Axon guidance 1

Axon guidance 1

1. How does connectivity arise?

   • 2 hypotheses
     • Chemoaffinity hypothesis
       • Sperry
         • 1939
       • Directed and specific outgrowth occurs through axons following 'individual identification tags' created by the cells and fibres of the embryo
     • Resonance theory
       • Stochastic and diffuse neuronal outgrowth occurs to all targets followed by elimination of non-functional connections
       • Weiss
         • 1928
     • Who was right?
       • Sperrys frog experiment
         • Cut optic nerve
           • Remove temporal retina
             • Allows just nasal axons to grow back
         • Grew back directly to the right place
           • Importantly, the regrowing 'red' axons grew through- yet ignored the territory normally innervated by the 'blue' axons
       • Axon pathways in embryos are highly stereotyped
         • If weiss were right
           • Would expect to see random patterns of outgrowth in embryos
         • Actually
           • Patterns of axon outgrowth are highly organised, reproducible and stereotyped
       • Motor axons
         • Guided specifically to targets in the chick embryo

2. Guidance cues

   • Where are they?
     • Early experiments
       • Done in insects
         • Relatively simple nervous systems
         • Embryos easy to observe and manipulate
         • In the larger insects (e.g. grasshopper) individual cells could be ablated with lasers
       • Found that pathways taken by axons are stereotyped
         • From embryo-embryo
         • From segment-segment
       • In grasshopper - map of axon projections made
         • Reproducibility of axon behaviours suggested that growth cones were responding to cues or labels
         • Pathways seem to change when specific axons are encountered
           • Tested by ablating cells that might carry potential cues
   • Labelled pathway hypothesis
     • Axons can selectively fasiculate with other axons
     • Axon surfaces carry labels or cues
       • Different axon growth cones express different sets of receptors for such cues
     • Axon scaffolds
       • Early axons form a scaffold on which later axons can extend
       • Establishes axon surfaces as potential source of guidance cues
       • Axon scaffolds are also Important in vertebrates
     • Stereotyped pathways in grasshopper embryo
       • Reproducibility of axon behaviour suggested that growth cones were responding to cues or labels in the environment.
       • Pathways seem to change when specific axons were encountered
       • Tested by ablating cells that might carry potential cues:
         • G axon “stalls” in absence of P axon
           • Not due to lack of axons on which to extend (e.g. A axon still present, C axon still extends)
           • Not due to reduction in numbers of axons (e.g. not affected when A axons ablated)
           • Must be because the G axon growth cone is ‘looking’ for specific cues on the P axon.

3. Pioneer axons

   • Axons follow stereotyped paths
     • Patterning information from early embryo predicts where axon tracts will form
       • Forebrain
         • Axons follow boundries of domains of patterning gene expression
       • Spinal cord
         • Axons follow boundries of floor plate
       • Hindbrain
         • Axons follow boundries of rhombomeres
       • Take home message is that axon guidance cues are located, not just in other axons, but in many cell types in the early embryo
     • E.g. In grasshopper embryo limb
       • pioneer Ti1 (tibial1) growth cone makes a specific turn at the limb boundary, and then again as it approaches a specific cell, Cx1.
         • Ablation of Cx1 causes the Ti1 growth cone to stall at the other side of the limb boundary.
           • Neither Cx1 nor the limb boundary cells are distinguished by any obvious morphological features.
           • Cx1 and the other cells are sometimes referred to as ‘stepping stones’ or ‘guidepost cells’
           • Implies that there must be molecular differences in the environment
     • Despite this apparently featureless environment, the pathways of pioneer axons are also stereotyped and growth cones appear to react at specific points in the pathway.

4. Four forces of axon guidance

   • Chemoattraction
   • Chemorepulsion
   • Contact attraction
   • Contact repulsion
   • The experiments we have seen suggest that axon guidance cues may be both positive and negative e.g.
     • Cell ablations lead to growth cones stalls, as if an attractive force has been lost
     • Ti1 growth cone seems to avoid the limb boundary as if it were inhibitory

Images and files in the map