How does an axon grow?

How do axons grow during development, and why do they fail to regrow when injured? In the complicated mesh of our nervous system, the axon is the information superhighway, carrying all of the data we use to sense our environment and carry out behaviors. To wire up our nervous system properly, neurons must elongate their axons during development to reach their targets. This is no simple task, however. The complex morphology of axons and dendrites puts neurons among the most intricate and beautiful cells in the body. Knowledge of how neurons extend axons and dendrites, elongate at a particular rate, and stop growing at the proper time is critical to understanding the development of our nervous system, yet the regulation of these processes is poorly understood. Considerable attention in recent years has focused on understanding how growth cones are guided and steered through complicated pathways (for review, see Schmidt and Hall 1998; Mueller 1999), and even how neurons initiate new processes (for review, see Da Silva and Dotti 2002), but what mechanisms are involved in elongation itself? Are environmental signals needed to drive axon growth and, if so, what are the intracellular molecular mechanisms by which they induce elongation? What controls the rate of axon growth? How do CNS neurons know whether to extend axons or dendrites? Is the signaling of axon versus dendrite growth attributable to different extracellular signals, different neuronal states, or both? All of these questions bear critically on our understanding of axon growth and here I review recent answers and approaches to the above questions, and highlight their relevance during development, injury, and disease. How does an axon grow? Axons are generally on the scale of 1 µm thick and some are more than 1 m long, making neurons the largest cells in the body by both volume and surface area. Sustaining such massive growth, however, requires the interplay of many factors—producing membrane and cytoplasmic elements in the proper proportions; shipping these building blocks to the right compartment and inserting them into the growing axon; and coordinating the timing of all of these with the growth cone’s moment-by-moment de