Exercise your brain

In a second paper, published in the January 21 on-line issue of Stem Cell Express, Rietze and his colleagues confirm that neural stem cells are extremely rare entities in the mouse brain - and by inference, in the human brain.

The study, led by Dr Daniel Grant, and Mohammad Golmohammadi, a visiting PhD student from Isfahan University of Medical Sciences in Iran, was the first to employ the new assay to analyse the number and distribution of neural stem cells in the mouse brain.

The QBI team made thin sections of the mouse brain, from front to back, and counted stem cells.

"There may be as few as 51 true stem cells in the entire mouse brain, when you would think there would be thousands," Rietze says.

"Research groups using an indirect measure of stem cells - a clonal lineage analysis - were reporting around 1200 stem cells in the ventricles of the mouse brain."

The latest study explains the discrepancies in reported numbers of neural stem cells from one study to another, by showing that the prevalence of the cells in the lining of the ventricles varies markedly along the neuraxis from the front to the back of the brain.

Rietze says stem cells are completely absent from the cephalic flexures, which form the boundaries between the forebrain, which is specialized for higher cognitive functions, and the midbrain and lower regions of the brain.

"Developmentally, these different stem cell populations appear to be unique, because when you infuse factors to stimulate division, they respond differently, according to where they are in the brain," he says.

"So it's not only the prevalence of neural stem cells that varies, but their competence as they age. That's astonishing - the brain ages at different rates in different regions."

Rietze says the widest variance in stem cell numbers occurs in the forebrain.

The human brain is different - for instance, Professor Arturo Alvarez-Buylla of the University of California, San Francisco, has found that neural stem cells appear to be restricted to the lateral ventricles.

"Nobody has yet published convincing evidence that they reside anywhere else, including the spinal cord, but these are still early days, and the jury is still out on that one," Rietze says.

The significance of this distribution is unclear, but Rietze said it makes sense that the cortex, the youngest and most plastic region of the human brain, would contain the most neural stem cells.

Neural stem cells are rare, and their replication rate declines with age. But the effects of ageing may not be inexorable.

The QBI researchers believe it may be possible to develop a biochemical cocktail to restore neural stem cells to youthful vigour - or even to make amendments for nature's frugality, by preserving and expanding the original endowment of stem cells in the brain.

While awaiting further developments in the brain stem cell story, readers should self-medicate with regular exercise.