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TOKYO?- A team of scientists has reported producing viable sperm using the stem cells of mice in an experiment that researchers hope could one day lead to treating infertile men.

The Kyoto University researchers managed to induce mice stem cells into creating sperm precursors, which were transplanted into infertile male mice. The mice then produced sperm that was successfully used to fertilize eggs in a lab dish.

The offspring were healthy and fertile, according to a paper published online Thursday in the scientific journal Cell.

Members of the research team, led by Mitinori Saitou, said they believe their success may help in the development of infertility treatments in humans, although they said many hurdles remain.

"We have high hopes, but it's not that easy," Saitou told the Associated Press by telephone Friday from Kyoto, in western Japan. "There are many difficult issues ahead in applying this to humans. But it is a first step."

Experts outside the group say it's an important first step toward infertility treatment although there would be a long way.

"This is a very good experiment for thinking about treatment of infertile man but a very very long way," said Toshio Suda, developmental biology professor at Keio University.

He said the Kyoto team's findings were great work but it is not easy to prepare the mature sperm cells at present. If some hurdles are cleared, it could successfully identify "which gene is very important to prepare the sperm."