The new treatment, developed in London, is targeted not only at degenerative diseases but at the much more common conditions where brain cells die from lack of oxygen - such as strokes and heart attacks. Its economic effect, as well as its human impact, could be enormous: such disorders affect more than 5 million people in the US and UK alone, costing health services in both countries more than pounds 60bn a year.
Up to now, neurological disorders of the brain have been practically impossible to combat because brain cells, unlike normal cells, are not replaced by the body.
But the team of psychologists from the Institute of Psychiatry at London's Maudsley Hospital, led by Professor Jeffrey Gray, injected rats brain- damaged from simulated heart attacks with embryonic mouse brain cells. Then the rats which had suffered from total amnesia and severe cognitive impairment recovered completely and were able to perform complex tasks, such as navigating through milky water to a platform to avoid drowning.
The team found that the injected brain cells - neuroepithelial stem cells or NESCs - migrated to various damaged sites in the rats' brains. There, they adopted the characteristics of the dead cells. The scientists' success led them to set up a company, ReNeuron, to sell their research. Experiments on humans are due to start in three years and a treatment could be available by early next century.
According to Professor Gray, the injected cells act as a first-aid kit for the brain. "I was overjoyed about the recovery of function. But what really surprised me was that the cells moved to the damaged sites." On top of that, the team has found a novel way of force-growing millions of foetal brain stem cells in the laboratory, using a cancer gene which switches on below body temperature. Dr John Sinden, co-founder of ReNeuron said: "We should be able to grow cells in the lab, keep them in the fridge and give them to the neurosurgeon when he needs them."
The only other method to transplant brain cells, currently practised in Sweden on patients with Parkinson's, depends on extracting partially developed brain cells which have to be taken from a large number of foetuses and at precisely the right time. This severely limits the conditions that can be treated. Some disorders need cells that do not appear until late in pregnancy when it would be illegal to abort.
"The ethical news is almost all good," said Professor Gray. "All the issues about using aborted foetuses will disappear. The need for human foetal tissue will dramatically reduce. We can treat a wide range of conditions. And this science also has great advantages over animal organ transplants."
Professor Gray, 63, a fellow in psychology at Oxford University for almost 20 years, and colleagues Dr Sinden, 46, and Dr Helen Hodges, 56, started work in 1984, setting up ReNeuron in 1997 as a private company. Initially funded by the Wellcome Trust and Medical Research Council, the scientists had to turn to industry, to tobacco giant BAT Industries in particular, to back their increasingly radical work.
"The lifeblood of science is asking the unlikely question. But no one wants to support research, no matter how important, that might fail," Professor Gray said. "As psychologists we took an imaginary leap that the biologically better-educated would not have risked."
This week ReNeuron has secured pounds 250,000 from Merlin Ventures, the seed capital trust founded by Chris Evans, the colourful Welsh entrepreneur, who has launched a string of technology companies on the London stock market.
ReNeuron's research raises difficult ethical issues. The prospect of transplanting brain tissue raises the spectre of Frankenstein. Already cryogenics - freezing the head or body after death in the hope that science may be able to resurrect them - is a multi-million dollar industry.
Professor Gray is worried about such wild imagining. But he emphasises that his approach is not about improving the brain, but restoring it.
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