Blood clots interrupt the supply of oxygen to the brain and cells die. Two years ago, scientists learned how to manipulate stem cells the natural machinery that allows a fertilized egg to turn into a human composed of 100 trillion cells of more than 200 different kinds. But stem cells are a challenge to science. They make new blood, liver, or nerve tissue, but there is nothing to distinguish them from the cells around them. We define them on the basis of their behavior and what we know now is that they exist throughout our lives, said Dr. John Sinden, chief scientist of ReNeuron.
Stem cells are normal cells.
They keep dividing but they can only divide a limited number of times, so that we gradually lose them through life. Stem cells also exist in the brain. Most of the body rejects transplanted tissue, but other experiments have shown that the brain will accept foreign cells. The ReNeuron researchers injected mouse brain stem cells into rats damaged by stroke, and watched the rodents recover. The hope is that under the right conditions, fetal brain stem cells implanted into the brains of adult stroke victims will move around and begin repairing damaged parts of the brain.