May 3, 2018
When I started in neuroscience, the dogma was that you were born with all the neurons you were ever going to have, and it was all downhill from there as you aged. About 20 years ago, this belief started to change as investigators found evidence of newly formed neurons, particularly in the cerebellum and the hippocampus—but only in younger brains. Thus, the new dogma was that the ability to form new neurons gradually diminished as you got older, until it vanished completely. Most of this early work was first completed in animal studies, then later reproduced in humans.
Now the dogma is being challenged again. Maura Buldrinis and J. John Mann of Columbia University studied the brains of 28 healthy individuals, ranging from 14 to 79-years-old. Subjects were carefully screened to make sure they were free of disease processes that might alter neurogenesis (the formation of new neurons). The investigators concentrated on the hippocampus, a place in the brain they felt more likely to find neurogenesis. In this hippocampal tissue they found thousands of immature and newly formed neurons in all participants. There was no significant drop-off with age.
However, there were other changes in the older brains. The supporting vascular structure was less developed. Thus, these new neurons might be relatively starved for oxygen and other nutrients. There were also lower levels of proteins needed for developing plasticity as neurons mature. These latter two findings suggest that the new neurons may be there in older brains, but they are less able to become functional.
How added neurons take on functions in the recipient brain is not well understood. It is not like what occurs when you add liver cells to a liver, or muscle cells to muscle. In those organs, most cells are very similar in their shapes and functions. In the brain, cells are remarkably diverse. The big divisions are between input cells, which take in sensory information, process it, and then deliver that information to other sensory cells, or to output cells that provide messages to motor cells, glands, and skin. In many regions of the brain, neurons function in complex interactive circuits, such as what occurs in the basal ganglia or cortex. How do you introduce an outside neuron to one of these circuits? Is the development of the added neuron influenced by its new neighbors? If some of the cells of the circuit are motor output cells, can added neurons take on those properties?
The answers to these questions, and a myriad of other questions, are approachable and will be known in the next few years.
Guy McKhann, M.D., is professor of neurology and neuroscience at the Zanvyl Krieger Mind/Brain Institute, Johns Hopkins University, Baltimore. He serves as scientific consultant for the Dana Foundation and scientific advisor for Brain in the News.