Stem cells are added to a portable nitrogen cooler at StemExpress in Placerville, Calif., on May 5. (StemExpress for The Washington Post)

Federal officials are proposing to end a moratorium on funding for research that involves transplanting human stem cells into animal embryos, a controversial practice that produces organisms know as "chimeras."

If approved after a 30-day comment period, the new policy would allow the National Institutes of Health to fund researchers who want to put stem cells in early-stage animal embryos to study disease, possible therapies and organ transplants. Stem cells can become any kind of human tissue, including organs.

NIH imposed its temporary ban on funding last September, citing ethical concerns. These include worries over animals whose brains might contain human brain cells and what might happen if chimeras were able — and allowed — to reproduce.

But on Thursday, Carrie D. Wolinetz, NIH's associate director for science policy, announced the new policy proposal, saying it would "enable NIH research community to move this promising area of science forward in a responsible manner." The request for public comment was also published in the Federal Register.

Animals have long been used in research on human cells, often as part of testing drugs that might attack disease. But because stem cells can become any kind of tissue, human-animal research in the field of "regenerative medicine" raises greater ethical issues and adds, for some, a visceral unease about the organism that could be produced.

Under the proposed NIH policy, taxpayer funds would be allowed for experiments in which human cells are added to early-stage embryos of all animals except nonhuman primates, such as chimpanzees and monkeys, because they are so similar to humans. For those species, the human cells could be added at a later stage of embryonic development and would require an extra layer of scrutiny by a special NIH committee.

Research that would introduce a substantial amount of human cells to a mammal's brain or would significantly modify the animal's brain also would require the extra review. That requirement does not extend to rodents.

The agency will continue its ban on funding research that would include breeding of animals that could make human eggs or sperm.

Sheng Ding, who studies the generation and maintenance of stem cells at the J. David Gladstone Institutes at the University of California at San Francisco, greeted NIH's announcement cautiously. While he does not favor the current moratorium, Ding said, he believes scientists in this field must move slowly because "we don't know how to precisely control where and how [cells] might contribute" to different organs.

"I will say I am certainly cautious about this," Ding said. "I'm not totally against opening up the discussion and figuring out the guidelines."

The moratorium was imposed at a time when NIH had no grants out for this type of research, which provided an opportunity for the agency to take stock of scientific and ethical considerations, Wolinetz said. Despite the attention that has been paid to the potential of regenerative medicine, it is very much a "niche" area of research that probably would generate only a "handful" of research grant requests if the moratorium is lifted, she said.

NIH would consider funding studies proposed by its own investigators as well as research by others outside of the Bethesda, Md., campus, she said.