For the human brain, which contains some 100 billion neurons, migration is a remarkable process that relies on chemical communication between many different cells. In general, the first neurons to migrate form interior brain structures. They are followed by waves of other neurons that travel farther outward. During the early 1970s, researchers examining the developing brains of monkeys discovered that neurons often clung to long fibers of cells called glia. Neurons use these glial fibers, which radiate from the brain's inner to outer surfaces, as a highway to carry them through the brain to their destination.
Some neurons also use the axons of other nerves to migrate from one brain area to another.
Scientists found special molecules on the surface of neurons called adhesion molecules. These substances bind to similar molecules on nearby glial cells or nerve axons. Scientists believe these and other molecules may act as chemical labels telling neurons which paths to follow. Learn how neurons migrate normally. Describe the development of neuron migration disorders, including some types of epilepsy and mental retardation.