© Mehau Kulyk/Photo Researchers, Inc.
A typical neuron's axons and dendrites, when loaded with dye and magnified, resemble long, untended tresses on an extremely bad hair day. They extend wildly, usually to one side, and then bend at weird angles as their ends split into branches and sub-branches.
This neuronal coiffure must appear even more chaotic before the nervous system has undergone the developmental equivalent of a crew cut crossed with a topiary trimming. From the late embryonic to early postnatal stage, this pruning process drastically thins out the branches in many axonal and dendritic arbors. Long neuronal offshoots that grew to inappropriate targets simply vanish.
Pruning occurs in probably all vertebrates and in many lower animals; neuroscientists have been aware of it for decades. Nevertheless, this phenomenon has garnered much less attention than that paid to other forces and events shaping the nervous system, such as axon guidance and...