The US ban on research involving chimeras – human-animal hybrids developed by adding stem cells from a person into the embryo of a different species – could soon be at least partially lifted, according to a proposal released Thursday by the National Institutes of Health (NIH).

That proposal would lift the ban, which according to Nature has been in place since September of last year, in specific cases while also establishing a panel to review the ethics of projects that apply for grants. It would also reduce the time frame in which human cells can be introduced to non-human primate embryos to before the development of the central nervous system.

By doing so, the NIH hopes to limit the number of human stem cells which would become part of the chimera's brain, the publication added. Furthermore, the new rules would any the breeding of animals with human cells to prevent a human-like embryo in the womb of a primate, and/or the birth of offspring that is closer to human than the creatures which gave birth to it.

"Biomedical researchers have created and used animal models containing human cells for decades to gain valuable insights into human biology and disease development," explained Dr. Carrie D. Wolinetz, Associate Director for Science Policy at the NIH. "To advance regenerative medicine, it is common practice to validate the potency of pluripotent human cells – which can become any tissue in the body – through introducing them into rodents."

"With recent advances in stem cell and gene editing technologies, an increasing number of researchers are interested in growing human tissues and organs in animals by introducing pluripotent human cells into early animal embryos," she added, noting that chimera research "holds tremendous potential for disease modeling, drug testing, and perhaps eventual organ transplant" but also raises "ethical and animal welfare concerns."

Researchers have mixed reaction to the proposed guidelines

Under the new program, Wolinetz said that any grant applications that would potentially raise ethical or animal welfare-related issues would undergo panel review. Specific attention will be paid to experiments that involve primates at any stage of development and project in which an animal's brain could be affected by human cells.

Research involving rats would be exempt from review, Nature explained because NIH scientific advisors are confident that the substantial differences between the brains of people and rats make it impossible for such rodents to become human-like following stem cell injection. The agency is currently accepting public comment on the planned changes to their chimera research policy.

Reaction to the proposal has been mixed, according to the publication. University of Rochester neuroscientist Steven Goldman told the journal that he was relieved that the funding ban would be lifted and called the guidelines "more intelligent from the standpoint of where the science is."