Scientists at the University of California, Davis are working towards growing human organs inside pigs.
The team has injected human stem cells into pig embryos, to produce hybrid embryos known as chimeras.
"They believe that the animals, which would have a human internal organ, may look and behave like any other pigs and could be a ready source of organs for life-saving transplants," The Guardian says.
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The complex procedure involves two stages, researchers say. In the first step, known as Crispr gene editing, scientists take a newly fertilised pig embryo and remove the DNA that would ordinarily allow the pig to grow a pancreas. Once that "niche" has been created, human stem cells are injected into the embryo to replace the missing DNA.
"Our hope is that this pig embryo will develop normally but the pancreas will be made almost exclusively out of human cells," says Professor Pablo Ross, the reproductive biologist leading the research.
The procedure is still highly controversial in the US. Last September, the National Institutes of Health imposed a ban on funding research in which human cells were introduced into non-human vertebrate animal embryos.
"The main concern is that the human cells might migrate to the developing pig's brain and make it, in some way, more human," says the BBC. Other scientists in the US have created chimeric embryos but have not allowed the foetuses to be born.
Compassion in World Farming's Peter Stevenson has also raised concerns about animal suffering. "Let's first get many more people to donate organs," he said.