What is a head transplant?
A head transplant is taking a living head and putting it onto a new body.
But actually, it's a body transplant, as the head will be gaining a new body to control. However, as the term "whole body transplant" is already used to mean transferring the brain between bodies, calling it a "head transplant" makes it clear that the whole head is to be switched, brain included.
How would Canavero's human head transplant work?
The donor body and the head to be attached are first cooled down to 12-15˚C to ensure that the cells last longer than a few minutes without oxygen. The tissue around the neck is then cut, with the major blood vessels linked with tiny tubes. The spinal cord on each party is then severed cleanly with an extremely sharp blade.
At this point, the head is ready to be moved, and the two ends of the spinal cord are fused using a chemical called polyethylene glycol, encouraging the cells to mesh.
After the muscles and blood supply are successfully connected, the patient is kept in a coma for a month to limit movement of the newly fused neck, while electrodes stimulate the spinal cord to strengthen its new connections.
Following the coma, Canavero anticipates that the patient would immediately be able to move, feel their face and even speak with the same voice. He believes physiotherapy would allow the patient to walk within a year.
What does the scientific community make of the human head transplant?
Dr Hunt Batjer has attracted headlines for being particularly blunt: "I would not wish this on anyone. I would not allow anyone to do it to me as there are a lot of things worse than death."
Dr Jerry Silver witnessed the 1970s monkey head transplant experiment and describes the procedure as "bad science", adding that "just to do the experiments is unethical".
Dr Chad Gordon, professor of plastic and reconstructive surgery and neurological surgery at Johns Hopkins University, agrees that Canavero's claims are scientifically implausible.
Dr Paul Myers, associate professor of biology at the University of Minnesota at Morris, puts it even more explicitly: "This procedure will not work... Try it with monkeys first. But he can't: the result would be, at best, a shambling horror, an animal driven mad with pain and terror, crippled and whimpering, and a poor advertisement for his experiment. And most likely what he'd have is a collection of corpses that suffered briefly before expiring."
Others wonder whether Canavero might simply be enjoying the limelight with a PR stunt, including Dr Arthur Caplan, director of ethics at the NYU Langone Medical Centre. Describing the doctor as "nuts," he explained to CNN: "Their bodies would end up being overwhelmed with different pathways and chemistry than they're used to, and they'd go crazy."
Has a head transplant been tried before?
No-one has ever attempted a human head transplant before, and attempts on animals have had little success.
The first time a straight swap of heads was "successful", was by Dr Robert White, in an experiment on a rhesus monkey in 1970. However the monkey lived for a mere eight days after the operation.
More recently, Chinese doctor Ren Xiaoping claims to have conducted head transplants on more than 1,000 mice. But none of these mice have lived longer than a few minutes.
What are the challenges involved in a head transplant?
1. The surgery requires the brain to be chilled to 12-15˚C, and only 30-40 percent of brains can survive that.
2. Fusing a spinal cord has never been done before, and may not be possible. This is probably the main objection people have – this hasn't even been attempted on animals, and it sounds hugely unlikely that millions of nerves will be able to connect perfectly.
3. The transplant could be rejected. Immune system rejection occurs when the body sees a new part as foreign and attacks it.
What are the possible side effects of a human head transplant?
If the surgery goes ahead as planned, the most pressing risk is that the body will reject the head and the person will die. Of all the animals this kind of thing has been tried on – monkeys, mice and dogs – the longest any has lived is eight days.
Writing for Forbes, NYU's Dr Arthur Caplan put this matter-of-factly: "I think the most likely result is insanity or severe mental disability."
