Axon structure and key mechanisms of axon growth. The image shows a neuron which extends an axon into a zone of an attractive factor. Parallel bundles of MTs fill the axon and splay in the growth cone (plus end-out polarity indicated for some MTs by encircled + and - signs). F-actin networks are prominent in growth cones, and scarce but highly organized into evenly spaced rings in axons. Close-ups illustrate the following molecular mechanisms contributing to axon growth: (1) MT plus end-associated factors (for example, EB1, CLASP, type 13 kinesins, XMAP215; blue ellipses) regulate elongation and shrinkage of MTs (blue arrows); (3) molecular motors (orange Y structures) mediate cargo transport; (2,4) the MT severing protein katanin (scissors) generates MT fragments which are moved anterogradely through MT-sliding; (5) proteins that bind along MTs (for example, tau, MAP1B; brown L's) protect MTs from depolymerization or severing factors and organize MTs by cross-linking them and regulating their spacing; (6–8) F-actin networks influence MT behaviors through antagonizing MT advance into lamellipodia and filopodia via retrograde flow (red arrows), through forming structures that can be supportive (for example, radial bundles) or inhibitory (for example, transverse arcs), through MT cross-linkage (yellow stars), through contractile activity (not shown), or through F-actin clearance from protrusions (shown in 8). For detailed information see [3, 23].