Labs in the US are growing human-animal hybrids that could one day be used to grow organs for transplants. The idea of human-animal chimeras is not new – in 2003, scientists in China announcing they had successfully fused human cells with rabbit eggs. Ethical questions have surrounded the issue ever since, and America's National Institutes of Health put a ban on funding into such research until scientific and social implications had been fully assessed.

However, MIT Technology Review has reported some research centres in the US are pushing ahead with experiments, finding there have been 20 pregnancies of pig-human or sheep-human chimeras in the last 12 months. As of yet, there have been no scientific papers describing the work, and no animal has been brought to full term.

Chimeras (which in Greek mythology are fire-breathing hybrid creatures) are created by adding human cells to animal embryos, which are then implanted into a host for gestation. To grow human organs, for example, scientists could engineer an embryo so it is missing a specific organ. Human stem cells can be injected into it so when it grows in the womb, a human organ replaces that which has been removed. This organ could then be harvested from the animal for use in transplants.

While promising, the technology is highly controversial. Animal welfare groups say it is cruel, while others cite concerns over regulation. Animals could turn out to be too human (say brain cells were introduced, for example), as David Resnik from the NIH explains: "The spectre of an intelligent mouse stuck in a laboratory somewhere screaming: 'I want to get out' would be very troubling to people."

Another issue is the moral and legal status of these animals. Would any human-animal chimera have human rights? If a chimera was created that was mostly human, would it still be owned by the laboratory? These are among the issues that prevent research moving forward.

Instead, scientists working on such projects are currently collecting foetuses to gather preliminary information on how much human cells contribute to the animals' bodies. University of Minnesota scientists developed a 62-day-old pig foetus that appeared to have a reversed a congenital eye defect thanks to the addition of human cells. "We can make an animal without a heart. We have engineered pigs that lack skeletal muscles and blood vessels," Daniel Garry, a cardiologist who leads a chimera project at the University of Minnesota, is quoted as saying.

Pablo Ross, a veterinarian and developmental biologist at the University of California, Davis, said at present they do not want the animals to fully develop as they need to ascertain if human cells are contributing to the organs: "We don't want to grow them to stages we don't need to, since that would be more controversial.

"My view is that the contribution of human cells is going to be minimal, maybe 3%, maybe 5%. But what if they contributed to 100% of the brain? What if the embryo that develops is mostly human? It's something that we don't expect, but no one has done this experiment, so we can't rule it out."

According to The Times, the UK Home Office will publish its own guidelines on human-animal chimeras this week, possibly paving the way for researchers here to carry out their own experiments. It is expected the guidelines will be in line with rules regarding other animal research.