Editing Axon (section)

====Extracellular signaling====
The extracellular signals that propagate through the [[extracellular matrix]] surrounding neurons play a prominent role in axonal development.<ref name="pmid17311006">{{cite journal | vauthors = Arimura N, Kaibuchi K | title = Neuronal polarity: from extracellular signals to intracellular mechanisms | journal = Nature Reviews. Neuroscience | volume = 8 | issue = 3 | pages = 194–205 | date = March 2007 | pmid = 17311006 | doi = 10.1038/nrn2056 | s2cid = 15556921 }}</ref> These signaling molecules include proteins, [[neurotrophic factors]], and extracellular matrix and adhesion molecules.
[[Netrin]] (also known as UNC-6) a secreted protein, functions in axon formation. When the [[UNC-5]] netrin receptor is mutated, several neurites are irregularly projected out of neurons and finally a single axon is extended anteriorly.<ref name="A">[[Neuroglia]] and [[pioneer neuron]]s express UNC-6 to provide global and local netrin cues for guiding migrations in [[Caenorhabditis elegans|''C. elegans'']]</ref><ref>{{cite journal | vauthors = Serafini T, Kennedy TE, Galko MJ, Mirzayan C, Jessell TM, Tessier-Lavigne M | title = The netrins define a family of axon outgrowth-promoting proteins homologous to C. elegans UNC-6 | journal = Cell | volume = 78 | issue = 3 | pages = 409–24 | date = August 1994 | pmid = 8062384 | doi = 10.1016/0092-8674(94)90420-0 | s2cid = 22666205 }}</ref><ref>{{cite journal | vauthors = Hong K, Hinck L, Nishiyama M, Poo MM, Tessier-Lavigne M, Stein E | title = A ligand-gated association between cytoplasmic domains of UNC5 and DCC family receptors converts netrin-induced growth cone attraction to repulsion | journal = Cell | volume = 97 | issue = 7 | pages = 927–41 | date = June 1999 | pmid = 10399920 | doi = 10.1016/S0092-8674(00)80804-1 | s2cid = 18043414 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Hedgecock EM, Culotti JG, Hall DH | title = The unc-5, unc-6, and unc-40 genes guide circumferential migrations of pioneer axons and mesodermal cells on the epidermis in C. elegans | journal = Neuron | volume = 4 | issue = 1 | pages = 61–85 | date = January 1990 | pmid = 2310575 | doi = 10.1016/0896-6273(90)90444-K | s2cid = 23974242 }}</ref> The neurotrophic factors{{Snd}}[[nerve growth factor]] (NGF), [[brain-derived neurotrophic factor]] (BDNF) and [[neurotrophin-3]] (NTF3) are also involved in axon development and bind to [[Trk receptor]]s.<ref>{{cite journal | vauthors = Huang EJ, Reichardt LF | s2cid = 10217268 | title = Trk receptors: roles in neuronal signal transduction | journal = Annual Review of Biochemistry | volume = 72 | pages = 609–42 | year = 2003 | pmid = 12676795 | doi = 10.1146/annurev.biochem.72.121801.161629 }}</ref>

The [[ganglioside]]-converting enzyme plasma membrane ganglioside [[sialidase]] (PMGS), which is involved in the activation of [[TrkA]] at the tip of neutrites, is required for the elongation of axons. PMGS asymmetrically distributes to the tip of the neurite that is destined to become the future axon.<ref name="pmid15834419">{{cite journal | vauthors = Da Silva JS, Hasegawa T, Miyagi T, Dotti CG, Abad-Rodriguez J | title = Asymmetric membrane ganglioside sialidase activity specifies axonal fate | journal = Nature Neuroscience | volume = 8 | issue = 5 | pages = 606–15 | date = May 2005 | pmid = 15834419 | doi = 10.1038/nn1442 | s2cid = 25227765 }}</ref>