A controversial experiment aimed at growing tailor-made human organs in pigs has echoes of the fictional horror story "Never Let Me Go", say animal welfare campaigners.
In the dystopian novel by Kazuo Ishiguro, adapted for the hit 2010 movie of the same name, a group of English children are cloned so that as young adults their organs can be used for transplants.
For the real-life research, US scientists at the University of California, Davis, have injected human stem cells into pig embryos to produce "chimeric" embryos that are part pig, part human.
One of the organs in the chimeras, the pancreas, will be composed almost entirely of human cells.
The experiment is the start of a programme that could lead to patient-compatible transplant organs being harvested from pigs.
But critics have raised strong ethical objections to the work, arguing that the research could lead to macabre "organ farms".
Dr Katy Taylor, director of science at Cruelty Free International, formerly the British Union for the Abolition of Vivisection (BUAV), said: "Reports of this gruesome experiment, to harvest organs grown inside a pig to transplant into a human body, reads like something out of the dystopian science fiction novel Never Let Me Go, but with animals instead of people.
"We are completely opposed to the use of animals to obtain organs for human transplantation. The procedure has been attempted thousands of times and has monumentally failed because of problems with safety, species differences and organ rejection.
"Animals are biologically very different to humans, and no amount of 'humanising' them will overcome this."
The US scientists plan to allow the "human" organs to develop in sows for 28 days before the pregnancies are terminated and the tissue removed for analysis.
Professor Pablo Ross, who is leading the research, told the BBC: "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."
The pancreas, which makes insulin, was chosen for the study because there is a pressing need for donated organs that can be transplanted into patients with Type 1 diabetes.
In the UK, only about 200 pancreas transplants are performed each year, and there are more than 300 people on the waiting list.
The new research is so controversial that last year the US National Institutes of Health announced it would not fund such experiments.
Peter Stevenson from Compassion in World Farming told the BBC: "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."
Other concerns relate to the possibility that the implanted human cells might migrate to the developing pig's brain and make it more human.
Professor Ross said this was unlikely but was a key reason why the research was proceeding with caution.
"We think there is very low potential for a human brain to grow, but this is something we will be investigating," he said.
To create the chimeras - named after the creatures from Greek mythology made up of more than one animal - scientists first used a new gene editing technique called CRISPR to remove pig DNA necessary for making a pancreas.
This left a genetic "void" which can be filled by a human substitute. In the next stage of the process, human induced pluripotent (iPS) stem cells were injected into the embryo. The iPS cells, derived from adult skin cells, are "blank slate" cells with the ability to develop into any kind of body tissue.
The scientists expect the stem cells to take advantage of the genetic niche they find themselves in and grow into a human organ.
Its tissue would be compatible with whoever donated the stem cells, raising the prospect of growing patient-specific organs that would not be rejected by a recipient's immune system.
:: The research will be outlined in Panorama - Medicine's Big Breakthrough: Editing Your Genes today (Monday) at 8.30pm on BBC One.