It is often the quiet, supportive types who wield real power. Star-shaped brain cells called astrocytes, usually dismissed as support cells for the attention-grabbing neurons, now seem to control the growth of new neurons in adult brains. The find brings scientists closer to understanding the forces that control neuron growth in adults and could lead to ways to treat neurodegenerative diseases or spinal cord injuries.
Astrocytes reside throughout the nervous system, filling in the spaces between neurons, the "wires" that pass the system's messages. Astrocytes were thought to constitute protective scaffolding and a nutrition source that help keep neurons healthy. But recently astrocytes have begun to shed their inert image. In the last few years, scientists have found that the supposedly placid cells help neurons form connections with each other (ScienceNOW, 26 January 2001) and might even be stem cells themselves (ScienceNOW, 15 June 1999).
Now neuroscientists have evidence for another crucial role for astrocytes. While trying to determine whether stem cells from adult brains can become mature neurons, neuroscientists Charles Stevens, Hongjun Song, and Fred Gage of the Salk Institute in La Jolla, California, noticed something surprising: When they grew neuronal stem cells from adult rat brains together with astrocytes, the stem cells produced more neurons. At first they suspected that the astrocytes might be encouraging more stem cells to divide or preventing new neurons from dying. But further experiments revealed that cultures with astrocytes produced six times as many neurons as cultures without astrocytes, they report in the 2 May issue of Nature. The astrocytes, they argue, actively encourage the immature cells to become neurons.
Astrocytes from the hippocampus, one of the places new neurons are born in adults, encouraged this proliferation of neurons. Astrocytes from the spinal cord, in contrast, didn't have an effect. That observation "suggests a reason why only certain regions of the adult brain retain the ability to generate new neurons," says Ben Barres of Stanford University. And although he predicts it won't happen in his lifetime, Stevens says doctors may someday be able to use astrocytes or the signals they produce to encourage reluctant spinal cord neurons to regrow in injured patients.