It is now well established that certain regions of the brain, such as the subventricular zone and the hippocampus, continue to generate neurons throughout life, whereas others lose this ability as the brain matures. A new study by Song et al., reported in Nature, indicates that a star-shaped glial cell — the astrocyte — might hold the key to this variation.

Even parts of the brain that do not normally generate neurons often contain stem cells, and these cells can be made to differentiate into neurons if they are transplanted into neurogenic regions. So, the local tissue environment must be important in determining whether or not a region is neurogenic. To find out which components of the brain tissue are responsible for creating a neurogenic environment, Song et al. extracted stem cells from the adult rat hippocampus and tried to culture them on a feeder layer of either hippocampal astrocytes or neurons.

The cells that were grown with astrocytes gave rise to a high proportion of neurons, whereas those grown with neurons generated large numbers of oligodendrocytes. The effects of the astrocytes were twofold: they increased the rate of proliferation of the stem cells and they biased their specification towards a neuronal fate. Adult hippocampal astrocytes were less effective in promoting neurogenesis than those from the neonatal brain, perhaps reflecting the general age-related decline in the brain's ability to generate new neurons. The origin of the astrocytes was also important; for example, astrocytes from the adult spinal cord, which is non-neurogenic, could not induce hippocampal stem cells to differentiate into neurons.

For a long time, astrocytes were considered to have a rather passive supporting role in the central nervous system, but it seems that they are much more interesting than we previously imagined. Last year, it was shown that they participate in the formation and maintenance of synapses, and Song et al. now show that hippocampal astrocytes can promote neurogenesis from adult neural stem cells. In the developing brain, on the other hand, the onset of neurogenesis precedes gliogenesis, so it is likely that embryonic neural stem cells differentiate in response to different signals than their adult counterparts.

Of course, it remains to be shown that hippocampal astrocytes can stimulate neurogenesis in vivo, and it will also be interesting to find out whether this is a general property of astrocytes in neurogenic regions of the brain. If this proves to be the case, it raises the possibility that astrocytes from neurogenic regions could be used to stimulate the production of neurons in non-neurogenic regions, simply by mobilizing stem cells that are already present. This could have significant implications for brain repair, possibly leading to the development of new strategies that emphasize the importance of providing the right environment to promote neuronal differentiation.