President Barack Obama lifted previous restrictions on the field in March, but left it to the National Institutes of Health to decide just what stem cell research was ethically appropriate: Only science that uses cells culled from leftover fertility clinic embryos - ones that otherwise would be thrown away - the agency made clear in draft guidelines.
But the final rules issued Monday settle a big question: Would new ethics requirements disqualify many of the stem cells created over the past decade, even the few funded under the Bush administration's tight limits?
The NIH came up with a compromise, saying it deems those old stem cell lines eligible for government research dollars if scientists can prove they met the spirit of the new ethics standards. Further, NIH will create a registry of qualified stem cells so scientists don't have to second-guess if they're applying to use the right ones.
"We think this is a reasonable compromise to achieve the president's goal of both advancing science while maintaining rigorous ethical standards," acting NIH Director Raynard Kington said Monday. "We believe that judgment is necessary."
He wouldn't speculate on how many old stem cells ultimately would qualify, but scientists welcomed the change.
"I expect that most existing lines will be found to have been ethically derived," said Dr. Sean Morrison, director of the University of Michigan Center for Stem Cell Biology. "This will eventually make hundreds of new stem cell lines available for use."
The issue: Trying to harness embryonic stem cells - master cells that can morph into any cell of the body - to one day create better treatments, maybe even cures, for ailments ranging from diabetes to Parkinson's to spinal cord injury.
Culling those stem cells destroys a days-old embryo, something many strongly oppose on moral grounds. Once created, those cells can propagate indefinitely in lab dishes.
The Bush administration had limited taxpayer-funded research to a small number of stem cell batches, or lines, already in existence as of August 2001. This spring, Obama lifted that restriction, potentially widening the field - there now may be as many as 700 stem cell lines around the world - but letting NIH set its boundaries.
Federal law forbids using taxpayer money to create or destroy an embryo. At issue here are rules for working with cells that initially were created using private money.
NIH sifted through 49,000 comments from the public in finalizing the rules, which take effect Tuesday. The draft changed little: Stem cells created solely for research in whatever manner, including cloning, won't qualify.
Any newly made stem cells must come with documentation that the woman or couple who donated the original embryo gave full informed consent. For example, they must have been told of other options for leftover embryos, such as donating to another infertile woman, and the donation must have been voluntary.
That kind of documentation may not exist for stem cell lines created years ago, Kington said, but "some and perhaps many of those lines might be eligible" on a case-by-case evaluation.