[1] In order to develop into a properly functioning nervous system, there must be an extremely high degree of accuracy in which cell the growth cone forms neural connections.
[1] Although the target cell selection must be highly accurate, the degree of specificity that the neural connectivity achieves varies based on the neuronal circuitry system.
[1] The target selection process of an axon to develop synaptic connections with specific cells can be broken down into multiple stages that are not necessarily confined to exact chronological order.
Sperry studied how the neurons in the visual systems of amphibians and goldfish form topographic maps in the brain, noting that if the optic nerve is crushed and allowed to regenerate, the axons will trace back the same patterns of connections.
Sperry hypothesized that the target cells carried "identification tags" that would guide the growing axon, which we now know as recognition molecules that bind the growth cone along a gradient.
An important family of adhesion molecules is constituted by the cadherins, whose different combination on targeting cells allow the traction and guidance of the forming axons.
For example, semaphorin is a substance with a repulsive effect that has been shown to have a fundamental role in layering between different somatosensory modalities in the spinal cord system.
[5] Different classes of adhesion molecules, like SynCAM, cadherins and neuroligins/neurexins play an important role in synapse stabilization and enable synaptic formation.