Anatomy of a Growth Cone

Growth cones vary in shape and size, and can be found on the ends of axons and dendrites. The image shows and example of a “Filopodial” growth cone, made of polarised F-actin bundles (green spikes), and a “Lamellipodial” growth cone, made of cross-linked F-actin (green netting). The red fluorophore is marking tubulin which makes up microtubules in the axon. Both of these conformations are highly motile, allowing the growth cone to move quickly in response to guidance cues.

Image courtesy of Wikipedia under the creative commons license.

Typically growth cones have both filopodia and lamella structures. When growth cones are in culture, with no guidance cues (at rest), they are still very active structures. Tubulin from the axon microtubules is sporadically dragged into the filopodia. This is termed treadmilling.

Push and Pull Forces

Attraction of Growth Cones

When a growth cone comes into contact with an attractive cue, the growth cone does not turn – it reorganises. The treadmilling of f-actin in the filopodia slows, allowing the f-actin to accumulate and stabilise the filopodium in contact with the cue. This is further stabilised by microtubules joining onto the base of the filopodium. The rest of the growth cone re-organises to this new orientation, establishing a new direction of growth.

Evidence:

In culture, when a growth cone comes into contact with a bead soaked in an attractive cue, the reaction of the growth cone can be observed in real time. Tubulin is drawn into the filopodia with contact, where treadmilling has slowed. Reorganisation is then observed.

Repulsion of Growth Cones

When a growth cone comes into contact with a repulsive cue, there is a localised collapse of the filopodia in contact with the repulsive cue, which initiates collapse of other filopodia, reorganisation of the growth cone and a new growth direction.

Evidence:  

When retinal ganglion cells and sensory neurons are mixed in culture they will only fasiculate with their own kind. This is because the axons carry cues which are repulsive to other kinds of axon – leading to growth cone collapse.

Guidance Cues

Repulsive Guidance Cues

Growth cones require a permissive substrate on which to attach and grow on. Non-permissive factors are often membrane bound repellents which cause growth cones to collapse, or channel axons down specific routes.

Examples:

• Semaphorin I is a short range, membrane bound inhibitory guidance cue, which is non-permissive for axon growth. Semaphorin I is found on retinal axons, repelling other axon types as described above.
• Semaphorin 3A is a secreted, long-range, inhibitory guidance cue.
• Ephrins are another non-permissive, contact-repulsion factor. If an axon expresses the ephrin receptor, Eph, it will be repulsed. This creates a reciprocal pattern of expression of Eph’s and Ephrin’s which help to compartmentalise the embryo.
• BMP acts as a chemo-repellant, causing commissural sensory relay neurons to be repulsed away from the roof plate.

Attractive Guidance Cues

Permissive substrates promote axon attachment and growth, and are normally contact-attractants. Permissive substrates create pathways through the ECM that the axons can follow towards their target.

Examples:

• Laminin is a growth promoting ECM protein found in the optic nerve. It works in a concentration dependant manner, as it is only permissive for growth in a specific concentration range. If laminin receptors are blocked in a model organism, such as a grasshopper, axon growth is slowed.
• Netrin is a secreted chemo-attractant, which associates with the ECM and attracts commissural sensory relay neurons in the spinal cord towards the floor plate.