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Blue stained stem cells and green stained fibrin on black background

Scientists say they have taken a potentially important — and possibly controversial — step toward creating human eggs in a lab dish.

A team of Japanese scientists turned human blood cells into stem cells, which they then transformed into very immature human eggs.

The eggs are far too immature to be fertilized or make a baby. And much more research would be needed to create eggs that could be useful — and safe — for human reproduction.

But the work, reported Thursday in the journal Science, is seen by other scientists as an important development.

"For the first time, scientists have been able to convincingly demonstrate that we are able to make eggs — very immature eggs," says Amander Clark, a developmental biologist at UCLA who wasn't involved in the research.

The technique might someday help millions of people suffering from infertility because of cancer treatments or other reasons, Clark says.

But the prospect of being able to mass-produce human eggs in labs raises a host of societal and ethical issues.

Theoretically, babies someday could be made from the...