The birth, growth and development of new brain cells – neurogenesis – happens throughout life. Or does it? This has been one of the liveliest debates among neuroscientists for over half a century.
If the answer is yes, then we can regenerate our aging brains.
The argument goes back and forth. Last year, researchers at the University of California San Francisco (UCSF) dashed our hopes. They found this process declines throughout childhood and is undetectable in adults.
Not so fast, respond scientists from Spain. Their just-published study demonstrates growth of new brain cells well into the ninth decade of life.
And they also came up with a totally unexpected finding that links neurogenesis to Alzheimer’s. . .
Hippocampus Keeps Birthing Neurons
Dr. Maria Llorens-Martin, from the Molecular Biology Center in Spain, led a study to investigate neurogenesis in a region of the hippocampus called the dentate gyrus. It’s a key area for learning and memory and one of the most affected by Alzheimer’s.
Direct evidence that the adult hippocampus generates new neurons has eluded scientists, so the research group set out to see whether this phenomenon is real.
To do so they examined the brains of 13 people between the ages of 43 and 87 who were neurologically healthy when they died.
Using state-of-the-art tissue processing methods under tightly controlled conditions, the researchers were able to identify thousands of immature neurons even in the oldest subjects, although neuron production did tail off with aging.
The number of young brain cells fell from around 40,000 per cubic millimeter in the youngest, to 30,000 in the oldest. (Aside: I’m forever amazed and humbled at how tiny these things are.)
Generate New Brain Cells – and New Memories
Commenting on the findings, Dr. Llorens-Martin said, “These results suggest that when people lose brain cells in old age, they might be able to generate new ones to replace them. That opens the possibility of generating new memories using new brain cells.
“I believe we would be generating neurons as long as we need to learn new things. And that occurs during every single second of life.”
These findings support research carried out by a scientific team from Columbia University in New York last year. They examined postmortem brain samples from people aged 14 to 79 and found even the oldest produced new brain cells in the dentate gyrus.
But how do the UCSF findings fit in with these newer studies? The authors of the latest study believe the immature neurons were there, but the San Francisco researchers couldn’t see them, because of chemical preservatives used when brains are stored in brain banks.
Reduced Neurogenesis Linked to Alzheimer’s
In a further experiment, the Spanish scientists looked at the brains of 45 people aged between 52 and 97 who were at different stages of Alzheimer’s when they died.
The Spanish team still found fresh neurons but in much smaller amounts. The fall was particularly marked in late-stage dementia patients, but also fell by a third — from 30,000 to 20,000 per cubic millimeter — in patients in the earliest stage of the disease. This occurred before the formation of amyloid beta plaques and tau tangles which are said to be the hallmarks of Alzheimer’s.
On this unexpected finding Dr Llorens-Martin commented, “It’s very surprising for us. It’s even before the accumulation of amyloid beta, and probably before symptoms; it’s very early.
“This is very important for the Alzheimer’s disease field because the number of cells you detect in healthy subjects is always higher than the number detected in Alzheimer’s disease patients, regardless of their age.
“It suggests that some independent mechanism, different from physiological aging, might drive this decreasing number of new neurons.”
Brain Plaque Theory Takes Another Hit
If impaired neurogenesis takes place in Alzheimer’s before the build-up of amyloid, it drives another nail in the coffin of the “amyloid hypothesis,” which says the accumulation of these plaques in the brain is the primary cause of the disease.
Skeptics of the “plaques and tangles theory” received another boost on the 20th of March with the announcement of yet another failure of a large amyloid-targeted human drug trial. Going after the plaques has not worked.
Impaired neurogenesis would be a far more relevant mechanism to research than amyloid, which has proved fruitless for decades.