The federal government is planning to lift a moratorium on funding for research studying the effects of injecting animal embryos with human stem cells, creating half-human half-animal hybrids.

The National Institutes of Health policy would allow federal funding to go toward research creating human-animal embryos known as “chimeras.”

Carrie D. Wolinetz, NIH’s associate director for science, announced Thursday in a blog post that the agency is requesting public comment on expanding funding for such research, signaling the end of a moratorium on the experimentation.

Ms. Wolinetz said development of “these types of human-animal organism … holds tremendous potential for disease modeling, drug testing, and perhaps eventual organ transplant.”

“I am confident that these proposed changes will enable the NIH research community to move this promising area of science forward in a responsible manner,” she wrote.

The change in protocol was proposed nearly a year after the NIH issued a reprieve on funding for chimera research.

Rob Stein, a health correspondent for NPR, said injecting human stem cells into animal embryos holds the potential to grow human organs capable of being transplanted into sick patients.

Additionally, scientists believe they can make progress toward cures for various diseases by isolating and closely studying human organs, he said.

“The idea here is that if you have, let’s say, pigs or sheep or cows that have partially or fully formed livers or kidneys or pancreases or even parts of their brain, then you might be able to study that to learn many new things that you could use to prevent and treat human diseases, like Alzheimer’s disease, Parkinson’s, diabetes,” Mr. Stein said Thursday on NPR.

But such experimentation raises difficult ethical questions, such as how to treat potentially partially human species.

“Critics say that this is dangerous because it blurs the line between humans and other species and starts to raise questions about what are these creatures,” Mr. Stein said. “Are they animals, or are they partially human? And if they are partially human, how do we treat them?”

To alleviate those concerns, the NIH said it would not permit funding for experiments on primate embryos.

Chimera experiments also would have to go through extra layers of scrutiny, especially if they run the risk of drastically changing the brains of the animals, which researchers worry could induce a humanlike state of sentience.

Experiments that could result in an animal sperm and human egg to breed, or vice versa, also would be prohibited even after the moratorium is lifted.

But some critics remain worried about the development of chimeras.

Writing in the pages of National Review on Thursday, Wesley J. Smith, a senior fellow at the Discovery Institute’s Center on Human Exceptionalism, expressed doubt about whether the scientific community or general public has the willpower to impose ethical boundaries on this sort of research.

“I don’t think this work can be stopped,” Mr. Smith wrote. “But identifying the lines that should not — and which we will not allow — to be crossed is urgently needed so that legally enforceable standards can be delineated.”

“I just don’t see anyone currently in power within the symbiotically connected science, government, and big business sectors much interested in giving such work more than placating lip service at the moment,” he wrote.

Scientists have been conducting cross-species experiments for decades to advance the state of medicine.

It is standard to grow human tumors on mice to study drugs that might cure cancer. Transgenic organisms — animals that have been modified to contain genes from other species — have been found to possess valuable medical properties in their milk without changing the essential nature of the animal.

“Biomedical researchers have created and used animal models containing human cells for decades to gain valuable insights into human biology and disease development,” Ms. Wolinetz wrote.

But Rod Dreher, a senior editor at the American Conservative, said creating partially human creatures is a step too far.

“Signs of the times, my friends,” Mr. Dreher wrote Thursday. “Read them, and prepare. We are a perverse and wicked generation who deserves what we are going to get.”

The general public reportedly will have one month to comment on the proposed change before the moratorium is lifted.

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