Human stem cells, when injected into pig embryos, are to create human-pig embryos called chimeras.
To combat a worldwide shortage of transplant organs, scientists in the US have grown their own inside human’s closest genetic relative, the pig.
The spliced embryos are kept developing within the pig for 28 days, then terminated and collected for analysis.
According to the team from the University of California, when the pigs are born they should look and act like normal, while having a specific organ made of human cells growing inside them.
The first stage to this process, known as CRISPR gene editing, is to remove the DNA from a freshly fertilized pig embryo to enable the foetus to grow a human pancreas.
This creates a genetic void which allows human stem cells to be injected back into the embryo. The group at UC Davis hopes that the resulting foetus will grow a human pancreas and later on something more complicated.
Pablo Ross, the biologist who is leading the research said: "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."
But the work is controversial. Last year, the main US medical research agency, the National Institutes of Health, imposed a moratorium on funding such experiments.
The 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 says this is unlikely but is a key reason why the research is proceeding with such caution: "We think there is very low potential for a human brain to grow, but this is something we will be investigating."