There’s a one-in-five chance you’ll die of heart disease. But imagine if as soon as your heart showed signs of disease, you got whisked into hospital and given a healthy young heart that was a perfect match for you.
If we had a cheap and unlimited supply of healthy organs for transplant, it wouldn’t just transform the lives of desperately ill people who wait for years for organs – often only to get ones in a poor condition. It would also help people who doctors today don’t even consider for a transplant. It could become normal to have 80-year-olds running around with organs as healthy as those of people a quarter their age.
Will this vision ever become a reality? Governments and companies are spending billions on research into stem cells and regenerative medicine. One of the aims is to grow replacement organs outside the body.
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But for all the progress being made, we are nowhere near to tissue-engineering complex organs good enough for transplantation. And even if it does become possible, it is hard to see how it can ever be done cheaply. The that involve just one kind of cell range from $100,000 to $500,000 a shot. The cost of creating entire organs with dozens of cell types will be even more jaw-dropping.
If tissue engineering is unlikely to deliver on cost, how about animal organs? If we could create genetically modified animals with organs suitable for human transplantation, they could be bred cheaply.
The problem is that first step: engineering animals with organs that don’t trigger severe immune rejection has proved so difficult that many researchers gave up. The  because the rise of CRISPR gene editing has made it more feasible to undertake the extensive modifications required – but it is still far from clear whether animal organs will ever be as good as the real thing.
A third way
So the prospects for a plentiful supply of cheap, healthy organs don’t look great. Or do they? There is a third possibility: growing human organs inside animals.
The idea is to create a pig embryo that is incapable of developing, say, its own heart. Instead, human stem cells would be injected into the embryo, and grow into a human heart inside the pig.
This idea has been around for decades, but only recently has a series of experiments by Hiromitsu Nakauchi at the University of Tokyo, Japan, shown the basic concept to be sound. His team managed to grow a rat pancreas inside a mouse, for instance.
Nakauchi’s success has galvanised the field, and several teams have already begun experimenting with adding human stem cells to pig or sheep embryos. For instance, Pablo Ross at the University of California, Davis, is adding human cells to pig embryos at the five-day stage and then destroying the embryos after 28 days to see where the human cells end up.
But just as the field was taking off came a major blow: last year, the world’s biggest funder of health research, the US National Institutes of Health (NIH), said it would not support work that involves creating this kind of human-animal chimera. This not only cut off a potentially huge source of funding, but also cast a shadow over the entire field.
Researchers in the US are still free to get money from elsewhere, but that’s not easy. “I don’t know how to seek private funds,” says Ross. And such funds often come with strings attached: this field is going to advance much faster if it’s done by publicly funded researchers rather than companies trying to tie up intellectual property rights.
Ethical concerns
Yes, there are clearly serious ethical issues with this approach. The biggest concern is that if a lot of human cells end up in an animal’s brain, it will become some sort of semi-intelligent, half-human beast.
Many biologists point out that they have been creating human-animal chimeras for decades, such as mice with human immune systems, and that all the evidence suggests that a mouse with, say, a few human brain cells is still just a mouse.
But we don’t need to take any chances. If studies like Ross’s show that human cells do get into a pig’s brain, there are various way we could stop this from happening: for example, human stem cells for a particular organ could be modified so they cannot turn into brain cells.
The , after the nervous system has begun to form, and this should minimise any human contribution to the brain. The good news is that it is consulting the public about its proposals and could reverse its funding ban next month, depending on the outcome.
We should all back this kind of research, despite the “yuck factor”. Sure, it’s far too soon to know whether human-animal chimeras really can help solve the transplant crisis – but the potential is so enormous that it would be very wrong not to find out.
