The researchers from the Yale University School of Medicine in the US believe this is a significant move towards a treatment for people with spinal cord injuries.

The GM cells were able to stimulate the production of myelin, the protein which forms a sheath spinal nerves and allows them to conduct electrical nerve messages to and from the brain.

The experiments were carried out on primates whose spinal cords had been deliberately damaged.

Alexion Pharmaceuticals, which is funding the research, is confident that trials on humans may now have moved a step closer.

Ethical dilemma

However, the transplantation of animal tissue into humans, or xeno-transplantation, will involve overcoming major ethical and clinical barriers.

The cells would be taken from GM pigs

Many fear that the husbandry procedures needed to raise pigs for xeno-transplantation use involve extremely inhumane practices.

And there are still worries that unknown viruses previously restricted to the animal population could be spread into humans.

In the UK, experts discussing possible safeguards for xeno-transplantation have suggested that people receiving animal tissue would have to agree never to have children, or even unprotected sex.

However, the US scientists are upbeat about the results.

Stephen Squinto, from Alexion, said: "This promising data demonstrates that immunoprotected transgenic pig cells can survive and regenerate myelin sheaths around damaged neurones within the spinal cords of non-human primates.

"It suggests that this approach may lead to the development of a new therapy for spinal cord injury patients."

The major obstacle to the use of animal tissue in humans is the body's immune response, which identifies foreign matter and destroys it.

The scientists managed to genetically modify the pig cells so that the level certain sugars on the cell which trigger an immune system attack is reduced or even eliminated.

Also, the surface of the cells was covered with proteins that act to hold back an immune response.

'Treatment milestone'

Dr Jeffrey Kocsis, who led the study, said it represented a "particularly important milestone."

Treatments for people with spinal cord injuries are currently extremely limited.

Spinal injuries causes paralysis, or loss of some function, proving more disabling the higher up the spine they occur.

Neck injuries can causes loss of sensation and motor function in all four limbs, and in the most severe cases, even the ability to breathe unaided can be affected.