Editing Neuron (section)

==Development==
{{Main|Neurogenesis}}
Neurons develop through the process of [[neurogenesis]], in which [[neural stem cell]]s divide to produce [[Cellular differentiation|differentiated neurons]]. Once fully differentiated they are no longer capable of undergoing [[mitosis]]. Neurogenesis primarily occurs during [[embryonic development]].

Neurons initially develop from the [[neural tube]] in the embryo. The neural tube has three layers – a [[ventricular zone]], an [[Development of the cerebral cortex|intermediate zone]], and a marginal zone. The ventricular zone surrounds the tube's central canal and becomes the [[ependyma]]. Dividing cells of the ventricular zone form the intermediate zone which stretches to the outermost layer of the neural tube called the pial layer. The [[gray matter]] of the brain is derived from the intermediate zone. The extensions of the neurons in the intermediate zone make up the marginal zone when [[myelinated]] becomes the brain's [[white matter]].<ref name="Caire">{{cite journal |last1=Caire |first1=Michael J. |last2=Reddy |first2=Vamsi |last3=Varacallo |first3=Matthew |title=Physiology, Synapse |url=https://www.ncbi.nlm.nih.gov/books/NBK526047/ |website=StatPearls |publisher=StatPearls Publishing |access-date=10 July 2024 |date=2024|pmid=30252303 }}</ref>

Differentiation of the neurons is ordered by their size. Large [[motor neuron]]s are first. Smaller sensory neurons together with [[glial cell]] differentiate at [[birth]].<ref name="Caire"/>

[[Adult neurogenesis]] can occur and studies of the age of human neurons suggest that this process occurs only for a minority of cells and that the vast majority of neurons in the [[neocortex]] form before birth and persist without replacement. The extent to which adult neurogenesis exists in humans, and its contribution to cognition are controversial, with conflicting reports published in 2018.<ref>{{cite journal | vauthors = Kempermann G, Gage FH, Aigner L, Song H, Curtis MA, Thuret S, Kuhn HG, Jessberger S, Frankland PW, Cameron HA, Gould E, Hen R, Abrous DN, Toni N, Schinder AF, Zhao X, Lucassen PJ, Frisén J | title = Human Adult Neurogenesis: Evidence and Remaining Questions | journal = Cell Stem Cell | volume = 23 | issue = 1 | pages = 25–30 | date = July 2018 | pmid = 29681514 | pmc = 6035081 | doi = 10.1016/j.stem.2018.04.004 }}</ref>

The body contains a variety of stem cell types that can differentiate into neurons. Researchers found a way to transform human skin cells into nerve cells using [[transdifferentiation]], in which "cells are forced to adopt new identities".<ref name=twsX33>{{Cite journal |doi=10.1038/news.2011.328 | last = Callaway | first = Ewen |title= How to make a human neuron | journal = Nature |quote= By transforming cells from human skin into working nerve cells, researchers may have come up with a model for nervous-system diseases and perhaps even regenerative therapies based on cell transplants. The achievement, reported online today in ''Nature'', is the latest in a fast-moving field called transdifferentiation, in which cells are forced to adopt new identities. In the past year, researchers have converted connective tissue cells found in the skin into heart cells, blood cells, and liver cells.
 |date= 26 May 2011 }}</ref>

During [[neurogenesis]] in the mammalian brain, progenitor and stem cells progress from proliferative divisions to differentiative divisions. This progression leads to the neurons and glia that populate cortical layers. [[Epigenetics|Epigenetic]] modifications play a key role in regulating [[gene expression]] in differentiating [[neural stem cells]], and are critical for cell fate determination in the developing and adult mammalian brain. Epigenetic modifications include [[DNA methylation|DNA cytosine methylation]] to form [[5-methylcytosine]] and [[DNA demethylation|5-methylcytosine demethylation]].<ref name=Wang2016>{{cite journal | vauthors = Wang Z, Tang B, He Y, Jin P | title = DNA methylation dynamics in neurogenesis | journal = Epigenomics | volume = 8 | issue = 3 | pages = 401–14 | date = March 2016 | pmid = 26950681 | pmc = 4864063 | doi = 10.2217/epi.15.119 }}</ref>  [[DNA methylation|DNA cytosine methylation]] is catalyzed by [[DNA methyltransferase|DNA methyltransferases (DNMTs)]]. Methylcytosine demethylation is catalyzed in several stages by [[TET enzymes]] that carry out oxidative reactions (e.g. [[5-methylcytosine]] to [[5-hydroxymethylcytosine]]) and enzymes of the DNA [[base excision repair]] (BER) pathway.<ref name=Wang2016/>

At different stages of mammalian nervous system development, two DNA repair processes are employed in the repair of DNA double-strand breaks. These pathways are [[homologous recombination]]al repair used in proliferating neural precursor cells, and [[non-homologous end joining]] used mainly at later developmental stages<ref>{{cite journal | vauthors = Orii KE, Lee Y, Kondo N, McKinnon PJ | title = Selective utilization of nonhomologous end-joining and homologous recombination DNA repair pathways during nervous system development | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 26 | pages = 10017–22 | date = June 2006 | pmid = 16777961 | pmc = 1502498 | doi = 10.1073/pnas.0602436103 | bibcode = 2006PNAS..10310017O | doi-access = free }}</ref>

Intercellular communication between developing neurons and [[microglia]] is also indispensable for proper neurogenesis and brain development.<ref>{{cite journal |last1=Cserép |first1=Csaba |last2=Schwarcz |first2=Anett D. |last3=Pósfai |first3=Balázs |last4=László |first4=Zsófia I. |last5=Kellermayer |first5=Anna |last6=Környei |first6=Zsuzsanna |last7=Kisfali |first7=Máté |last8=Nyerges |first8=Miklós |last9=Lele |first9=Zsolt |last10=Katona |first10=István |title=Microglial control of neuronal development via somatic purinergic junctions |journal=Cell Reports |date=September 2022 |volume=40 |issue=12 |pages=111369 |doi=10.1016/j.celrep.2022.111369|pmid=36130488 |pmc=9513806 |s2cid=252416407 }}</ref>