The National Institutes of Health announced on Thursday that it was planning to lift its ban on funding some research that injects human stem cells into animal embryos.

The N.I.H. announced its proposal in a blog post by Carrie Wolinetz, the associate director for science policy, and in the Federal Register.
The purpose is to try to grow human tissues or organs in animals to better understand human diseases and develop therapies to treat them.
Researchers have long been putting human cells into animals — like pieces of human tumors in mice to test drugs that might destroy the tumors — but stem cell research is fundamentally different. The stem cells are put into developing embryos where they can become any cells, like those in organs, blood and bone.
If the funding ban is lifted, it could help patients by, for example, encouraging research in which a pig grows a human kidney for a transplant.But the very idea of a human-animal mix can be chilling, and will not meet with universal acceptance.
In particular, when human cells injected into an animal embryo develop in part of that animal’s brain, difficult questions arise, said Paul Knoepfler, a stem cell researcher at the University of California, Davis.
“There’s no clear dividing line because we lack an understanding of at what point humanization of an animal brain could lead to more humanlike thought or consciousness,” he said.
The N.I.H.’s plan will most likely go into effect in the fall — perhaps with some modifications — after a 30 day comment period  that is now open to the public and researchers.
The N.I.H., which would be a major source of federal funds for this type of work, imposed the moratorium in September to consider concerns about the research. The studies were just beginning, and the N.I.H. did not have any projects underway involving human-animal chimeras, a term derived from mythological creatures that were part goat, lion and snake. But Renate Myles, a spokeswoman, said, “We watch the state of the science and knew that this was where the science was heading.”
For scientists, the moratorium was “a little jarring,” said Dr. George Q. Daley, a Harvard professor and the director of the stem cell transplantation program at Boston Children’s Hospital. Two months later, the N.I.H. convened a workshop to hear from researchers and experts in animal welfare.
Two types of experiments that are being considered for funding would still have to undergo a review by an N.I.H. advisory committee. The first involves the addition of human stem cells to the embryos of animals before the embryos reach a stage when organs are starting to develop. Because nonhuman primates like monkeys and chimpanzees are so genetically close to people, researchers working with such primates would have to wait until an embryo was further developed before adding human stem cells, according to the proposal.
The second type of study introduces stem cells into embryos of animals other than rodents where the cells could get into and modify the animals’ brains. Of particular concern is creating chimeras with human cells in the brain.

The N.I.H. would continue its ban on funding any research that could result in an animal with human sperm or eggs that would then be bred.

All of the N.I.H.’s proposals, though, apply only to the work that is financed with taxpayer money. Research supported by private donors or companies would not be affected.
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