Astrocytes and Neurogenesis

By Melissa Lee Phillips Neuroscience for Kids Consultant May 21, 2002 Nerve cells (neurons) and glial cells (glia) are the two major types of cells present in the nervous system. For many years, glial cells were thought to only support the activities of neurons. Researchers thought that astrocytes -- glial cells named for their star-like appearance -- had only a supportive role in the central nervous system. It is well established that astrocytes help form a network on which neurons grow. Astrocytes fill in the space between neurons, take up neurotransmitters that are released by neurons, and help maintain the correct chemical concentrations around neurons.

Astrocyte Image courtesy of Biodidac

Within the past few years, however, attention has focused on the role of astrocytes in adult neurogenesis. Neurogenesis, the growth of new neurons from neural stem cells, has recently been shown to occur in adult mammals. In adults -- including humans -- neurogenesis has been observed in two specific areas: the subventricular zone (the area immediately surrounding the brain's ventricles) and the hippocampus. A study by researchers at the Salk Institute in California shows that astrocytes play a more important and active role in neurogenesis than previously thought. The researchers took stem cells from the hippocampus of adult rats and engineered these cells so that they would express a green fluorescent protein (GFP). The GFP makes it easy to see these cells and all of the cells' offspring...it's a bit like making the cells wear a glow-in-the-dark shirt. The stem cells were cultured with astrocytes from the hippocampus of newborn rats. The scientists found that neurogenesis was about eight times higher in cells that were cultured with astrocytes compared to those that were cultured without astrocytes. This increase in neurogenesis is consistent with previous studies. In those studies, however, it was unclear whether the astrocytes simply provided an environment that encouraged the survival and division of neural stem cells or if the astrocytes actually caused cells to become neurons. In this experiment, the researchers found that not only were more stem cells growing and maturing, but that the cells actually became neurons (instead of glia) about six times as often when they were in an astrocyte-enriched environment. Research involving stem cells is promising because it has the potential to develop new treatments for neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. If scientists can engineer stem cells to grow into neurons -- perhaps involving signals from astrocytes -- they might be able to repair or even prevent damage caused by these diseases.