The bad news: Dr Rod Rietze's research group at the Queensland Brain Institute in Brisbane has found that the neural stem cells that renew high-maintenance regions of the brain through life are much rarer than originally thought, and their activity declines steeply with age.

The good news: Rietze and his colleagues believe that recent research showing that exercise stimulates neurogenesis in mice - an effect that almost certainly involves activation of neural stem cells - raises the exciting possibility that exercise, or drug therapy, can also make ageing human brains young again.

The new QBI findings trace to the momentous 1992 discovery by a Canadian research team that the mouse brain contains a self-renewing population of neural stem cells.

In 1887, the great Spanish neuroanatomist and Nobel laureate Ramon y Cajal proposed, that, unlike other cells, neurons cannot regenerate, and are not replaced when they die.

US researchers knew by 1990 that Cajal's conjecture was wrong, but it was a young Canadian PhD, Brent Reynolds, who discovered the elusive precursors of new neurons, in the lining of the fluid-filled ventricles towards the centre of the brain.

Isolated and grown in culture, the undifferentiated cells divided and formed small ball-like clusters, or neurospheres.

The discovery yielded the neurosphere assay (NSA), which soon became standard in neural stem cell research laboratories around the world. It was crucial to researcher's ability to isolate neural stem cells and study their activity.

However, after Reynolds joined his friend and compatriot Rietze's lab at the Queensland Brain Institute in 2004, they discovered that not all neurosphere-forming cells are stem cells.

In fact, only as few as five in 100 neurosphere-forming cells are true stem cells, with the potential to continue replicating, throughout life, and regenerate the full diversity of specialised progeny cells that form the working brain.

The rest are neural precursor cells (NPCs) that have already taken the first step towards specialisation and terminal differentiation.