Brain's support cells, always on the go.

When the brain is injured or infected, cells called microglia leap into action. A new study indicates that these cells, which researchers previously had thought were quiescent in life's less-stressful moments, are constantly sweeping their surroundings for signs of danger.

Most microglia research has been confined to lab dishes, in which the cells are constantly moving, changing shape, and otherwise active. But scientists presumed that the activity was a result of the abnormal and stressful context of being in the lab instead of in the brain.

To determine how microglia behave inside live animals, Frank Kirchhoff of the Max Planck Institute for Experimental Medicine in Gottingen, Germany, and his colleagues studied mice whose microglia were genetically modified to glow green. After anesthetizing the mice, the researchers imaged the glowing cells for up to 10 hours using an innovative optical technique that didn't require surgically invading the brain. Kirchhoff's team found that during the cells' supposed resting period, microglia were constantly extending tiny appendages into their surrounding areas to clean out debris and, presumably, check for problems.

Kirchhoff says that these results, to be published in an upcoming Science, suggest that microglia perform a complete checkup on the brain several times a day.--c.B.