What is Axon Guidance?
The human brain is often compared to a computer, with its intricate wiring system connecting the 1011 neurons in the brain. Added to the neuronal connections in the rest of the body, these connections are over 10 times more numerous than the stars in our galaxy. Even the world’s most powerful computer seems simple in comparison! Not only are there millions of these connections in our body, they are highly organised and stereotyped. How are these connections made with such reproducibility?
This wiring system results in the incredible specificity of neuronal connections. It is achieved by the ability of axons to accurately navigate their way to their target during development. This navigation is directed by a specialised region on the leading tip of the axon called the growth cone. The growth cone is a highly motile and senses attractive and repulsive signals in its surrounding environment, guiding the axon to its correct position.
Axon guidance has been studied for well over a century, but it is Ramón y Cajal who first suggested that axons are guided to target cells by secreted signals. However, it is only recently that the molecular mechanisms behind this process have started to be understood.
Image of a fluorescently-labelled growth cone, extending from an axon. F-actin (red) microtubules (green). Courtesy of Wikipedia under the creative commons license.
Title image courtesy of Flickr under the creative commons license.
Did you know?
The growth cone was first discovered and described by Santiago Ramón y Cajal in 1890. He described the function of the growth cone in 1909 as “a sort of club or battering ram, endowed with exquisite chemical sensitivity, with rapid ameboid movements, and with certain impulsive force, thanks to which it is able to proceed forward and overcome obstacles met in its way, forcing cellular interstices until it arrives at its destination". His pioneering work had lead to him being recognised as one of the founders of modern developmental neurobiology.
Interesting Fact
The longest neuron in the adult human body is the sciatic nerve, which runs from the base of the spine to the foot - averaging a length of three feet. Whilst the developing axon will only have to navigate a fraction of this length, this is still quite a feat for a single cell!