For the first time, scientists have grown an embryo that is part-pig, part-human.

The experiment, described Thursday in the journal Cell, involves injecting human stem cells into the embryo of a pig, then implanting the embryo in the uterus of a sow and allowing it to grow. After four weeks, the stem cells had developed into the precursors of various tissue types, including heart, liver and neurons, and a small fraction of the developing pig was made up of human cells.

The human-pig hybrid — dubbed a “chimera” for the mythical creature with a lion's head, a goat's body and a serpent's tail — was “highly inefficient,” the researchers cautioned. But it's the most successful human-animal chimera and a significant step toward the development of animal embryos with functioning human organs.

In a study published a day earlier, an international team of researchers demonstrated that organs for transplant can be grown in chimera embryos that are part-mouse, part-rat. Writing in Nature, the researchers reported Wednesday that they were able to grow a mouse pancreas inside a rat embryo, then transfer insulin-secreting tissue from that organ into diabetic mice, alleviating their illness without triggering an immune response.

It was the first demonstration that such an interspecies organ transplant is possible. Researchers hope that one day doctors may be able to grow human tissue using chimera embryos in farm animals, making organs available for sick human patients who might otherwise wait years for a transplant.

The technique is already the subject of a vigorous debate about the ethics of introducing human material into animals; since 2015, the National Institutes of Health has had a moratorium on funding for most human-animal chimera research. (The new study was performed in California at the Salk Institute without federal funds.) Some argue that, since stem cells can become any kind of tissue, including parts of the nervous system, chimeras raise the specter of an animal with a human brain or reproductive organs. Others feel there's a symbolic or sacred line between human and animal genetic material that should not be crossed.

But Vardit Ravitsky, a bioethicist at the University of Montreal's School of Public Health, said that the two studies published this week could help make a case for further human-animal chimera research by demonstrating the field's potential benefits. 

“I think the point of these papers is sort of a proof of principle, showing that what researchers intend to achieve with human-non-human chimeras might be possible,” she said. “The more you can show that it stands to produce something that will actually save lives … the more we can demonstrate that the benefit is real, tangible and probable — overall it shifts the scale of risk-benefit assessment, potentially in favor of pursing research and away from those concerns that are more philosophical and conceptual.”

In an effort to address the world's growing organ shortage — an estimated 22 people a day die waiting for transplants, according to U.S. Department of Health — scientists have been trying to grow organs outside of the human body. But organs developed in petri dishes are not identical to the ones that grow inside a living thing.

“That's where the rationale of this kind of experiment comes in,” said Juan Carlos Izpisua Belmonte, a developmental biologist the Salk Institute and the senior author on the study of the human-pig chimera. “What if we let nature do the work for us? What if we just put human cells inside the embryo and the embryo knows what do to?”

The model for using chimeras for organ transplant would probably look something like the technique reported in Nature. In that experiment, researchers took induced pluripotent stem cells (ordinary cells that have been reverted to an early embryonic state, so that they have the potential to develop into any tissue type) from mice. These cells were then injected into rat embryos that had been genetically modified so that they were unable to grow their own pancreas — “emptying a niche” for the mouse stem cells to fill.

The embryonic rats developed normally and were born healthy. Each had a rat-sized pancreas made of mouse cells. The whole pancreases were too big to transplant into tiny mice, so the researchers extracted just the islets — the region of the pancreas that produces hormones like insulin — and planted them in mice that had been induced to have diabetes.

Because the transplanted cells were grown from stem cells taken from mice, the animals required just five days of immunosuppressive drugs to keep their bodies from rejecting the new tissue. After that, they were able to live normally with healthy blood glucose levels for over a year — half a lifetime in human terms.

The study showed that interspecies organ transplants are not only possible, but they can be done effectively and safely, said Hiromitsu Nakauchi, a stem cell researcher at Stanford University and the University of Tokyo who is the senior author of the study. 

“This is a form of transplantation we could do in the clinic with human patients some day,” he said. 

Nakauchi also conducts research on human-chimera embryos, but his efforts to inject human stem cells into sheep embryos have largely been unsuccessful — the evolutionary distance between humans and livestock may be making it difficult to get human stem cells to take hold in those animals. Other researchers have achieved human-mouse chimeras that developed to full size and grew to adulthood, but there is debate about how substantially human cells can contribute to mice, which are much more distantly related.

He said he was cheered to read the Cell study, which represents the most significant progress on human-animal chimeras yet, though the technique is still nowhere near ready for an experiment like the one performed in Nakauchi's mice.

“If you read the paper, the contribution of human cells is very limited, is very, very minor, and only in the early embryonic phase so we’re still not sure if we can make human chimeras,” he cautioned. “But I'm glad that they're doing this research.”

The Cell study was the result of four years of work involving some 1,500 pig embryos. These embryos were not genetically modified, like Nakauchi's rat embryos, but the Salk scientists used a similar technique to inject human stem cells.

Pigs are an ideal animal for chimera research, said co-author Pablo Ross, an associate professor in the department of animal science at the University of California, Davis. Their organs are roughly the same size as those of humans (recall that the pancreases grown in Nakauchi's rats were rat-sized, even though they were grown with mouse cells), but they reach their full size far more quickly than humans and other primates. 

“You go from one cell [at] fertilization to 200 pounds, the average size of an adult [pig], in nine months,” Ross said. “I think that's very reasonable, when you think about the fact that the average wait for a kidney transplant is about three years.”

Still, pigs' rapid gestation means that their organs develop much more rapidly than those of humans. If researchers want to create a successful chimera, they have to consider timing.

So Ross and his colleagues used three different types of stem cells for their experiment: “naive” cells that were at the very earliest stages of development, “primed” cells that have developed further (but are still pluripotent), and “intermediate” cells that are somewhere in between.

Dozens of cells of each type were injected into pig embryos, which were then implanted in sows and allowed to develop for three to four weeks (about a quarter of a pig's gestation period). The primed cells never really took hold in the host embryo. The naive cells were initially incorporated into the growing animal, but were indistinguishable in the developing pig four weeks later.

The intermediate cells were most successful; by the time the embryos were removed from the sow and analyzed, about one in every 100,000 cells was human rather than pig, lead author Jun Wu estimated. The human cells were distributed randomly across the chimera: many wound up in what would become the heart (where they made up about 10 percent of tissue), some in the kidneys and liver (1 percent or less). A few developed into the precursors of neurons, a fear of bioethicists who worry about creating an animal with human or even humanlike consciousness.

But Izpisua Belmonte said that prospect is still a long way off. The contribution of human cells to the chimera was tiny, and research protocols were in place to prevent the development of any human-animal chimera to maturity.

“We were just trying to answer the yes or no question of, can human cells contribute at all?” he said. “And the answer to that question is yes.”

The Cell study researchers also discussed progress with rat-mouse chimeras. Though they have not performed an interspecies organ transfer, they were able to grow hearts, eyes and pancreases in chimeric embryos. They also grew a rat gall bladder inside a mouse embryo, even though rats don't grow gall bladders during normal development — suggesting that rats have the genetic coding for gall bladders but those genes are suppressed by their developmental environment.

That's another important aspect of chimera embryo research, Izpisua Belmonte said, one that is sometimes overlooked in the focus on organ transplants. Chimera embryos can be used to understand development, examine genetic diseases and test drugs without risking the health of humans.

In August, the NIH released a draft of a policy that would change the guidelines to allow funding of certain human animal chimeras. Under the proposed new rule, the taxpayer funds could be used for experiments that introduced human stem cells to early stage embryos of all animals except other primates. Some nonhuman primate research would also be allowed, but only using embryos at later stages of development and only after an extra layer of review by a special NIH committee. But the policy change is still under review.

Neither Nakauchi's nor Belmonte's study was funded by NIH grants. Nakauchi said he hoped that recent progress in the field might garner support for easing the ban.

“Finally we’re able to provide a proof of principle that of this approach of making organs … is possible and also safe and efficient,” he said. “So I hope people will understand this.”

He continued, “Many people think this is a kind of science fiction story. But this is becoming reality.”