Until recently, a commonly held view in neuroscience was that neurogenesis does not occur in the adult brain. However, this idea was overturned by the discovery that new neurons are generated in the subventricular zone (SVZ). These neurons have been shown to colonize several structures, including the olfactory bulb and the hippocampus, and the list of possible destinations continues to grow, as a report in the Proceedings of the National Academy of Sciences illustrates.

Bernier et al. used 5-bromodeoxyuridine (BrdU), a thymidine analogue that is incorporated into the DNA of dividing cells, to identify sites of neurogenesis in adult monkey brains. They detected BrdU-labelled cells in the amygdala and piriform cortex, and they used immunostaining techniques to confirm that these cells were neurons. In addition, they identified a stream of BrdU-labelled cells that stretched from the temporal horn of the lateral ventricle (tLV) down to the dorsal amygdala. By labelling the SVZ with the lipophilic dye DiI, and tracking cell migration from the tLV, the authors provided further evidence for the existence of this migratory stream, which they termed the temporal stream.

The amygdala is probably best known for its role in mediating the fear response, but it also acts as a relay centre for olfactory information, as does the piriform cortex. Therefore, Bernier et al. speculate that new neurons might be generated in these regions to complement the turnover of olfactory bulb neurons that project to them. Of course, the real test will be to show that the new neurons integrate functionally into the neuronal circuitry, and the next step will be to find out whether this is the case.