Recent research has revealed that brains continue to produce new neurons throughout life, helping create new neural networks. This neurogenesis only takes place in a few specific areas, such as the area in which the brain and spinal column meet. The new cells, however, can migrate throughout the brain and turn up as far away as the olfactory bulb--a cluster of nerve cells at the front surface of the brain responsible for the sense of smell. A recent study in mice has revealed that these neurons make the long and complicated journey by going with the flow of spinal fluid circulating in the brain.

Neurologist Kazunobu Sawamoto at Keio University in Japan and an international team of his colleagues used fluorescent dye and India ink to trace the flow of spinal fluid in mice and found that it followed the whiplike waving of hairlike projections known as cilia from cells lining the route. They then tracked neurons as they migrated from region to region of the brain and found that new neurons oriented in the direction of fluid flow rather than the direction of their ultimate destination in the olfactory bulb.

But this did not provide definitive proof of how neurons migrate through the brain. So the researchers turned to mutant mice with cells that lacked cilia. In addition to suffering from a host of other problems like an abnormally large accumulation of this spinal fluid in the brain, these mice developed new neurons that did not have a clear sense of direction, pointing and moving in multiple orientations. Whereas 65 percent of new neurons in wild mice ended up in the olfactory bulb, little more than 9 percent of the mutants' neurons were able to complete the journey.

The scientists also found that the cilia's motion was important for the amount and efficiency of proteins in the spinal fluid that usher the new neurons on their way by chemically repelling them from certain sites. The research appeared online yesterday in Science.