Many researchers are working on adult stem cells - the immortal cells that regenerate old or damaged tissues in our bodies - as they have great potential for treating many diseases, with fewer ethical issues than stem cells taken from embryos.
Now scientists at the Schepens Eye Research Institute in Boston have found the chemical triggers that can wake up stem cells in the brain, making them multiply and repair damaged brain areas, or send them back to sleep again.
The story started earlier this year, when the researchers, led by Dr Dong Feng Chen, found that stem cells are scattered throughout the brain, but are usually kept 'asleep' by chemical signals sent from neighbouring cells.
It had previously been thought that stem cells were restricted to two specific parts of the brain, known as the subgranular zone of the hippocampus, the area responsible for learning and memory, and the subventricular zone which makes nerve cells that help us smell. So it was believed that when nerve cells died in other areas of the brain, they were lost forever.
First, the team showed that stem cells could be found all over the brain by growing tissues from different regions of mouse brains in the lab alongside supporting cells called astrocytes taken from the hippocampus (where stem cells do regenerate). They found that nerve cells could regenerate, proving the presence of stem cells.
Next, the researchers compared the chemical signals produced by astrocytes from the hippocampus with those made by astrocytes from other areas of the brain. They have now discovered that in areas where stem cells are 'sleeping', astrocytes make high levels of two molecules called ephrin-A2 and ephrin-A3.
These molecules are produced in much lower amounts by astrocytes in the hippocampus - instead they make a protein called sonic hedgehog (named after the computer game character). The scientists have shown that removing these ephrin molecules, or adding sonic hedgehog, can reawaken stem cells.
This new research adds hope to the idea that we might be able to reactivate stem cells in damaged brains, helping to rebuild them from within. This could potentially treat diseases such as Parkinson's or Alzheimer's, as well as brain or spinal cord injuries.
The next step for the team will be to try and wake up sleeping stem cells in animals with diseases such as Parkinson's, to see if this works in whole brains, rather than cells grown in the lab.