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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Neuron | 8/8 | https://en.wikipedia.org/wiki/Neuron | reference | science, encyclopedia | 2026-05-05T07:31:09.374333+00:00 | kb-cron |
Neurons develop through the process of neurogenesis, in which neural stem cells divide to produce 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 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. Differentiation of the neurons is ordered by their size. Large motor neurons are first. Smaller sensory neurons together with glial cell differentiate at birth. 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. 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". 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. 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 cytosine methylation to form 5-methylcytosine and 5-methylcytosine demethylation. DNA cytosine methylation is catalyzed by 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. 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 recombinational repair used in proliferating neural precursor cells, and non-homologous end joining used mainly at later developmental stages Intercellular communication between developing neurons and microglia is also indispensable for proper neurogenesis and brain development.
== Nerve regeneration ==
Peripheral axons can regrow if they are severed, but one neuron cannot be functionally replaced by one of another type (Llinás' law).
== See also ==
== References ==
== Further reading ==
== External links ==
IBRO (International Brain Research Organization). Fostering neuroscience research especially in less well-funded countries. NeuronBank Archived 2021-04-13 at the Wayback Machine an online neuromics tool for cataloging neuronal types and synaptic connectivity. High Resolution Neuroanatomical Images of Primate and Non-Primate Brains. The Department of Neuroscience at Wikiversity, which presently offers two courses: Fundamentals of Neuroscience and Comparative Neuroscience. NIF Search – Neuron Archived 2015-01-22 at the Wayback Machine via the Neuroscience Information Framework Cell Centered Database – Neuron Complete list of neuron types according to the Petilla convention, at NeuroLex. NeuroMorpho.Org an online database of digital reconstructions of neuronal morphology. Immunohistochemistry Image Gallery: Neuron Khan Academy: Anatomy of a neuron Neuron images