Figure 8.

Model: CLIPs enable microtubule engorgement of the growth cone during axon formation. A, Axon elongation consists of three repeating steps. First, filopodia and lamellipodia enable the growth cone to extend distally (Protrusion). Then, dynamic microtubules engorge from the central growth cone area into the former transitional zone, where actin filaments disassemble and the remaining arcs reorient toward the growth axis (Engorgement). Protruding microtubules transport vesicles and organelles into the engorged area. The extension of the neurite consolidates by the bundling of microtubules in the central domain; the proximal part of the growth cone resumes cylindrical shape and becomes part of the axonal shaft (Consolidation). B, Nonfunctional CLIPs lead to a decrease in microtubule stability. Hence, while filopodia and lamellipodia still protrude, microtubules fail to subsequently engorge into the actin rich area. As a result, the growth cone area increases, but the axon fails to grow.