New Brain Cells in Adult Cortex are Unlikely, Yale Researcher Shows

There is no evidence that adult primates are able to create new neurons in the neocortex, the most sophisticated part of the brain, Yale researchers have found in a study published in the December 7 issue of Science.

The results do not support widely publicized research two years ago that reported the first discovery of neurogenesis-formation of neurons-in the neocortex of adult macaque monkeys. That study found that considerable numbers of new neurons are continuously added to areas of the neocortex during adulthood. The neocortex is responsible for the highest human brain functions, such as language, perception and memory.

“Our study shows that all neurons of the cerebral cortex are created during restricted periods of development before birth and during the neonatal period,” said Pasko Rakic, M.D., the Dorys McConnell Duberg professor of neuroscience and chair of the Department of neurobiology at Yale School of Medicine. “Therefore we have to live our entire lives with the cortical neurons that we were born with.”

This is not necessarily bad news, according to Rakic. “We use neurons to store our life experiences and if we change neurons every season like canaries do, then we would lose a lot of our life experiences. Neurogenesis in the neocortex could eliminate crucial, learned cognitive functions and long-term memories. Although we did not detect new neurons in the primate neocortex, we did find neurogenesis in the olfactory bulb and the hippocampus. These two brain structures serve as important model systems from which we can learn how to introduce new neurons into more resistant parts of the brain such as the neocortex and spinal cord.”

Rakic and his colleague, David Kornack now at the University of Rochester, used the most sophisticated methods for detection of new neurons available. “We used markers of DNA synthesis for cell-division and special neuronal and glial labels to identify the presence or absence of new neurons,” said Rakic. “We then used laser-based confocal microscopy to look closely at every new cell.”

Even though they found thousands of new cells in the cortex, Rakic said they were not neurons. They were mostly blood vessels and various types of glial cells that provide support for neurons.

“Now that we’ve demonstrated that there is no neurogenesis in the normal adult neocortex, some might question what this means for the future of research in neurodegenerative diseases and brain repair following brain trauma,” said Rakic. “One strategy is to find ways to replace damaged neurons, and this can be done by learning how to overcome the primate brain’s natural resistance for acquiring new neurons in adulthood. Future studies could also focus on understanding why neurogenesis in each brain structure ceases at the end of specific developmental milestones.”