Back in September 2015, the US-based National Institutes of Health (NIH) issued a ban on its researchers from adding human stem cells to animal embryos for the purpose of creating chimeras – hybrid creatures with physical features of both. This was largely done for ethical reasons, with the NIH at the time suggesting that they were being cautious and holding back until more evidence of the medical benefits of chimeras came to light.

Now, a NIH proposal this month has suggested that this moratorium should be lifted by early September, but with a few notable caveats and restrictions still in place. A panel designed to review both the ethics of chimera creation and any grant applications relating to the subject will also be set up.

“Since the moratorium was issued, NIH has reviewed the state of the science and also convened a workshop in November 2015 to bring together leading experts in the field of chimera research and animal welfare,” Carrie Wolinetz, the associate director for science policy at NIH, said in a statement.

“[This August], the NIH has published in the Federal Register and the NIH Guide to Grants and Contracts a proposal to make two changes to our policy in this area, for which we are seeking public comment,” Wolinetz continued. “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.” 

The organization has declared that its researchers cannot introduce human stem cells into animal embryos – in this case, nonhuman primate embryos – before the development of the central nervous system, in order to prevent human cells dominating the chimera’s brain. The development of breeding animals containing human cells is also not allowed, which means that any concerns regarding human embryo growth within other animals are unfounded.

This topic is not without its controversy, and some are worried that this technology could be used to create human-like embryos growing in animal wombs, or perhaps induce animal offspring to look slightly human in some way. This new proposal by the NIH hopes to quell these fears, and highlight the medical benefits of such experiments instead.

^{Pigs have been used in chimera trials before. Igor Stramyk/Shutterstock}

Chimeras, once merely consigned to historical legends, are now possible thanks to the discovery and manipulation of stem cells, those that can theoretically differentiate into any type of cell. By adding human stem cells to animal embryos, genuine hybrids can be created that would allow researchers to study early embryonic development, to create animal models of human diseases, and even grow human organs in animals that could later be harvested and used in organ transplants.

In June of this year, the CRISPR gene-editing tool was used to inject human stem cells into pig embryos by scientists at the University of California, Davis. This created human-pig chimeric embryos that were allowed to develop for 28 days before they were aborted, with the tissues subsequently harvested for investigation.

The team behind these experiments ultimately hope to produce pigs that contain human organs. Specifically, they hope to grow a human pancreas that can be transplanted into a human patient in need of one. This is a far cry from the monstrous half-pig half-human beast that critics of the practice are worried about seeing, but it’s fair to say that there are some ethical concerns that are worth considering.

This new NIH proposal will make trials like these much easier to run and fund. It’s a brave new world, one with plenty of moral pitfalls and medical advancements.

“There are no hard and fast lines,” Wolinetz told Nature. “There’s going to be some on-the-job learning.”

^{A mythological chimera, the sort that definitely will not emerge from real-life chimera experiments. Vuk Kostic/Shutterstock}