Growth Cones

 What is a growth cone?

Growth cones were discovered in 1980 by Ramon y Cajal, a Spanish neuroanatomist. After studying the tips of axons in fixed embryonic material, he described the growth cone as “a concentration of protoplasm of conical form, endowed with amoeboid movements”.

Figure 7: Ramon y Cajal (1 May 1852 – 17 October 1934) looking through a microscope. See source.

 As Cajal said, a growth cone is a specialised distal tip of a growing axon that is responsible for axon growth and guidance. It is a dynamic mechanism that changes the direction and speed of an axon. 

It does this by sending out long cytoplasmic projections called filopodia which constantly probe the environment, and these are joined by thin fan-shaped sheets called lamellipodia that undulate in rhythmic waves (see 'Anatomy of a Growth Cone'). 

Rapid retraction of these processes enables the growth cones to move. This process was observed in vivo by Spiedel who published a paper in 1941. He used a transparent growing tail fin of a tadpole to observe the ability of growing nerve fibers to branch, change direction and respond to mechanical and chemical stimulation.

 Figure 8: A growth cone of an axon. See source.