Scientists inject human stem cells into a pig blastocyst. A laser beam (green circle with a red cross inside) was used to perforate an opening to the outer membrane of the pig blastocyst to allow easy access of an injection needle delivering human iPS cells. (Salk Institute)

Paul Knoepfler is a stem-cell biologist at the University of California at Davis and writes about innovative science at the Niche. His most recent book is “GMO Sapiens: The Life-Changing Science of Designer Babies.” You can watch his TED talk on that topic here and find him on Twitter: @pknoepfler.

A new study out of California unsettled a lot of people last week after revealing that scientists had, for the first time, made part-human, part-pig embryos — referred to as “chimeras.” That should be expected: The debate over the technology is a mixed bag of difficult issues not unlike the fire-breathing hybrid Chimera from Greek mythology.

But on balance, the promise of this biotechnology should outweigh our fears and ethical questions. Chimeras could be a game-changer in terms of organ transplants in coming decades, and for that reason, scientists should carefully proceed with the research.

More than 100,000 people in the United States currently sit on organ waiting lists, struggling to stay alive long enough to get a new liver or kidney. With few realistic alternatives to the limited supply of cadaver-based transplants, about 22 Americans die each day. Hundreds more die daily at the global level.

Our recent renaissance of cutting-edge biotechnologies — particularly based on the utilization of pluripotent stem cells — gives real hope for these people in need of transplants. What exactly is a human chimera? It’s a mixture of a small number of human cells within an otherwise predominantly animal embryo, such as a pig. The hope is that, if allowed to grow, a chimera embryo would develop entirely as animal except for one harvestable organ that is human. It might even be possible for that organ to be produced from the patient’s own stem cells, making it a perfect match.

In the past, other researchers have made similar chimeric embryos, mixing human stem cells with mouse cells. But a mouse-size kidney or liver — even if made of human cells — cannot help a human, because these organs would be about the size of a small kidney bean. Pigs, on the other hand, are relatively closer to humans on the evolutionary tree, perhaps bringing us a small step closer to actual clinical use.

Even so, there’s a long road ahead. The California researchers found that many of the human-pig chimeric embryos did not grow properly. And even if organs in pig chimeras ended up 100 percent human at a cellular level, they are certain to contain other factors — such as pig proteins — that could spark a patient immune reaction leading to organ rejection. Still, every cutting-edge biomedical technology faces technical obstacles at first, and there is a good chance that researchers might overcome these hurdles in the future.

It’s understandable if people imagined full-grown, human-pig creatures when reading about this new research. In reality, though, the chimeras produced were only embryos — just tiny collections of cells. If the technology progresses further, chimeras would have to be taken to term or near-term before full-size organs could be harvested. Inevitably that means there may be large chimeras produced and photographed for the world to see; but remember, these animals wouldn’t look any different from ordinary animals, because only a single organ would be human.

Animal rights advocates were quick to raise ethical questions: Should we allow chimeric pigs to be used as a biomedical incubator of sorts and then sacrificed to obtain a human organ? But this ignores the fact that people are eating billions of animals each year.

Tougher questions focus on the human side of chimeras and include the dilemma of what makes an animal a human in terms of cells. How many human cells within a chimera overall would make that chimera too close to a human being? How many human brain cells and in particular neurons in a human-pig chimera would be too many? What should we do if a human-pig chimera accidentally ended up with an abundance of human cells in its brain? What if a human-pig chimera made human sperm or eggs?

Fortunately, there are some simple technological answers to many of these questions. We could agree, for example, to prevent all chimeras from being born. We could also use animals that are sterile as the basis for making chimeras and closely monitor human cell numbers in chimeras (including in the brain) during early research studies. We could also ban organ production if human-cell levels consistently fall outside acceptable parameters.

Overall, though, the global shortage of organs for transplants is too urgent a problem to refuse to explore innovative solutions. We should pursue more human-chimera technology while from the start acknowledging and addressing the important bioethical considerations it faces. We should also carefully plan outreach efforts to the public as the technology advances.

Human chimeras not only have potential to address the organ shortage; they also could educate us about unexplored questions of human development. Groundbreaking biomedical technologies might be unnerving, but they have real potential to positively change our world.

Read more:

Eight questions to ask before human genetic engineering goes mainstream

What’s the difference between genetic engineering and eugenics?

In defense of transhumanism