People aren't going to be getting head transplants in two years - here's why

Whether you're simply looking to extend your life or legitimately trying to survive a deadly disease, the procedure is nowhere near ready for real life, despite the recent claims of Italian neurosurgeon Sergio Canavero.

Here are a few of the reasons head transplants aren't going to take place in 2017, or likely anytime in the next few decades.

Heads can't stay alive on their own

In any transplant, the donor organ (the one that's been taken from a donor's body) has to be kept alive until it can be placed into the recipient's body.

As soon as an organ gets removed from a body, it begins to die.

For things like heart or kidney transplants, doctors cool the organ to keep it viable for as long as possible. Cooling the organ helps reduce the amount of energy its cells need to stay alive.

Doctors accomplish this by bathing the organ in a solution of cold salt water (saline). This process preserves kidneys for 48 hours, for example, livers for 24 hours, and hearts for about 5-10 hours.

But a head would be a far more difficult process.

A head isn't just an isolated organ. It's the heaviest and one of the most complex parts of the body - it houses not just your brain but your eyes, ears, nose, mouth, and skin, as well as two separate gland systems: the pituitary, which controls the hormones that circulate throughout the body, and the salivary, which are responsible for producing saliva.

More than a century of disturbing animal research has shown that at the moment of decapitation, blood pressure in the head drops dramatically. The resulting loss of fresh blood and oxygen pushes the brain into a coma, soon followed by death.

Which brings us to the next problem.

The immune system has to be coaxed into accepting a foreign head

As with any transplant, one of the main issues facing patients is that of their own body: if the immune system flags the foreign organ (or organs, in this case) as foreign, it can unleash a full scale attack.

What happens is that the immune system of the person receiving the new organ detects immune-triggering substances, called antigens, on the cells of the new organ that don't match the substances that would be on a non-foreign organ. This is why almost all transplant patients take immune-suppressing drugs after their procedure.

Because the head is so complex and includes so many organs, the risk of rejection is much higher.

The surgery has to happen in under an hour

In a 1970s experiment that would never be allowed today, neurosurgeon Robert White transplanted the head of a monkey onto the body of a donor monkey. He maintained the monkey bodies by cooling them to about 59 degrees Fahrenheit for the duration of the procedure. The monkey with the transplanted head survived until the immune system rejected the head 8 days after the surgery, and the monkey died.

One big catch, though: the whole thing has to be done in under an hour, according to White's experiments and Canavero's paper.

Canavero notes in his paper that both of the heads would have to be removed from their bodies at the same time. Working swiftly, the surgeons would have to re-attach the head of the person they want to keep alive to the circulatory system of the donor's body while both bodies are under total cardiac arrest. All of this would happen in less than 60 minutes.

Spinal cords are very tricky to fuse - it's never been done before

In order for the preserved head to be able to communicate with and control its new body, the spinal cord and the brain must be seamlessly connected, or fused.

This didn't happen with the monkey transplant. While the monkey who's head was transplanted onto a donor body was able to see, move its eyes, and eat, it was paralyzed from the neck down.

"The greatest technical hurdle" to a head transplant, Canavero writes in his paper, "is of course the reconnection of the donor's (D)'s and recipients (R)'s spinal cords. It is my contention that the technology only now exists for such linkage."

Canevero's technology is "a special biological glue," he said in a recent TED talk, called polyethylene glycol. In the 1930s and 40s, some experimental surgeons have used this material, which is a type of plastic, to fuse the spinal cords of dogs, but these experiments typically involved attaching a foreign head to the complete body of a dog (artificially giving it two heads), not replacing one dog's head with that of another. These procedures were also done in less than an hour.

After the procedure, one of Canavero's patients would be placed in a coma for up to a month to allow the spinal cords to fuse. Otherwise, the "spaghetti" (as he calls it) that makes up the spinal cord could become gnarled or twisted.

But such a long coma is a potential problem as well, University of California, Davis professor of neurosurgery Harry Goldsmith told Popular Science, because medically-induced comas often result in infection, blood clots, and reduced brain activity.

The procedure has to work in animal trials before being done in humans

Before head transplantation moves into trials in people, all of these problems have to be addressed in animal trials. And there are hurdles to getting animal experiments approved that involve so much cruelty. The bar for approving any such procedure, then, would have to involve proving that it is both helpful and necessary.

So far, there's no evidence that any of this is happening yet, which means any type of surgery approaching this level of risk is decades off, at the very least.