Implanting human stem cells to animal embryos, which creates non-mythological "chimeras," has been banned in the United States since September 2015.

However, a major U.S. federal agency released on Aug. 4 a new proposal that would lift the ban on funding these types of research, except for certain situations.

Advocates believe that chimera research could help experts understand complex human diseases and determine more efficient ways to treat them.

Human-animal hybrids might also address the shortage of livers, lungs and hearts for organ transplant, experts say.

New Policies, New Limits

In its proposal, the National Institutes of Health (NIH) has released certain new policies for human-animal hybrid research.

The NIH will allow the transfer of human stem cells into non-human embryos, except for non-human primate embryos, up to the 14-day mark.

According to Nature, the new policies cut the developmental window in which stem cells can be added into non-human primate embryos, preventing it before the phase of development where the central nervous system starts to form.

This is designed to limit the number of human stem cells that would comprise the chimera's brain.

Furthermore, the policies prohibit the breeding of animals that contain human stem cells in order to avoid the formation of a human-like embryo from developing in a non-human womb.

Experts also aim to prevent the birth of an animal that is more human than its parents.

The Ethics Of Chimera Research

These types of research will be thoroughly reviewed, says bioethics associate professor Insoo Hyun from Case Western Reserve University.

Hyun spoke to the National Public Radio (NPR) about the ethics of such studies, saying that one large concern is the welfare of animals.

Researchers do not exactly know what effect these experiments will have in terms of animal suffering, says Hyun.

He says there is also apprehension about the level of hybrid, which might result in animals that are not 100 percent animals but also not 100 percent human.

Hyun says scientists may be "flirting" with a line of human dignity that they do not want to cross, even if the medical or scientific value is quite high.

Still, Hyun says he is a supporter of the research, although on certain conditions.

For instance, it has to have scientific merits, and there must be no other methods to perform the experiment but with this approach.

"I really take it on a case-by-case basis," says Hyun.

Gray Area

To address issues of integrity, the NIH plans to set up a panel that would review ethics and oversee grant applications for chimera research.

Carrie Wolinetz, NIH associate director for science policy, says the panel will become an extra set of eyes that will ascertain research is not triggering animal welfare issues.

The NIH panel will pay heed to applications that involve mammals at early stages of development, primates or those research in which human stem cells could affect the brain of an animal.

Wolinetz says beyond a certain point of development, rodent embryos with human stem cells that could influence the brain development are excluded from panel review.

Scientific advisers say that rodent brain is significantly different than human brain and would not become human-like.

Meanwhile, the NIH's new policies are open for public comment within 30 days. The NIH will then issue a final rule and lift the ban.

Wolinetz hopes that the new rule will be ready in time for the grant cycle that starts in January 2017.

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