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Cerebral cortex
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====Radial glia==== [[File:Neurogenesis and Differentiation of Cortical Layers.jpg|thumb|Neurogenesis is shown in red and lamination is shown in blue. Adapted from (Sur et al. 2001)]] The cerebral cortex is composed of a heterogenous population of cells that give rise to different cell types. The majority of these cells are derived from [[Radial glial cell|radial glia]] migration that form the different cell types of the neocortex and it is a period associated with an increase in [[neurogenesis]]. Similarly, the process of neurogenesis regulates lamination to form the different layers of the cortex. During this process there is an increase in the restriction of cell fate that begins with earlier [[Progenitor cell|progenitors]] giving rise to any cell type in the cortex and later progenitors giving rise only to [[neuron]]s of superficial layers. This differential cell fate creates an inside-out topography in the cortex with younger neurons in superficial layers and older neurons in deeper layers. In addition, laminar neurons are stopped in [[Cell cycle|S]] or [[G2 phase]] in order to give a fine distinction between the different cortical layers. Laminar differentiation is not fully complete until after birth since during development laminar neurons are still sensitive to extrinsic signals and environmental cues.<ref>{{cite journal | vauthors = Sur M, Leamey CA | title = Development and plasticity of cortical areas and networks | journal = Nature Reviews. Neuroscience | volume = 2 | issue = 4 | pages = 251β262 | date = April 2001 | pmid = 11283748 | doi = 10.1038/35067562 | s2cid = 893478 }}</ref> Although the majority of the cells that compose the cortex are derived locally from radial glia there is a subset population of neurons that [[cell migration|migrate]] from other regions. Radial glia give rise to neurons that are pyramidal in shape and use [[Glutamate (neurotransmitter)|glutamate]] as a [[neurotransmitter]], however these migrating cells contribute neurons that are stellate-shaped and use [[Gamma-Aminobutyric acid|GABA]] as their main neurotransmitter. These GABAergic neurons are generated by progenitor cells in the [[medial ganglionic eminence]] (MGE) that migrate tangentially to the cortex via the [[subventricular zone]]. This migration of GABAergic neurons is particularly important since [[GABA receptor]]s are excitatory during development. This excitation is primarily driven by the flux of chloride ions through the GABA receptor, however in adults chloride concentrations shift causing an inward flux of chloride that [[Hyperpolarization (biology)|hyperpolarizes]] [[postsynaptic neuron]]s.<ref name="Sanes_2012">{{Cite book |title=Development of the Nervous System | vauthors = Sanes DH, Reh TA, Harris WA |publisher=Elsevier Inc. |year=2012 |isbn=978-0-12-374539-2}}</ref> The glial fibers produced in the first divisions of the progenitor cells are radially oriented, spanning the thickness of the cortex from the [[ventricular zone]] to the outer, [[Pia mater|pia]]l surface, and provide scaffolding for the migration of neurons outwards from the [[ventricular zone]].<ref>{{cite journal | vauthors = Rakic P | title = Evolution of the neocortex: a perspective from developmental biology | journal = Nature Reviews. Neuroscience | volume = 10 | issue = 10 | pages = 724β735 | date = October 2009 | pmid = 19763105 | pmc = 2913577 | doi = 10.1038/nrn2719 }}</ref><ref>{{cite journal | vauthors = Rakic P | title = Extrinsic cytological determinants of basket and stellate cell dendritic pattern in the cerebellar molecular layer | journal = The Journal of Comparative Neurology | volume = 146 | issue = 3 | pages = 335β354 | date = November 1972 | pmid = 4628749 | doi = 10.1002/cne.901460304 | s2cid = 31900267 }}</ref> At birth there are very few [[dendrite]]s present on the cortical neuron's cell body, and the axon is undeveloped. During the first year of life the dendrites become dramatically increased in number, such that they can accommodate up to a hundred thousand [[synapse|synaptic connections]] with other neurons. The axon can develop to extend a long way from the cell body.<ref name="Gilbert2">{{cite book | vauthors = Gilbert S |title=Developmental Biology |date=2006 |publisher=Sinauer Associates Publishers |isbn=978-0-87893-250-4 |pages=394β395 |edition=8th}}</ref>
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