Axon guidance

Axon guidance is a process in embryological brain development.

Axonal projections
After neurons have formed from the neuroectoderm's semiconservatively dividing neural precursor stem cells, they project axons that must find reciprocating and correct dendritic targets to form electrical synapses in the mature brain. Neurons must also form connections with the spinal cord to form the spinal nerve pathways, and often, axons, 20 microns in diameter, must travel 5 meters to synapse with spinal cord dendrite targets to form the appropriate motosensory pathways.

Growth cones
Soma project "growth cones", cytoplasmic filopodia that project outwards from the soma and "feel" the surrounding microenvironment's chemical signals to determine where axonal projections should extend.

Cellular adhesion markers
Cellular adhesion molecules are markers left behind by axons that have previous transversed an area that promote subsequent and further axonal growth in their footsteps.
 * Cadherins: A family of chemicals left by previous axons. Cadherins bind to cadherins found on the growth cone surface and in a Ca+2-mediated response, promote growth.
 * CAMs (Ca+2-independent cellular adhesion molecules): A family of chemicals that are generic growth molecules common to many developing pathways.
 * Ng-CAM: Promotes axonal growth along astrocytes and Schwann cells.
 * N-CAM: Promote axonal bundling.

Extracellular chemicals
Extracellular chemicals are chemicals mounted on the extracellular matricies or in the extracellular fluid that were secreted by the cells propulating a microenvironment that either promote or discourage axonal growth in that area.


 * Laminin, fibronectin: Two chemicals mounted on the extracellular matricies. Growth cones adhere to these, which sets of an intracellular cascade that promotes axon extension towards this direction.
 * Netrins: A family of chemicals found on the base plate of the developing neural tube and commisural neurons. These tremendously promote axonal growth towards them.
 * IN-1: A chemical secreted by oligodencrocytes and mounted on an extracellular matrix or plasma membrane that discourages growth towards it (chemorepellant).
 * Semaphorins: A family of chemorepellants, including collapsin, which causes nearby growth cones to collapse and prevent grwoth.