Control of Axon Selection


The term polarity is used to define asymmetry. In cell biology, it refers to describe the shape of a cell, the distribution of organelles, molecules and thus the intracellular trafficking of cellular components. How this asymmetry is attained in neurons is a question that has been under scrutiny as the shape of a neuron provides valuable clues to its function: Mature neurons extend dendrites and an axon to receive, process and propagate signals. A longā€standing question in neurobiology is how neurons decide where the axon would form. One possibility is that external cues determine the position of axon extension. However, it is also possible that the intracellular organisation of the neuron determines where the axon will grow. Existing data provide evidence for both mechanisms playing a role during the axon selection, either concomitantly and/or sequentially.

Key Concepts

  • Neurons are highly polarised cells with generally one axon and several dendrites.
  • Neurons developing in vitro form axons and dendrites, which resembles the in vivo situation.
  • Axon selection is the result of synchronous and/or sequential effects of intrinsic and extrinsic cues.

Keywords: neuronal polarity; axon selection; axon formation; centrosome; Golgi apparatus; neuronal development

Figure 1. Axon selection is predicted. (a) Time‐lapse analysis of a recently plated neuron with the first deformation of the neuronal sphere marked by an asterisk. (b) From the place of the first sphere breakage the axon (arrows) elongates 2 days after plating. (c) The axonal identity of this neurite is confirmed with Tau‐1 immunostaining. Scale bar 10 µm. (modified from de Anda, F.C. and L.H. Tsai ().
Figure 2. The first protrusion in developing neurons in vitro contains polarised growth information. (a) Neurons with one protrusion (stage 1) treated with cytochalasin D for 24 h develop a polarised phenotype with preferentially one Tau‐1 positive neurite. (b) Neurons with several neurites (stage 2) treated with cytochalasin D for 24 h develop several long processes positive for Tau‐1. Scale bar 10 µm.
Figure 3. Axon selection in situ. (a) Sensory neurons from Drosophila define polarity by a cluster of N‐cadherin (adhesion complex) and furrow markers from the last mitosis of the neuroblast (pIIIb cell). The N‐cadherin cluster recruits the centrosome and triggers the initial outgrowth that leads to the formation of the dendrite. The axon forms opposite to the dendrite (modified from Pollarolo et al., ). (b) In the developing mammalian cortex, axon formation mainly occurs in multipolar cells located in the IZ. At this multipolar stage, the centrosome and the Golgi apparatus rotate towards the neurite that eventually grows as an axon. Finally, the multipolar cell becomes bipolar with the centrosome and the Golgi at the base of the leading process or future apical dendrite (opposite to the axon) and migrates towards the CP where it will form dendrites. Modified from de Anda, F.C., et al. () © Society of Neurosciece.


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Further Reading

Bradke F , Fawcett JW and Spira ME (2012) Assembly of a new growth cone after axotomy: the precursor to axon regeneration. Nature Reviews Neuroscience 13: 183–193.

Caceres A , Ye B and Dotti CG (2012) Neuronal polarity: demarcation, growth and commitment. Current Opinion in Cell Biology 24: 547–553.

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Richter, Melanie, and de Anda, Froylan Calderon(Feb 2015) Control of Axon Selection. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0025967]