Italian neurosurgeon Sergio Canavero made headlines around the world two years ago with the bold claim that he would soon be stitching one person's head onto another's body.

Despite the numerous obstacles Canavero faces from ethical and scientific standpoints, he appears to be moving full-steam ahead. Last year, he performed what he called a "proof-of-concept" procedure in which he severed a dog's spinal cord then reconnected it — one of the major obstacles to completing the procedure in people.

And now Canavero has published another equally repellant study that details how he and another surgeon, Xiaoping Ren, attached the head of a mouse to the body of a rat. The team then repeated the procedure in several other animals, effectively creating a series of two-headed rats.

Most of the animals in Canavero's studies lived for only several days — the two-headed rats carried on for an average of 36 hours, and Canavero hasn't released details on the dog. But keeping them alive wasn't Canavero's goal; instead, he says each experiment is meant to stand as evidence that a complete procedure in people is on the horizon.

That far-fetched idea that has been explored by several scientists with mostly noble aims, such as saving the lives of people with deadly diseases like spinal muscular atrophy. But experts say there are many reasons to be doubtful. "These papers do not support moving forward in humans," Case Western Reserve University neuroscientist Jerry Silver, told New Scientist last year about Canavero's dog paper.

Here are the five major obstacles a successful head transplant would face.

1. Heads can't stay alive on their own.

In any transplant, the donor organ (the one that has 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 is removed from a body, it begins to die. So doctors cool down the organ to help reduce the amount of energy its cells need to stay alive. Using a solution of cold saltwater (saline), they can preserve kidneys for 48 hours, livers for 24 hours, and hearts for about five to 10 hours.

But a head isn't an isolated organ — it's one of the most complex parts of the body. In addition to housing your brain, eyes, ears, nose, mouth, and skin, it holds two gland systems: the pituitary, which controls the hormones that circulate throughout the body, and the salivary, which produces 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.

In the most recent mouse procedure, Canavero and his team claim they addressed this problem by keeping a steady blood supply flowing between the rodent who received the head, the rodent who donated it, and a third rodent.

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

In any transplant, one of the main issues is the reaction of a patient's own body. The immune system of the person receiving the organ detects immune-triggering substances, called antigens, on the cells of the new organ. These antigens don't match those found in their own body, which can lead the immune system to unleash a full-scale attack. That's why almost all transplant patients take immune-suppressing drugs after their procedures.

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

3. 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 for eight days after the surgery, until the immune system rejected the head.

According to White's experiments and Canavero's paper, the entire transplant surgery has to be done in under an hour. Canavero notes that both of heads would have to be removed from their bodies at the same time. Working swiftly, the surgeons would have to reattach 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.

4. Spinal cords are incredibly tricky to fuse.

For a head to be able to communicate with and control a new body, the spinal cord and the brain must be seamlessly connected.

This didn't happen in the monkey transplant — the monkey head that was transplanted onto the donor was able to see, move its eyes, and eat, but the body was paralyzed from the neck down. It did appear to happen in the dog procedure, but experts have said they need more concrete proof of Canavero's claims.

According to Jerry Silver, Canavero's team claims "they cut the cervical cord 90% but there’s no evidence of that in the paper, just some crude pictures."

TED

Canavero disagrees, of course. In a TED Talk, he described "a special biological glue" called polyethylene glycol that he says could be the key to successfully completing the procedure. Some experimental surgeons in the 1930s and 1940s used this material, which is a type of plastic, to fuse the spinal cords of dogs. But those experiments typically involved attaching a foreign head to the complete body of a dog (giving it two heads), rather than replacing one head with another.

Canavero's plan would also involve placing the patient 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, Harry Goldsmith, professor of neurosurgery at the University of California at Davis, told Popular Science — medically-induced comas often result in infection, blood clots, and reduced brain activity.

5. The procedure has to work in animal trials before being considered for humans.

Before head transplantation is ever considered for trials in people, all of these problems have to be addressed in animal tests. And such experiments would face many hurdles in getting approved (in the US, at least) because they involve so much cruelty.

Nevertheless, Canavero and his team appear to be moving ahead in other countries where the procedures are legal.