Despite a funding ban by the top American health agency, U.S. research institutions are running experiments that could result in human-animal chimeras.

The scientists’ goal is to grow human organs, such as hearts and livers, usable for transplants inside other animals, like pigs and sheep.

"We can make an animal without a heart. We have engineered pigs that lack skeletal muscles and blood vessels," Daniel Garry, a cardiologist leading a chimera project at the University of Minnesota, told the MIT Technology Review.

The human-animal hybrids are produced by injecting human stem cells into an animal embryo, which is then implanted into female livestock for gestation. A specific organ in the animal embryo is removed during the process and replaced with the scientifically engineered human stem cells. The MIT Technology Review estimated that “about 20 pregnancies of pig-human or sheep-human chimeras have been established during the last 12 months in the U.S., though so far no scientific paper describing the work has been published, and none of the animals were brought to term.”

Details of the process were presented at the NIH’s Maryland campus in November, where researchers showed unpublished data and photographs of pig embryos containing human cells, one of which even appeared to have cured a congenital eye defect during the process. The experiments would mean a lot for the future of stem-cell biology and genetic engineering. Scientists can modify genes to create organs and remove them as needed, saving multitudes of hospital patients waiting for organ transplant donors.

The National Institutes of Health (NIH) decided to take action after it learned that scientists had begun such experiments using funding from other sources, including from California’s state stem-cell agency. Last September, the NIH stated it would not support research involving “human-animal chimeras” until the social and scientific implications of the research would be more thoroughly reviewed. Here is an excerpt from their statement:

Given the rapid expansion of potential research models employed beyond the scope described above, NIH would like to undertake a deliberative process to evaluate the state of the science in this area, the ethical issues that should be considered, and the relevant animal welfare concerns associated with these types of studies.

While the procedures could be beneficial for human patients who are in need of organ donors, the NIH is concerned that it would blur lines between human and animal species, as well as the chance that animals’ “cognitive state” might change if they ended up with human brain cells.

Animal rights organizations also oppose the research, expressing concerns that it could lead to animals with human characteristics after being injected with human brain cells.

"The spectre of an intelligent mouse stuck in a laboratory somewhere screaming, 'I want to get out,' would be very troubling to people," David Resnik, an NIH ethicist, told the International Business Times.

Resnik said he is concerned that animals might end up frighteningly similar to humans. “We are not near the island of Dr. Moreau, but science moves fast,” he said at a meeting last November.

Another issue is the legal status of the animals. If the human-animal chimera are acting similar to humans, would the laboratory still have the rights to own them, or could the chimera be eventually granted human rights? These and other questions are plaguing scientists and bioethicists during the process.

Scientists outside of the U.S. have reportedly tried similar experiments. But while the NIH ban will mean nothing for them, it could lead to challenges by their peers and the need to explain the purpose of their research. Currently, the research is not advanced enough to produce usable human transplant organs, but it does have the power to determine whether that is a scientific possibility for the future.