BBC Panorama followed a team from the University of California, Davis, which used the genome-editing technique to cut out the section of the pig DNA that codes for the pig's pancreas. They then injected pig embryos with human induced pluripotent stem cells. The embryos were examined at 28 days.
'Our hope is that this pig embryo will develop normally but the pancreas will be made almost exclusively out of human cells and could be compatible with a patient for transplantation,' explained Dr Pablo Ross, lead researcher on the study.
Where the pig embryo lacks genetic instructions for producing a pancreas, human stem cells would substitute this, allowing development of a human pancreas. 'You are basically creating a vacuum, a hole, so that the human cells respond to the right cues; they make a pancreas. The pig cells can't,' explained Professor Robin Lovell-Badge of the Francis Crick Institute, London, who was not involved in the study.
He added: 'What we don't know – and this is what they need to look at – is whether the human cells can also contribute substantially to other tissues, and particularly they are worried about the brain.'
Professor Ross said that the research was proceeding with caution for this reason: 'We think there is very low potential for a human brain to grow, but this is something we will be investigating.'
Currently there is a worldwide shortage of organs for transplantation. Professor George Church of Harvard Medical School, who was not involved in the study, told BBC News that 'gene editing could ensure the [pig] organs are very clean, available on demand and healthy, so they could be superior to human donor organs'.
There is also a risk of rejection of human organs grown from pig embryos. As Professor Lovell-Badge explained: 'There are other cell types that are going to be present in the pancreas which come from the pig – including blood vessels. Those would be a big problem and they would be rejected by a human.' Another potential concern is that animal viruses could be transmitted to humans via the organs.
Sir John Burn, professor of clinical genetics at Newcastle University, who was not involved in the research, said: 'I think this is exciting because it is a sensible, practical application of revolutionary new science in the form of induced pluripotent stem cells and gene editing, and I think it is very exciting to bring them together. But I also know it is a long way from exciting ideas to the clinic.'