Doctors in California plan to regenerate brain cells outside the brain and re-inject them into patients with spinal injuries, and diseases such as Alzheimer's or Parkinson's.

It was previously thought that brain cells did not grow and divide in the same way as other cells, and that once damaged they could not be repaired.

But a team from the Salk Institute for Biological Studies in California has produced evidence that human brain cells do grow and divide.

They looked at the brains of terminal cancer patients that had been treated with a chemical, bromodeoxyuridine, that is sometimes used to mark rapidly dividing cancer cells.

Working with colleagues at the Sahlgrenska University Hospital in Sweden, they found that the chemical had been taken up by the DNA in a region of the brain associated with learning and memory.

This, they reported in the journal Nature Medicine, indicated that cells in this region had divided and synthesised new DNA.

"Our study demonstrates that cell genesis occurs in human brains and that the human brain retains the potential for self-renewal throughout life," they wrote.

Regenerate cells

Brain cell: can divide and grow

A team at the Cedars-Sinai Medical Centre in Los Angeles hopes to use the discovery to help brain-damaged patients. They announced a plan to take damaged brain cells from patients, regenerate them in the lab, and then put them back in the brain.

Dr Michael Levesque, the neurosurgeon who will do the transplants, said: "Right now we will use cell harvesting and implantation to treat Parkinson's disease."

Parkinson's is marked by the death of brain cells that produce chemicals key to movement. Victims start out with shaking that can progress to paralysis and death. There is no cure.

"Treating stroke and spinal cord injuries with regenerated cells is infinitely more complex," Dr Levesque said.

"We have to identify, grow and reintroduce a complex mixture of cells to restore a damaged circuitry. We're working on a human protocol for spinal cord injury now and hope to start treating patients with regenerated cells within the next six months."

Researchers have already managed to make paralysed and brain damaged rats and mice walk again by growing human brain cells and implanting them into the creatures' brains.