IS IT ethical to grow human organs inside pigs? The first thing to bear in mind, one medical ethicist said this week, is that there is a large number of people in desperate need of organ transplants, and a shortage of available organs. Funny kind of ethics, that: the scale of the problem is far from the first thing to bear in mind. The primary ethical issue is: is it morally acceptable?

Of course, we must guard against yuk-factor arguments in responding to reports that scientists have inserted human stem-cells into pig embryos. Some things just don’t sound right. A mouse with a rat’s pancreas now exists; so do mice with human-cell livers. And it sounds like Frankenscience.

Utilitarians are dismissive. If something does more good than harm, overall, it must be OK. But the ends soon justify the means that way. Absolutists, in contrast, insist that there is something distinctive and sacrosanct about human DNA. For them, such procedures are repellent because they violate human dignity. Some animal-rights advocates extend that notion to animals, although that is very much a minority view in societies that routinely use animals for food and clothing.

Interestingly, Roman Catholic teaching is surprisingly liberal on xenotransplantation, and it regards stem-cell research as morally unproblematic, so long as the material is from adult tissue or umbilical cords rather than from human embryos. Catholicism insists that animals are there for our responsible use, since only humanity is created “in the image and likeness of God”, and sits at the summit of the creation hierarchy. Thousands of patients who have received heart valves made from pig or cow tissue would agree, even if they do not regard themselves as a type of human-animal chimera.

That said, there are three important precautionary issues. A human pancreas from a pig still has piggy blood vessels, which might result in rejection in the human body. Second is the risk that viruses latent in a pig will become active in a human. But the third is the most serious.

Human stem-cells that are intended to create a pancreas might migrate to a pig’s brain, ovaries, or testes. Researchers say that there is “very low potential” for this. It may be low probability, but it is high risk. The scientists say that they are working on mechanisms to prevent human stem-cells’ developing in such areas. “Working on” is not reassuring enough.

The worry is not some sci-fi scare about “a human mind trapped inside a pig’s body”. It goes to the question what makes humans different from animals. Might the qualities that elevate the human above the mere animal begin to be eroded by the impact of human neurons in pig brains? That is not far-fetched. Baby mice that received cells from human embryos learned much faster than their fellows, which suggests that human brain cells boosted mouse neural networks.

How many extra human characteristics would a pig need before slaughtering it for its pancreas was transformed from medical harvesting into murder? There are questions of nuance and degree here that require far more careful consideration.

Paul Vallely is a Visiting Professor in Public Ethics at the University of Chester.