They found that in young rats, the hippocampus contained 50,000 stem cells -- and, significantly, this number did not diminish with aging. This finding, the researchers said, suggested that the decreased production of new neurons in the aged brain was not due to a lack of starting material.
The researchers then used another fluorescent molecule to tag all stem cells that were undergoing division in the process of staying "fresh" in case they were recruited to become mature nerve cells.
They found that in young rats, approximately 25 percent of the neural stem cells were actively dividing, but only 8 percent of the cells in middle-aged rats and 4 percent in old rats were dividing. This decreased division of stem cells is what causes the decreased neurogenesis, or birth of nerve cells, seen with aging, the scientists said.
"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 aging in general," Hattiangady said.
The team now is searching for ways to stimulate the brain to replace its own cells in order to improve learning and memory function in the elderly.
One 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, Shetty said. The researchers also are grafting neural stem cells grown in culture dishes into the hippocampus, to stimulate those already present. Additional approaches include using behavioral modification techniques, such as physical exercise and exposure to an enriching environment, that are known to stimulate proliferation of stem cells.