updated 4/17/2009 1:04:36 PM ET 2009-04-17T17:04:36

When President Barack Obama eased limits on federally funded embryonic stem cell research, the big question became how far scientists could go. Friday, the government answered: They must use cells culled from fertility clinic embryos that otherwise would be thrown away.

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Draft guidelines released by the National Institutes of Health reflect rules with broad congressional support, excluding more controversial sources such as cells derived from embryos created just for experiments.

"We think this will be a huge boost for the science," said Acting NIH Director Raynard Kington. "This was the right policy for the agency at this point in time."

But the limit will disappoint some researchers who had hoped to use a broader variety of cells.

Scientists are trying to harness embryonic stem cells — master cells that can morph into any cell of the body — to one day create replacement tissues and better treat, possibly even cure, ailments ranging from diabetes to Parkinson's to spinal cord injury.

Those cells can propagate indefinitely in lab dishes, but initially culling them does destroy a days-old embryo, a result strongly opposed by many on moral grounds. The Bush administration had limited taxpayer-supported research to a handful of embryonic stem cell "lines" or groups, a policy that the NIH said was slowing the pace of potentially groundbreaking science.

Obama last month ended the Bush limit and widened the field — but he left it to the NIH to set ethics guidelines determining which cell lines now will qualify for government funding.

Many scientists had hoped that the guidelines would allow use of stem cells derived from embryos created just for science, perhaps even those created using cloning techniques that could make them genetically customized for a potential recipient. Some existing stem-cell guidelines that are used in privately funded research — including guidelines from the National Academy of Sciences — are open to all types.

"We discussed the pros and cons of every conceivable variation on this policy," Kington said.

Ultimately, the NIH proposed limiting new grants to research that uses stem cells originally derived from fertility-clinic leftovers, the extra embryos that couples wind up not needing and thus often are thrown out.

That's in line with legislation passed by the last Congress but never signed by then-President George W. Bush.

"There's compelling broad support both in the scientific community and the public at large" for that approach, Kington said. "There is not similar broad support for using other sources at this time."

The guidelines also demand that the woman or couple who donate the original embryo give proper informed consent. There are other options for such donors, such as donating the embryo to another infertile woman, and all must be explained. The donation must be voluntary, without pressure from scientists.

And the guidelines also clearly forbid some types of research using human embryonic stem cells — such as mixing them with embryos from monkeys and other primates.

The NIH will accept public comments on the guidelines for a month and assess those comments before issuing final rules by early July.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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