Reaching a long-sought milestone, Japanese researchers have demonstrated in mice that eggs and sperm can be grown from stem cells and combined to produce healthy offspring, pointing the new treatments for infertility.

If the achievement can be repeated in humans and experts said they are optimistic that such efforts will ultimately succeed the technique could make it easier for women in their 30s or 40s to become mothers. It could also help men and women whose reproductive organs have been damaged by cancer treatments or other causes.

About one in 10 American women of childbearing age have trouble becoming or staying pregnant, and more than one-third of infertile couples must contend with a medical problem related to the prospective father, according to the national Centers for Disease Control and Prevention in Atlanta.

Using current technology, only about one-third of attempts at assisted reproduction result in live births, CDC data show. Scientists, doctors and patients would like to boost that percentage.

"These studies provide that next level of evidence that in the future fertility could be managed with stem cell intervention," said Teresa Woodruff, chief of fertility preservation at Northwestern University Feinberg School of Medicine.

The prospect of using stem cells to grow new eggs is particularly tantalizing, since women are born with a set number and don't make more once they are gone. In a sense, the therapy would allow them to turn back their biological clocks, said Stanford stem cell researcher Renee A. Reijo Pera, who studies reproduction.

"This is a get-them-back strategy," she said.

Dr. Mitinori Saitou and colleagues at Kyoto University detailed how they generated the functional mouse eggs in a report published online Thursday by the journal Science. Last year, the researchers reported in the journal Cell that they had done the same thing with mouse sperm.

In both cases, the team started with embryonic stem cells, which have the potential to develop into all of the different types of cells in the body.

The scientists exposed the embryonic stem cells to stimuli that coaxed them to become egg and sperm precursors.

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Mouse stem cells used to produce eggs, Japanese scientists say