6.2 The growing axon: growth cones
The growth cone is a small area of active tissue at the tip of a growing axon (Figure 15). As the growth cone moves forward, it adds new material to the cell membrane and so extends the axon. (New axonal membrane is also added at other points along the axon, though to a much lesser extent.)
The growth cone moves forward through the extracellular matrix, as a consequence both of being pushed and of being pulled. The growth cone is pushed by the synthesis of microtubules and the arrival at the growth cone of material transported along the axon from the cell body. The growth cone is pulled by its own thin membrane extensions, filopodia (singular filopodium). The filopodia extend in various directions from the axon tip and adhere to the surroundings, such as extracellular matrix material, other axons or other cells. Within each filopodium, actin microfilaments are synthesised. These microfilaments are able to contract in a way similar to muscle (actin is one of the components of muscle cells). As the microfilaments contract, so the filopodia contract, pulling the rest of the growth cone along. The rate of advance of the growth cone is about 10–40 μm per hour.
Cytochalasin is a drug that disrupts the formation of actin microfilaments. Cytochalasin also disrupts the formation of filopodia.
What would you expect to be the effect of treating the growth cone with cytochalasin?
Movement of the growth cone would be impaired.
The filopodia are only part of the mechanism by which axons grow, so you would expect disrupting filopodia to slow growth, but not to stop it.
What is the other mechanism by which growth cones move forward, by which axons grow?
The other mechanism is microtubule formation.
Several studies have shown that growing axons treated with cytochalasin meander off course and lose their way.
What does this result suggest about the function of filopodia?
This result suggests that filopodia are important for guiding the growth cone.
One important component in the guidance of growth cones is adhesion; growth cones stick to greater or lesser extents to the surface over which they are growing, their substrate. This adhesion can be illustrated in tissue culture. In tissue culture neurons will extend axon-like processes (called neurites, because in tissue culture axons cannot be distinguished from dendrites) with growth cones at their leading edge. The growth cones adhere strongly to some substrates, e.g. laminin, and to some cells, e.g. guide post cells. The effect of differences in adhesion as a growth cone encounters a guide post cell is shown in Figure 16.
It takes just one filopodium to contact the guide post cell for the whole growth cone to change its direction of growth. The growth cone is continuously pulled in various directions by its filopodia. The amount of pull exerted by a particular filopodium will depend to some extent on its size, i.e. how many actin microfilaments it contains, but mostly on its adhesion to the substrate. The filopodium that sticks best to the substrate will pull the growth cone behind it.
What implication does this have for axon navigation?
It implies that the growth cone, and its trailing axon, will follow the path of greater adhesion.
For example, in tissue culture, retinal neurites will grow on two types of extracellular matrix molecules, laminin and fibronectin. (Extracellular matrix molecules are molecules secreted by cells into the surrounding spaces and so are not attached to cells. They also tend not to diffuse very far.) If given the choice, by growing on a surface with alternating fibronectin and laminin stripes, the retinal neurites will grow along the laminin stripes, and not the fibronectin stripes.