New Neurons in Neocortex? New Study Says NO!

By Ellen Kuwana
Neuroscience for Kids Staff Writer
December 30, 2001

This month scientists backed away from previous conclusions about neurogenesis -- that is, growth of new nerve cells -- when they found that nerve cells do not grow in the neocortex of adult primates. The study, done by Drs. David Kornack and Pasko Rakic, used a combination of techniques to look at cells in the neocortex, the brain's specialized outer layer where complex functions such as planning, reasoning, and language take place. The "neo" in neocortex means "new," signifying that this area evolved later and is not as primitive as other cortical areas (such as the "older" hippocampus).

The researchers used molecular markers to examine and identify different cell types in the adult brain. One marker (BrdU) tagged new cells, a second marker tagged neurons (nerve cells), and the third marker tagged glial cells (glia). Glia are cells that support nerve cells.

In addition to the molecular tools, the researchers used confocal microscopy to examine thousands of cells to determine cell type. Microscopic examination was important because often a cell with two markers may lead the researchers to think it was a new cell. However, the special optics of the confocal microscope can help researchers tell the difference when there are two cells instead of one.

The scientists concluded that neurogenesis does occur in the adult primate hippocampus and olfactory bulbs, but found no evidence of new nerve cell growth in the neocortex. However, they did find evidence of new glial cells in the neocortex.

A Brief History of Neurogenesis Research

In 1998, Princeton University scientists and others announced that they had observed neurogenesis in the hippocampus and olfactory bulbs of the adult primate brain. The hippocampus, important for memory and learning is an "older" part of the cerebral cortex. The olfactory bulbs are important for the sense of smell. In 1999, the same group published work showing that there were areas of the neocortex in adult monkeys where new nerve cell growth occurred. Later, researchers at the Salk Institute reported on neurogenesis in the hippocampus of adult human brains.

This work in humans strengthened the conclusions from earlier work the earlier studies -- neurogenesis does occur in the adult brain. This was a surprising finding because for many years it was believed that new nerve cells do not grow in the adult human brain. It was thought that humans were born with all the brain cells that they will ever have. That new neurons can grow and develop, even within limited areas of the brain, was amazing news because if, indeed, nerve cells could be replaced, this information would be valuable for developing new therapies for disorders such as Alzheimer's disease , Parkinson's disease, and stroke .

The next logical challenge for scientists is to discover why and how neurogenesis occurs in the hippocampus and olfactory bulb. Then perhaps that knowledge can be harnessed to prompt new cell growth in the neocortex and other areas of the brain, which could lead to promising therapies for a variety of brain diseases. In the meantime, use what you have wisely -- it may be all you get.

References:

  1. No Evidence of New Neurons in Adult Primate Neocortex by Tom Rickey.
  2. Kornack, D. and Rakic, P. "Cell Proliferation Without Neurogenesis in Adult Primate Neocotex," Science, Vol. 294, December 7, 2001, pages 2127-2130.
  3. David Kornack's homepage.
  4. Neurogenesis - from Neuroscience for Kids


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