Scientists in the United States are trying to grow human organs inside pigs.

They have injected human stem cells into pig embryos to produce human-pig embryos known as chimeras.

The embryos are part of research aimed at overcoming the worldwide shortage of transplant organs.

The team from University of California, Davis says they should look and behave like normal pigs except that one organ will be composed of human cells.

The human-pig chimeric embryos are being allowed to develop in the sows for 28 days before the pregnancies are terminated and the tissue removed for analysis.

The BBC's Panorama was given access to the research for Medicine's Big Breakthrough: Editing Your Genes.

Creating a chimera

Creating the chimeric embryos takes two stages. First, a technique known as CRISPR gene editing is used to remove DNA from a newly fertilised pig embryo that would enable the resulting foetus to grow a pancreas.

This creates a genetic "niche" or void. Then, human induced pluripotent (iPS) stem cells are injected into the embryo. The iPS cells were derived from adult cells and "dialled back" to become stem cells capable of developing into any tissue in the body.

Gene editing technique could transform future

Pablo Ross, a reproductive biologist who is leading the research told me: "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."

Animal viruses

Gene editing has revitalised research into xenotransplantation, and the concept of using animal organs for humans.

In the mid-90s there were hopes that genetically modified pigs might provide an endless supply of organs for patients, and that cross-species transplants were not far off.

But clinical trials stalled because of fears that humans might be infected with animal viruses.

Last year, a team at Harvard Medical School used CRISPR gene editing to remove more than 60 copies of a pig retrovirus.

Prof George Church, who led the research, told me: "It opens up the possibility of not just transplantation from pigs to humans but the whole idea that a pig organ is perfectible.

"Gene editing could ensure the organs are very clean, available on demand and healthy, so they could be superior to human donor organs."

Animal suffering

But organisations campaigning for an end to factory farming are dismayed at the thought of organ farms.

Peter Stevenson, from Compassion in World Farming, told me: "I'm nervous about opening up a new source of animal suffering. Let's first get many more people to donate organs. If there is still a shortage after that, we can consider using pigs, but on the basis that we eat less meat so that there is no overall increase in the number of pigs being used for human purposes."

In Greek mythology, chimeras were fire-breathing monsters composed of several animals - part lion, goat and snake. The scientific teams believe human-pig chimeras should look and behave like normal pigs except that one organ will be composed of human cells.

Scott Fahrenkrug, whose Minnesota-based company Recombinetics is teaming up on the chimera research with Prof Low, told me: "Perhaps the term chimera is going to take on a new meaning and it will be one that's much more affectionate: chimeras will be seen to be what they are which is a saviour, given that they will provide, life-saving, sustaining organs for our patients."

Seven thousand people in the UK are on the transplant waiting list and hundreds die each year before a donor can be found.