Scientists have successfully created part-pig, part-human embryos. How have they done this, and could viable human organs really be grown this way?

The sow carrying human-pig embryos at the University of California, Davis, and featured on BBC Panorama.
The sow carrying human-pig embryos at the University of California, Davis, and featured on BBC Panorama.
Photograph: BBC

What are the researchers doing?

Scientists at the University of California, Davis, are hoping to find a way of growing human organs inside pigs, which can then be transplanted into humans. The technique involves altering the genetic makeup of the pigs so that they do not develop a pancreas and then injecting human cells that will go on to make replacement organs inside the animals.

Why might this research be useful?

It is hoped the research will provide a solution to the shortage of organs available for transplant. According to the NHS, about three people die every day in the UK because replacement organs cannot be found for them. Organs grown in the pigs could, in addition, be genetically tailored to specific patients.

Are they creating piglets here?

Not at this research stage. It would take 114 days for a pig, containing a human organ, to be born but for now the embryos are only allowed to develop up to 28 days to permit scientists to monitor the human cells’ behaviour.

How does it work?

The scientists “knock out” key genes involved in pancreas development in the pig embryos using the gene-editing technique Crispr. To replace the pig pancreas, the scientists then inject human cells that can develop into any tissue type into the pig embryo. These are called induced pluripotent (iPS) stem cells.

Although genetically foreign, the human cells are not rejected by the pig embryo because its immune system has not yet developed. So the pig embryo goes on to develop into a foetus with a pancreas derived from the injected human cells.

Is this the same as xenotransplantation?

Not quite. Xenotransplantation involves putting an organ from one species into another – for example a pig’s liver into a human. With a chimera, the organ transplanted from the pig would primarily be composed of human cells, not pig cells.

Is this the first time this has been done?

Research with chimeras has been going on for decades. Among previous experiments, scientists using different techniques were able to produce a mouse with a rat’s pancreas, and mice with livers almost completely composed of human cells. In the latter case, the liver was not human-sized – one of the reasons why scientists are now using pigs.

What are the ethical worries surrounding this research?

There are several areas of concern, not least worries about animal welfare and the possibility that viruses could jump from animals to humans. What’s more, some are concerned that human cells could be involved not only in forming the pancreas of the pig, but also other tissues, including the brain. That proposition gives rise to a host of conundrums around whether the pigs could become human-like in some way. “It would have to be a major contribution either overall or to specific parts of the brain before I would expect it to have any significant effect on pig behaviour,” said Robin Lovell-Badge, a geneticist at the Francis Crick Institute in London.

Who is funding the research in the US?

The National Institutes of Health – the main agency for medical research in the US – imposed a moratorium on providing funds for research with chimeric embryos last year amid ethical concerns. But Pablo Ross from the University of California, Davis, one of the scientists involved in the current work with pigs, said their research was funded by a number of sources, including the California Institute for Regenerative Medicine, the University of California, Davis, the Universidad Católica de Murcia, Spain, and private funds. Researchers undertaking similar work at the University of Minnesota were funded by the US Department of Defense, said Ross.

How is this research regulated in the US?

Researchers must seek approval from a number of bodies before the work can be carried out. According to Ross, these include the Institutional Animal Care and Use Committee, the Institutional Biosafety Committee, and the Stem Cell Research Oversight Committee.

And in the UK?

Researchers in the UK who are planning to carry out such work need to apply to the Home Office for a licence.

What is a chimera?

According to Greek mythology, a chimera was a creature made up of different animals – the head of a lion, body of a goat and tail of a snake. But in science, a chimera is an animal that has cells or genes that come from two or more different species.