Axon growth and the factors that affect it play a significant role in determining the circuitry of the brain. Molecular gradients and concentrations, as well as firing patterns, help determine the direction for axonal growth and where that axon forms its synapses. A significant effort underway within the scientific community is aimed at cataloging and creating a comprehensive overview of all of the factors that determine axon growth. The process of axon growth and the associated synapse formation control which cells in the brain communicate directly with one another and how information is processed.
Semaphorins and plexins are classes of molecules expressed in the brain. They can be expressed as either ligands on cell surfaces or as free-floating, released molecules, forming molecular density gradients within the brain. At certain concentrations they have the ability to attract axons of particular cell types to grow toward them and at other concentrations they have the ability to act as repellants. This allows them to function as guides during axon growth phases, but to avoid simply pulling all local axons to grow toward the cell releasing the molecular guidance molecules.
There are still further axon growth associations expressed in the adage, "cells that fire together wire together." That is to say, cells of certain class types or cells that process a certain class of information will have a determined probability of forming synapses with other cells in neural circuitry motifs. This phenomenon creates repeated circuitry patterns within the brain, which scientists can use to help understand how the brain processes information.
Very early in cellular development, a neuron has many undifferentiated neurites. These cellular somatic processes remain undetermined until certain circumstances occur, like contact with other nearby cells or exposure to certain molecules or growth factors that cause differentiation, where one neurite becomes an axon and the remaining neurites on the cell soma develop as dendrites. When this happens, the neurite that becomes the axon begins to elongate and develop axon-like characteristics. These characteristics will include a lack of dendritic spines, a thinner appearance than the other neurites, and common terminal arborizations.
During brain development, axons also have a tendency to grow in directions that are later pruned away as the organism matures. This is considered an evolutionary throwback, as these inputs may once have been used but no longer are, much in the same way a human fetus develops a tail but that characteristic soon disappears. While it might seem redundant to have axon growth develop only to be consistently pruned back, one must consider the many factors that have determined the way neural circuitry has formed. A little seeming redundancy in development might have its own uses that are currently beyond scientific comprehension.