Microtubule-based filopodium-like protrusions form after axotomy.

The growth cone at the front of a growing neurite often has F-actin-rich structures--digitate filopodia and sheet-like veils and lamellipodia--whose protrusion advances the leading edge. Microtubules and other cytoplasmic constituents later fill the protruded area, transforming it into new neuritic length. Growth can be initiated from an axon by transecting it. We have used video-enhanced contrast-differential interference contrast microscopy to observe the early events following transection of Aplysia axons in culture. Many filopodium-like protrusions (FLPs) grew rapidly (average instantaneous velocity of 1.6 microns/sec) from the sides and end of the axon stump within minutes of transection. Some of these displayed bidirectional transport of swellings, at a rate similar to fast axonal transport. Dihydrocytochalasin B, which blocks actin polymerization, only halved the number of FLPs that formed within 10 min of transection, and actually increased the number of transporting FLPs. Nocodazole, a microtubule-specific drug, also halved the number of FLPs, but none of them displayed transport of swellings. No FLPs formed in the presence of both drugs. In transected axons that had not been exposed to either drug, removal of the plasma membrane revealed fibers in many of the FLPs; immunofluorescence showed these fibers to be microtubules. Thus, a substantial number of the FLPs that form soon after axotomy are microtubule based, rather than actin based, underscoring the potential of microtubules to drive the rapid extension of neuritic precursors.