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As bundled axons finish navigating through various neural circuits during neural development, the growth cones must selectively target with which cells it will synapse. This can be particularly well observed in the visual and olfactory systems of organisms. 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. 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. 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. The stages of targeting include axonal defasciculation, target entry prior to axonal branching without exit from target, topographic region location, specific layer location, and target cell connection. Additionally, synaptic refinement and synaptic pruning of axon projections is sometimes included as the final stage, because synaptogenesis leads to the commencement of synaptic activity. This synaptic activity then allows organisms to eliminate poorly functioning or irregular connections as a form of postsynaptic processing and functional verification.

Defasciculation

As pioneer axons forge a path utilizing extrinsic cues and guidepost cells, follower axons fasciculate into axonal bundles. As fasciculated axons are guided along this common path, specific axons or groups of axons will defasciculate for target entry as various potential targets are passed. In the process of defasciculation, there is a deactivation of the homophilic adhesion molecule known as the neural cell adhesion molecule (NCAM) or its homologs. Should NCAM or its homologs not be downregulated, issues may arise regarding whether axons will defasciculate near their target correctly. Two additional ways to decrease fasciculation is to secrete an anti-adhesive factor, such as Beat-1a, from the growth cone and to post-translationally alter adhesion molecules prior to their insertion into the growth cone membranes.

The actual target during target selection can also play an important role in the defasciculation of axons that will eventually innervate. In target muscle, the immunoglobulin superfamily protein Sidestep signals for the defasciculation of motor neuron branches and then attracts those branches to the target muscle. Importantly, Sidestep is present and expressed in all muscles. In parallel with Sidestep, the matrix metalloprotease Tolloid related 1 also facilitates nerve branch defasciculation.

References

Sanes, Dan H.; Reh, Thomas A.; Harris, William A. (2012). Development of the Nervous System (3rd ed.). Burlington, MA: Academic Press. pp. 143–169.

Holt, Christine E; Harris, William A (1998-02-01). "Target selection: invasion, mapping and cell choice". Current Opinion in Neurobiology. 8 (1): 98–105. doi:10.1016/S0959-4388(98)80013-5. ISSN 0959-4388.

Raper, Jonathan; Mason, Carol (September 2010). "Cellular Strategies of Axonal Pathfinding". Cold Spring Harbor Perspectives in Biology. 2 (9). doi:10.1101/cshperspect.a001933. ISSN 1943-0264. PMC 2926747. .

Van Vactor, D. (February 1998). "Adhesion and signaling in axonal fasciculation". Current Opinion in Neurobiology. 8 (1): 80–86. doi:10.1016/s0959-4388(98)80011-1. ISSN 0959-4388. .

Kristiansen, Lars V.; Hortsch, Michael (2010), Berezin, Vladimir (ed.), "Fasciclin II: The NCAM Ortholog in Drosophila melanogaster", Structure and Function of the Neural Cell Adhesion Molecule NCAM, Advances in Experimental Medicine and Biology, Springer, pp. 387–401, doi:10.1007/978-1-4419-1170-4_24, ISBN 978-1-4419-1170-4, retrieved 2020-03-12

Meyer, Frauke; Aberle, Hermann (2006-10-15). "At the next stop sign turn right: the metalloprotease Tolloid-related 1 controls defasciculation of motor axons in Drosophila". Development. 133 (20): 4035–4044. doi:10.1242/dev.02580. ISSN 0950-1991. .