The successful creation of human-pig chimera embryos could be the first step to growing transplantable human organs in pigs.
By: Sophie Weiner
Chimeras, animals with a mix of DNA and cells from more than one species, are a controversial topic to say the least. Up until last August, there was a ban on federal funding for any chimera research in the U.S. But that didn’t stop researchers at the Salk Institute, who have used private funding over the last four years to successfully create human-pig chimera embryos, according to their new paper in the journal Cell.
Creating chimeras is a project of high importance in science for a few reasons, but the most pressing is for organ transplants. Every day, 22 people die because they didn’t receive needed organ donations in time. In the future, it could get worse. Scientists have long imagined growing human organs in animals to augment the donor supply, but this research brings that sci-fi goal closer than ever to reality.
We can’t simply grow human organs in animals for the same reason we can’t transplant animal organs into our bodies: they’ll almost certainly be rejected by our immune system, which recognises the tissue as foreign. Stem cells, another way to grow human organs outside of a human body, are equally tricky, often refusing to grow into the required organ. Chimeras are a perfect middle ground—by injecting human cells into a young animal embryo, the organism that emerges is neither pig nor human, and thus less likely to have its organs rejected by either species.
Over four years, the Salk Institute researchers developed a method that allowed pig embryos injected with human cells to survive. Pigs are already known to be quite similar to humans in anatomical structure, but that doesn’t mean that creating human-pig chimeras was easy. To do so, the scientists tried and failed many times, injecting the human stem cells at different points in gestation until they found a time period that allowed the embryos to survive.
“We tried three different types of human cells, essentially representing three different [gestation] times” Salk Institute scientist and the paper’s author Jun Wu, told National Geographic. The researchers discovered that naïve pluripotent cells (the name for early stem cells that can develop into any kind of tissue) were less likely to survive as those that had developed a bit further. The embryos injected with these stem cells survived and were then injected into adult pigs and carried for four weeks before they were removed for scientists to examine. In total, the researchers created 186 chimera embryos that survived to this stage. “We estimate [each had] about one in 100,000 human cells,” Wu says.
That’s an extremely low percentage which would need to be upped considerably before the organs grown had a chance of surviving as a transplant. But it’s an encouraging first step. The end goal is to grow embryos using the stem cells from a patient who needs a transplant. If the percentage of human cells used was high enough, there’s almost no chance these chimeric organs would be rejected upon transplant.
That goal is still far off, but that doesn’t mean the experiment is useless for the time being. Researchers are likely to gain all kinds of important knowledge by studying these chimeric embryos. Their understandings could lead to better understandings of disease and of human embryo development, insights that could be just as valuable as creating transplantable chimeric organs in the long run.
This article was originally written for and published by Popular Mechanics USA.