"So far we have managed to produce several millions of new cells from the original stem cells. About 25 percent of them are (active) neurons," Ulf Westerlund, who presented his doctoral thesis on the subject last week, told the paper. Westerlund is researcher at the Stockholm Karolinska Institute in Sweden.

When the researchers added glutamate to the media, which is a neurotransmitter in the central nervous system, the new cells then communicated in a network, according to Westerlund.

"This means we had working synapse connections that are needed for nerve cells to work," he said.

For years now, researchers have attempted to find ways of replacing dead brain cells with healthy ones. The hope is with this to reverse the effects of such diseases as Alzheimer's, Parkinson's and multiple sclerosis, in which brain cells are slowly dying.

In cooperation with the University of California at Los Angeles, Westerlund and other Swedish researchers have inserted the extracted human stem cells into the spinal marrow of rats, revealing that also there the cells continued to divide and create new cell neurons.

The injection of stem cells into the rats also appeared to lead to quicker recovery for allodynia, or pain that results from a non-injurious stimulus to the skin, according to Westerlund.

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