Human cells being injected into a pig embryo.
But, concern has already surfaced that playing God in this way could open the door to pig’s developing with human intelligence and too much like us.
There are fears that mixing pig and human genes in this way coud see the animals develop looking like animals but with human brains or boosted intelligence.
The research is so controversial that last year, the main US medical research agency, the National Institutes of Health, imposed a moratorium on funding such experiments.
Its main concern is that the human cells might migrate to the developing pig’s brain and make it, in some way, more human.
Pablo Ross, a reproductive biologist who is leading the research, said this is unlikely but is a key reason why the research is proceeding with such caution.
He said: “We think there is very low potential for a human brain to grow, but this is something we will be investigating.”
Mr Ross, whose team is from University of California, Davis, says the hybrids should look and behave like normal pigs except that one organ will be composed of human cells.
But, amid concerns that the human-pig chimeric embryos could develop too much like us, the inch-long embryoes are only being allowed to develop inside the sows for 28 days before the pregnancies are terminated and the tissue removed for analysis.
One of the sows who is pregnant with a human-pig hybrid.
Other teams in the United States have created human-pig chimeric embryos but none has allowed the foetuses to be born, because of the fears of the hybrid creation.
Walter Low, professor in the department of neurosurgery, University of Minnesota, said pigs were an ideal “biological incubator” for growing human organs, and could potentially be used to create not just a pancreas but hearts, livers, kidneys, lungs and corneas.
He said if the iPS cells were taken from a patient needing a transplant then these could be injected in a pig embryo which had the key genes deleted for creating the required organ, such as the liver.
He said: “The organ would be an exact genetic copy of your liver but a much younger and healthier version and you would not need to take immunosuppressive drugs which carry side-effects.”
Alarmingly, his team is also trying to create dopamine-producing human neurons from chimeric embryos to treat patients with Parkinson’s disease.
These embryos have been allowed to develop for up to 62 days – the normal gestation period is around 114 days.
Like the team in California, Prof Low said they were monitoring the effects on the pig brain: He said: “With every organ we will look at what’s happening in the brain and if we find that it’s too human like, then we won’t let those fetuses be born”.
We think there is very low potential for a human brain to grow, but this is something we will be investigating.
Creating the chimeric embryos takes two stages.
First, a technique known as CRISPR gene editing is used to remove DNA from a newly fertilised pig embryo that would enable the resulting foetus to grow a pancreas.
This creates a genetic “niche” or void.
Then, human induced pluripotent (iPS) stem cells are injected into the embryo.
The iPS cells were derived from adult cells and “dialled back” to become stem cells capable of developing into any tissue in the body.
The team at UC Davis hopes the human stem cells will take advantage of the genetic niche in the pig embryo and the resulting foetus will grow a human pancreas.
In the mid-90s there were hopes that genetically modified pigs might provide an endless supply of organs for patients, and that cross-species transplants were not far off.
But clinical trials stalled because of fears that humans might be infected with animal viruses.
Organisations campaigning for an end to factory farming are also dismayed at the thought of organ farms.
Peter Stevenson, from Compassion in World Farming, told Panorama: “I’m nervous about opening up a new source of animal suffering.
“Let’s first get many more people to donate organs. If there is still a shortage after that, we can consider using pigs, but on the basis that we eat less meat so that there is no overall increase in the number of pigs being used for human purposes.”