Newly born cells (green) in the cortex express interneuronal markers (red).

Neurogenesis in the adult brain is generally considered to be restricted to the hippocampus and olfactory bulb—ancient parts of the brain that are found even in nonmammalian species. A few publications suggested that the neocortex—a well-developed region in primates that is implicated in higher thought—also supports neurogenesis. These reports have been met with skepticism, as other groups have been unable to replicate the findings. On page 415, Dayer et al. bolster the evidence for proliferating neurons in regions beyond the hippocampus and olfactory bulb.The authors used a battery of neuronal markers to confirm that neurogenesis exists in the neocortex. Although difficult to find in the sparsely populated expanse of the cortex, replicating cells with markers of neuronal precursors were present in the adult rat neocortex. Their numbers suggest that these interneurons in the cortex turnover as rapidly as granule cells in the hippocampus, where up to 6% of the cells are replaced within a month.

The various markers that the authors used also revealed for the first time that the neocortical precursors produce GABAergic interneurons, which are small inhibitory neurons that regulate the larger pyramidal cells of the cortex. Because most neurons in the cortex are pyramidal cells, which are large and unmistakably neuronal, the new interneurons can be easily overlooked or mistaken for glial cells.

The precursors originate from within the cortex itself, rather than migrating from the subventricular zone (SVZ), which provides new neurons to the olfactory bulb. The authors did note, however, that some SVZ precursors also found their way to the striatum—a region associated with motor skill learning—where they formed interneurons.

Depression has been correlated with decreased hippocampal neurogenesis and a decrease in small cells in the neocortex that look like glia. The new findings hint that some of those missing cells may be newly born interneurons and suggest that neurogenesis in the cortex will be an important event to examine in disease states and old age.