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Adults add almost no neurons
In rodents and some non-human primates, new neurons keep appearing in the olfactory bulb and in a part of the hippocampus called the dentate gyrus. For humans the evidence is more controversial. The olfactory bulb is the clear case: after a hard blow to the head shears the axons of smell neurons where they pass through a bony plate, the neurons can grow their connections back and new ones are added.
Rusty Gage's lab at the Salk Institute injected terminally ill cancer patients with a dye that is incorporated only into new neurons. Their brains were examined after death, and new neurons were there, but only very few cells. Several papers from labs at UCSF and elsewhere found that if the adult brain adds neurons at all, the number is infinitesimally small.
Adult change comes mainly from making existing synapses stronger and more reliable, and from removing others. Removal matters for working through grief or taking the emotional load off a traumatic experience. Early in life, space between neurons lets them move and sample connections. With age that space fills with extracellular matrix and glial cells, like pouring concrete between rocks.
Why do smell neurons regrow?
Neurons in the nose that detect odors are in direct contact with the outside world, so they are damaged regularly and replaced throughout life. They arise from stem cells in the nasal lining, and their axons grow back into the olfactory bulb. Researchers such as Richard Axel and Linda Buck mapped how the receptors work, work that earned the 2004 Nobel Prize in Physiology or Medicine. Mice and humans both show this turnover.
How did a dye reveal new neurons?
The dye is a compound called bromodeoxyuridine, or BrdU, which cells take up when they copy their DNA to divide. In a study published in 1998, Peter Eriksson and Fred Gage found BrdU-labeled neurons in the hippocampus of cancer patients who had received it for diagnosis. Neurons that retained the dye must have been born after the injection. The finding challenged the long-held belief that adult human neurons are never replaced.
What fills the space between neurons in adults?
Perineuronal nets are lattices of extracellular matrix that wrap around certain neurons as the critical periods of development end. They help hold synapses in place and restrict change. Experiments in rodents have found that breaking them down with an enzyme reopens plasticity in the visual cortex. Glial cells, including astrocytes and microglia, also shape which connections stay.
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