The researchers hope this biotechnological wizardry will help with two goals. One is to shed light on the complicated process of embryological development, which might eventually lead to treatments for some congenital diseases. Chimeras may offer a way around some of the ethical difficulties involved in experimenting on human embryos.
The other is the hope that chimeric animals might one day provide a source of organs to be transplanted into sick humans. In 2017 Japanese researchers demonstrated the principle by transplanting parts of a pancreas that had grown inside a mouserat chimera into a diabetic mouse, curing it. Whether that can work in people is, for now, unclear. And research into human chimeras is ethically fraught. America, for instance, forbids federal funding of such work. Most of the work reported in this latest paper happened in China.
But if chimeric human organs do become a reality, macaques are unlikely to be the animal of choice, says Dr Izpisúa Belmonte. The most likely donor would probably be pigs (this is why his 2017 experiment focused on the animals). Their organs are roughly the size of their human equivalents, and, fairly or unfairly, they seem to provoke fewer moral qualms. (Pigs already provide thousands of people with replacement heart valves, for instance.)
The advantage of working with monkeys, at least for now, is that they are much closer, in evolutionary terms, to humans. That may have helped smooth out any compatibility issues between the two sets of cells. The hope is that lessons from experiments with humanity's close cousins might allow the researchers to revisit their work with its more distant, porcine relatives—and get better results.