Imagine your brain is a garden,” says Prof. Dr. Marco Prinz. That might seem far-fetched, but the neuropathologist speaks in such enthusiastic terms of his work that you want to understand what he is so excited about, so you envision the follow- ing: Shrubs and trees flourish in the fertile humus of your head, the fields are a deep green, and the flowers bloom in the most vibrant colors. The trees, says Prinz, are the nerve cells. And they need to be tended. Sometimes they grow uncontrollably in one direction, sometimes individual branches fall off, and sometimes the entire tree dies. Were one to simply allow things to take their course in the brain’s garden, there would soon be chaos. Preventing this from happening is the job of the scavenger cells, the macrophages, which are found everywhere in the human body. A part of the immune system, macrophages play an important role in all immune defense reactions, the fight against tumor cells, and the healing of wounds. In connective tissue they are called histiocytes, in the bones osteoclasts, and in the skin Langerhans cells. In the brain, where they are known as microglia, they take on the function of a gardener, trimming the branches of the nerve cell trees and chopping them down altogether when they die off. In the case of major damage, for instance after a fall or an accident, there is Microglia keep house in the brain – when damaged, however, they can cause severe diseases Tending the Nerve Cell Garden by Claudia Füßler 12