Stem Cells and Old Age Memory Loss Explained

Cosmos Online, December 20, 2006

A shortage of neural stem cells in the brain was formerly thought to be the reason that older people exhibited a reduced capacity to learn and remember. However, new research reveals that the reduced ability may be caused by stem cells in brain dividing less frequently instead.

The findings suggest that by stimulating the stem cells' ability to divide and produce new nerve cells, it may be possible to treat degenerative disorders, including dementia, depression, and Alzheimer’s disease.

As the brain ages, fewer new nerve cells, or neurons, are born in the hippocampus - the brain's learning and memory centre – as previous studies indicate. But until now, the cause of this decline had remained a mystery.

The common assumption had been that due to a diminishing supply of neural stem cells in the aging hippocampus, the result had been a sort of “brain drain”.

It seems logical due to the fact that immature neural stem cells possess the ability to develop into all types of nerve cells in the brain.

However in this new study, stem cells in aging brains dividing less frequently, instead of reducing in number, resulted in dramatic reductions in the number of new neurons added to the hippocampus. The online journal Neurobiology of Aging reports the researcher’s findings in its latest issue.

Taking middle-aged and older rats, the researchers attached fluorescent tags to the neural stem cells in the hippocampus to begin their experiments.

The hippocampus contained 50,000 stem cells in young rats. A significant point was that this number did not diminish with age. The decreased production of new neurons in an aged brain is not due to a lack of starting material as indicated by this finding from the research team.

Then all stem cells that were undergoing division – to potentially become mature nerve cells – were tagged with another fluorescent marker. Only 8 percent of the cells in middle-aged rats and 4 percent in old rats were dividing, as opposed to the young rats, where approximately 25 percent of the neural stem cells were actively dividing.

According to the team, this finding suggests that decreased division of stem cells is what causes the decreased neurogenesis - or birth of nerve cells - seen with ageing.

"This discovery provides a new avenue to pursue in trying to combat the cognitive decline associated with conditions such as Alzheimer's disease and with ageing in general," said co-author Bharathi, of Duke University.

In order to improve learning and memory function in the elderly, the team is now searching for ways to stimulate the brain to replace its own cells.

Co-author Ashok, also of Duke University said that, “another approach being explored is to treat older rats with drugs designed to mimic the action of compounds called neurogenic factors, which encourage stem cells in the brain to divide.”

To stimulate those that are already present, neural stem cells that are grown in culture dishes are also being grafted into the hippocampus.

Additional approaches that are known to stimulate proliferation of stem cells are being looked at. These include behavioral modification techniques such as physical exercise and exposure to an enriching environment.