Axon-bound vesicles (green) caught in a plus-end trap help form a new growth cone. A minus-end trap keeps pinocytic vesicles (red) out of the way.

Microtubules set traps for vesicles in injured axons, according to Erez et al. (page 497). The traps are positioned to maximize membrane deposition into a new growth cone.

After an axon is severed, a new growth cone rapidly expands while the cell's overall surface area decreases. Erez et al. examined this membrane budgeting in severed Aplysia neurons. They found that vesicles traveling out from the cell body help swell a new growth cone ∼100 μm back from the cut site, perhaps to avoid the most heavily damaged membrane at the tip.

These membrane-donating vesicles are tugged along by a motor toward microtubule plus ends. Normally, nearly all plus ends point toward the end of the axon. But the group saw that a set of microtubules close to the new tip reoriented 180 degrees soon after injury. Their new arrangement created a plus-end trap, where axon-bound vesicles halted and a new growth cone emerged.

The exact mechanism that creates the trap is unclear, but proteins coming out from the cell body, perhaps kinesin or its cargo, were necessary.

A second trap—this one of apposing minus ends—formed between the plus-end trap and the very tip of the axon. This trap caught irrelevant, cell body–bound pinocytic vesicles, which are probably not competent for fusion with the plasma membrane.