eggs.

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READ A SUMMARY of the report on how women's stem cells can be turned into eggs: tinyurl.com/6w6kass

If Sunday's report is confirmed, harnessing those stem cells might one day lead to better treatments for women left infertile because of disease -- or simply because they're getting older.

"Our current views of ovarian aging are incomplete. There's much more to the story than simply the trickling away of a fixed pool of eggs," said lead researcher Jonathan Tilly of Harvard's Massachusetts General Hospital, who has long hunted these cells in a series of controversial studies.

A next step is to see whether other laboratories can verify the work. If so, then it would take years of further study to learn how to use the cells, said Teresa Woodruff, fertility preservation chief at Northwestern University's Feinberg School of Medicine.

Still, even a leading critic said such research may help dispel some of the enduring mystery surrounding how human eggs are born and mature.

"More than anything else, it's giving us some new directions to work in," said David Albertini, director of the University of Kansas' Center for Reproductive Sciences.

Scientists have long taught that all female mammals are born with a finite supply of egg cells, called ooctyes, that runs out in middle age. Tilly first challenged that notion in 2004, reporting that the ovaries of adult mice harbor some egg-producing stem cells.

But do they exist in women? Enter the new work, reported Sunday in the journal Nature Medicine.

Tilly collaborated with scientists at Japan's Saitama Medical University, who were freezing ovaries donated for study by healthy 20-somethings who underwent sex-change operations.

He had to figure out how to tell if he was finding true stem cells or just very immature eggs.

His team latched on to a protein believed to sit on the surface of only those purported stem cells and fished them out. To track what happened next, the researchers inserted a gene that makes some jellyfish glow green into those cells. If the cells made eggs, those would glow, too.

"Bang, it worked -- cells popped right out" of the human tissue, Tilly said.

Researchers watched through a microscope as new eggs grew in a lab dish. Then came the pivotal experiment: They injected the stem cells into pieces of human ovary. They transplanted the human tissue under the skin of mice, to provide it a nourishing blood supply. Within two weeks, they reported telltale green-tinged egg cells forming.

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Rare stem cells may produce new eggs, scientists say