Why human head transplants are still a long way from becoming a reality

Thanks to Frankenstein's monster, Robocop, and the cyborg commandos of Ghost in the Shell, extreme portrayals of transplant surgery have captured our imagination over the years. The story of a patient's hand grafted on to a leg in China after an industrial accident to preserve it before being restored to his arm is just the latest success story to gain worldwide attention.

So when Italian neurosurgeon Sergio Canavero first announced his intention to perform the first ever human "head transplant" by December 2017 – part of his "head anastomosis venture" or HEAVEN project – science fiction seemed to inch a little closer to science fact. Canavero's idea involves a 36-hour surgery during which the head of a patient suffering from a debilitating disease would be fused at the spinal cord to a brain dead donor with an otherwise healthy body.

Despite scientists and surgeons voicing some serious doubts that such a massive undertaking would be successful, Canavero is adamant that the technology now exists – by employing his novel GEMINI protocol, he argues, the likelihood of success is around 90%.

But just how well do his claims stand to scientific scrutiny? Below are just three of the many important issues that haven't been convincingly addressed.

Fundamental principles

First of all, let's look at how the surgery would be performed. The head of the patient and spinal cord of the donor body would be cooled below 20°C. This would give the surgical teams less than an hour to simultaneously remove both heads at the neck, transfer the head of the patient to the donor body, and reconnect the spine and blood vessels before nervous system cells begin to decay. The spines would be held together and stabilised, and a specialised compound known as polyethylene glycol (PEG) would be used to connect the bundles running through the spinal cords. After all the blood vessels, neck muscles and connective tissue are sewn up, the patient would be placed in a chemically induced coma for three to four weeks to allow the connections to seal and recover.

One of the fundamental principles behind this procedure is that severed spinal cords possess the ability to reconnect, but that spinal injuries smash up the millions of connections beyond repair. Canavero argues that by slicing through the spine with an extremely sharp knife, the mostly intact fibres could reconnect with the help of the PEG glue and electrical stimulation. He illustrated this concept at a TEDx talk this year, where he compared a banana squashed at the centre with one finely sliced with a sharp knife.

Serious flaws in Canavero's proposal include the failure of previous animal models or the implausibility of keeping the head alive during the procedure. Canavero, however, is not only convinced that the head could be connected, but that it could gain full control of the body. To understand whether his conviction is warranted, we should look at the neuroscience behind his arguments.

Glial scars: the Gandalf between bridges

Neurons in the brain sprout tails known as axons, which travel through the spinal cord to send and receive signals to and from the body. In a spinal cord injury, these axons are severed, preventing the signals from reaching their target. To some extent, Canavero is correct that the spine is equipped with the tools to repair axons, but these connections are actively blocked by the almost immediate formation of glial scars.

You shall not pass!
PhillieCheesie, CC BY-SA

Glial scars are clusters of immune cells that flock to the site of injury when the spinal cord is damaged. These scars patch up holes in the axons and protect against further injury, but they also release chemicals that stop the two ends from fusing. Canavero's GEMINI protocol makes no mention of glial scars, which would likely prove to be a fatal hindrance to his procedure.

Fixing a spine needs more than glue

If we ignore the glial scar problem, the next question is whether using polyethylene glycol to fuse the spinal cords would actually work. PEG has indeed been shown to promote axon repair to some extent, but most of these experiments were performed on cells isolated in a lab as opposed to in the body. Some promising results have, however, been shown in recent animal models. One group used PEG to treat rats with fully severed spinal cords and found that some axons did reconnect. They also restored some movement, with some rats gaining minor control of their hind legs.

No doubt, these findings offer some important implications for treating spinal cord injuries, but when considering how this could be applied to a head transplant, the devil is in the detail. Only the physical control of the body of the rats was tested, which means that we're still unsure as to whether they regained sensation in their lower bodies. The PEG treatment also offered only modest improvement in function and repair of cells in the spine. Most importantly, though, the rats' spines were disconnected at the thoracic level TH8/9. This is a region about half way down the spine, which is low enough to preserve the most important bodily functions.

In Canavero's procedure, the spine would be cut at the cervical region where axons carry signals involved in functions that keep the body alive. Unsuccessful fusion of these axons would leave a patient paralysed and breathing with a machine. Canavero claims in his proposal that a research group in China has already successfully carried out a similar operation in mice. This is partially true, in that 18 out of 80 mice operated on survived for three hours after being taken off a ventilator. Importantly, there was also no spinal fusion with these mice and the brain stem of the donor bodies were kept intact. As the brainstem controls vital functions including breathing, the mouse heads were essentially stitched onto a paralysed incubator.

Massive hurdles

Although Canavero's proposal is an exciting idea, the research simply doesn't support his claim that we now have the technology to pull it off. However, significant advances are being made in the way treating spinal cord injuries through stem cell therapy or forming bridges over glial scars. A more robust and stable development of this technology would be an important step in the spinal cord injury treatment and a possible first step towards realising Canavero's vision.

We still have a long way to go, however, before we can start swapping our bodies. Even after we figure out how to fuse spines and restore connectivity, we still don't know whether the brain can rewire itself to control a new body. We know from studies into hand transplants, which are several magnitudes less complex, that function can be restored to some degree, but even this varies in success with our current technology. More worryingly, though, episodes of the immune system attacking the transplanted hand are extremely common. This could be a catastrophic event following a head transplant, as the donor body's immune system could attack the head.

To convince neuroscientists that this procedure could work, more compelling evidence is needed. In Canavero's own paper he argues that a preliminary experiment would need to be performed on a primate model. Whether or not this would even be ethical given our current understanding would be another major question. For Canavero's ideal to be realised, science has some massive hurdles to jump. Before we move onto humans, we should start with fixing the banana.

Darren Ó hAilín is PhD candidate in Molecular Medicine at Freiburg University

This article was originally published on The Conversation. Read the original article.