Axon Growth

Abstract

During embryogenesis, the construction of the nervous system requires that nerve cells connect very precisely to each other and to peripheral tissues. Establishment of these connections involves the growth and extension of long thin processes, known as axons, from the main body of the cell. The tip of this growing axon displays a highly specialised and dynamic structure, called the growth cone. Growth cone sense information presented to it in the embryonic environment and respond by steering axon growth towards the correct targets.

Key Concepts:

  • The growth cones of growing nerves control axon outgrowth and direction.

  • Growth cones sense a variety of signals in the environment that initiate a variety of intracellular signalling events.

  • Growth signals act through the Rhoā€GTPase signalling pathways to regulate microtublule dynamics that in turn influences the actinomyosin cytoskeleton.

  • Once development is complete, axon growth may continue in discrete neurogenic regions of the brain and also following nerve injury.

  • Intracellular second messengers such as calcium are important in mediating the response of growth cones to attractive and repulsive cues.

Keywords: growth cone; guidance cues; nerve axon; development; microtubules; actin polymerisation

Figure 1.

Stylised representation of a normally highly dynamic growth cone with its major structural features indicated. The organisation of key cytoskeletal components is also indicated. Note that the peripheral region (dark green) has more actin filaments than the axon or central region (light green). Whereas most microtubules (red) are located within the axon and splay out into the central region, some do become stabilised within active filopodia. Growth would be towards the top.

Figure 2.

Representation of actin dynamics in the leading edge of a growth cone. Actin filaments (green) extend into the proximal edge of the advancing growth cone. Actin monomers (yellow) are assembled into filaments at the plus end (dark green) and disassembled at the minus end. The activity of myosin (blue) causes contraction of the actin filaments and the balance between continual polymerisation and depolymerisation results in translocation of incorporated actin monomers rearwards. Continued cycles of retrograde flow (or treadmilling) therefore drive the forward motion of the growth across a substratum of ECM proteins and guidance cues (orange), which are recognised by specific receptors in the growth cone (brown). The whole adhesion complex is linked through intracellular components to the cytoskeleton (dark brown). Note that the peripheral domain also contains some microtubules (red) that are linked to actin (purple), but these do not extend as far proximally as actin filaments.

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Further Reading

Lowery LA and Van Vactor D (2009) The trip of the tip: understanding the growth cone machinery. Nature Reviews. Molecular Cell Biology 10: 332–343.

Sanes DH, Reh TA and Harris WA (2005) Development of the Nervous System, 2nd edn. Burlington, MA: Academic Press.

Squire LR, Berg D, Bloom F, du Lac S and Ghosh A (2008) Fundamental Neuroscience, 3rd edn. San Diego, CA: Academic Press.

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How to Cite close
Toro, Carla, Sousa, Catia, and Tannahill, David(Dec 2010) Axon Growth. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000798.pub2]