The growth cone was found to have two major compartments. The central domain of the growth cone contains microtubules that extend the axon shaft and support mitochondria and other organelles (see Figure 15.23C). The peripheral domain contains two types of actin-associated membrane protrusions: the lamellipodia, broad membranous sheets containing short, branched actin networks, that act as the migratory network of the growth cone; and the filopodia, membranes extended by long bundles of filamentous actin, that act as the sensory network. A transition zone between the central and peripheral regions may coordinate actin and tubulin growth (Rodriguez et al. 2003; Lowery and Van Vactor 2009). Lone “pioneer” microtubules from the central zone’s core of microtubule bundles protrude through the actin arc that defines the transition zone and extend into the periphery of the growth cone. These pioneer microtubules dynamically associate with the actin filaments, and together the microtubules and actin filaments grow and shorten to perform the fingerlike movements characteristic of filopodia (Mitchison and Kirschner 1988; Sabry et al. 1991; Tanaka and Kirschner 1991, 1995; Schaefer et al. 2002). The microtubule- and actin-based membrane protrusions, coupled with selective adhesion and membrane recycling, provide the force that drives axon movement and directionality.
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