page 499, Lamoureux et al. show that any of the processes can become the axon when given the right stimulus. But this cue is not a hormone or growth factor. The stimulus that tells an axon to form is more physical than chemical.
What the neurite needs to become an axon is a good tug, which the authors applied by attaching a needle to the end of a process. In most cases, the processes that were pulled extended rapidly and expressed axonal markers. Thus, every process appears ready to form an axon, perhaps with stores of unassembled tubulin and vesicles ready to be secreted. Tension somehow elicits the rapid biochemical reactions needed to make an axon, including microtubule assembly and plasma membrane expansion, although how tension is transduced is unclear.
The experiments dispel theories suggesting that the first process to reach a critical length become the axon, as processes pulled and then released when still shorter than its neighbors continued to grow and mature into axons. Tension even elicited a second axon in cells that already had one. Thus, whatever factor prevents a second axon from forming is apparently overcome by tension. Perhaps tension, which in vivo is provided by the growth cone's tugging on the extracellular matrix, is tightly regulated in the developing organism by precisely placed growth factors. ▪