Tuesday, January 28, 2014 CC-BY-NC
Axon Guidance part 2

Maintainer: admin

1Growth cone cues

1.1Extracellular Matrix Molecules: Lamina

  • major compoment of basal laminae
  • promote axon outgrwoth
  • binds to integrin on nerve cells .
  • integrins bind to cytoskeletal components like actinin and talin
  • guide axons to the optic nerve head (exit the retina)

1.2Adhesion molecules: CAM

  • CAM promotes neurite growth.
  • two types :
    • calcium-dependent cell adhesion molecules
    • members of immunoglobulin superfamily


  • temperal RGC axons project to rostral tectum
  • nasal RGC project to caudal tectum
  • signaling of Ephrin

    • forward: downstream of Ephs (receptors of ephrin)
    • reverse: downstream of ephrinA and B
  • ephrins A :

    • high in posterior, low in anterior tectum
    • anterior-posterior tectum mapping
    • GPI-anchored, so reverse signaling possibly through recruitment of receptors
  • ephrin B :

    • high in dorsal/medial and low in ventral/lateral tectum
    • transmembrane protein: reverse signaling via signaling proteins recruited to intracelluar domain
  • axon guidance

    • ephrin A is repellant
      • those in nasal retina express low ephA and target toward posterior tectum
    • ephrin B is attractant
      • neurons in dorsal retina express low ephB and targets toward ventral tectum


  • causes growth cone to collapse
  • sema5A: expressed along developing optic nerve and repels TGC axons
  • prevents TGC from wandering outside of optic nerve


  • expressed at the optic chiasm
  • mutation of Slit1 or slit2 leads to axonal pathfinding defects
    • ectopic chiasm, wandering axons

2Commissural Axon guidance

  • commissural neurons project ventrally toward the floor plate, cross and turn longitudinally


  • binds to proteoglycans, allowing cells to create gradiance
  • exerts trophic effect on the axons and guide it to cross the floor plate
  • aka unc-6 in worms
  • unc-5 receptor: promote growth away from the midline by binding with DCC during netrin signaling
  • unc-40: DCC in mammals, promote growth toward midline

3Intracellular signaling

  • siganling through rho : growth cone collapse , repulsion
    • semaphorin, ephrin A
  • signaling through rac or cdc42: promote growth cones, attraction
    • netrin, slit