Will brain transplants ever be possible?

Organ transplantation has come a long way. It's now possible to transplant not just hearts and livers, but functioning uteruses, hands and even faces. 

But will it ever be possible to transplant a brain? 

The answer to that is maybe — but it's a long, long way from happening. And it's not really clear that it would be ethical, even if it were possible. After all, a brain transplant is really a body transplant, raising the question: Is it right to save one person's life with a full donor body, or should that donor's organs go to multiple people, potentially saving many lives at once? 

Ethics aside, the brain is a delicate organ, and the spinal cord, which connects to the brain, does not recover well after being cut. While there have been animal experiments involving transplanted heads, most animals that have been subjected to these experiments died within hours or days. The longest-lived have made it only a few months. Though a few researchers have made headlines by claiming head transplants in humans are just around the corner, the hurdles are massive. 

Related: Creating 'universal' transplant organs: New study moves us one step closer.

"I don’t think any serious scientist considers any of that truthful or scientific," said Dr. Fredric Meyer, a neurosurgeon at the Mayo Clinic. 

A brief history of head transplants 

Scientists have not attempted to transplant an isolated brain into any animal. The living brain is soft and squishy, and it is too easily damaged to attempt to scoop it out from one skull and plop it into another. Trying to transplant an isolated brain would also entail reconnecting numerous delicate cranial nerves, which would be challenging. The brain transplants that have been attempted are really head transplants.

The first attempt occurred in 1908, when scientists Alexis Carrel and Charles Guthrie transplanted a dog's head onto another dog, creating a Cerberus-like animal that lived for just a few hours, according to a 2015 article in CNS Neuroscience and Therapeutics.

It wasn't a success, exactly, but Carrel and Guthrie's work did contribute to medical science. Carrel was later awarded a Nobel Prize in Medicine for their work on blood vessel reattachment — a technique that would later lead to the possibility of organ transplantation and limb reattachment. 

In 1954, Soviet scientist Vladimir Demikhov experimented with grafting dogs' upper bodies to other dogs. The two-headed animals mostly lasted a few days, with one surviving up to 29 days, according to a 2016 review article in the journal The History of Neurosurgery. The grafted-on heads were functional, doing things like lapping up water and responding to visual stimuli. But immune rejection ultimately led to the dogs’ deaths. 

In the 1960s and 1970s, an American neurosurgeon named Robert White took the head transplant concept a step further. Using rhesus monkeys (Macaca mulatta), he experimented with transplanting only heads, not full upper bodies, and did the transplants head-for-head, rather than grafting an extra head onto a full body, according to the 2015 CNS Neuroscience and Therapeutics paper. The transplanted monkeys could chew and swallow food and track objects with their eyes. They were, however, quadriplegic, because their spinal cords had been severed and could no longer send nerve signals to their bodies. They also died within about 36 hours due to troubles with blood flow.  

The problem with head transplants 

Today, it's often possible to prevent immune rejection with cocktails of cutting-edge drugs, enabling even highly immune cell-rich tissue like skin to last decades following a transplant. Scientists have also made big strides in vessel reattachment and in theoretically keeping the blood supply to the brain flowing during a head transplant surgery. In 2015, researcher Xiaoping Ren, of Harbin Medical University in China, experimented with mice and reported a method of cutting just one of the two jugular veins in the neck and one of the two carotid arteries to connect a second mouse head to a first mouse body, leaving the other jugular and carotid to feed the original head.

But major problems remain. A big issue is that transplanting a head requires slicing and reattaching a spinal cord. Though Ren and his team have found ways to slice the spinal cord low enough in mice to enable the transplanted animals to breathe without a ventilator, there is no good evidence in humans that the spinal cord could heal, according to the History of Neurosurgery review. Some researchers are exploring nanomaterials and specialized polymers for spinal cord repair, but those methods have only been tested in animals with different nervous system physiology from humans.

Preventing the brain from losing oxygen during and after surgery would also be harder in humans than it is in mice, simply due to the size and logistics of moving around human body parts versus mouse body parts. There is little room for error: Brain cells begin to die within five minutes of losing oxygen, according to the National Institute of Neurological Disorders and Stroke.  

Finally, according to the History of Neurosurgery review, there is no research at all on how to control pain after a head transplant. This isn't just the pain that would result from having been essentially beheaded. It would also be central neuropathic pain — a type of chronic pain that often occurs after damage to the spinal cord or brain. This type of pain is "notoriously difficult to treat," according to a 2016 article in the journal Mayo Clinic Proceedings.

For all these reasons, the Ethico-legal Committee of the European Association of Neurosurgical Societies (EANS) declared head transplantation in humans unethical in 2016. (The committee has no legal power to stop head transplants from being performed, but produces professional guidelines for neurosurgery practice.)

"The risks involved for the patient undergoing a head transplant [are] enormous, including this risk of death," the committee concluded. "There is no solid evidence base for all steps of the procedure; for some, there is even lack of proof of concept."