Many cells in the human body can divide and reproduce, making healing possible. Neurons, however, cannot reproduce, which makes diseases affecting the brain particularly crippling. Why can't neurons divide - that is, what makes them different from "normal" cells? Are there any ways to artificially stimulate neuron cell division?
Neurons do not divide due to the reasons mentioned in Cornelius's answer. However, some new neurons can be generated in adults (Ref: Neuroscience, 2nd edition).
Generation of new neurons in adults was first demonstrated in birds, where labeled DNA precursors could be found in differentiated neurons. Experiments in mammals and humans demonstrated later that new neurons are created in the central nervous system (CNS) in adults, although it seems to be restricted to some particular regions: granule cell layer of the olfactory bulb and dentate gyrus of the hippocampus. These new neurons seem to be local circuit neurons and interneurons (i.e. no long distance neurons).
How are the new neurons produced if neurons cannot divide? They come from neural stem cells (NSCs) that were preserved in the sub-ventricular zone during development. NSCs are presumed to play a part in brain plasticity in the adult brain. However, they have therapeutic potential. Check the brief information in wikipedia), or this perspective in Nature neuroscience that may contain further details.
- Neuroscience, 2nd edition. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Sunderland (MA): Sinauer Associates; 2001.
- Gene therapy: can neural stem cells deliver? Müller FJ, Snyder EY, Loring JF. Nat Rev Neurosci. 2006 Jan;7(1):75-84. Review. Erratum in: Nat Rev Neurosci. 2006 Feb;7(2):167. PMID: 16371952 [PubMed - indexed for MEDLINE]
Morphological point of view
Neurons cannot divide because they lack centrioles.
Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell .
Functional point of view
New cells in the nervous system wouldn't do any good. The whole nervous system is based on interneuronal connections, so adding an extra neuron would mess up these connections and alter both the functionality and the "stored" information.
Each nerve cell has a specific place in our nervous system. Its job is all about taking a signal from one specific place to another one. Adding new nerve cells would mess up these very specific connections in a very complex system .
- SEER Training. NERVE TISSUE. Available from http://training.seer.cancer.gov/anatomy/nervous/tissue.html
- UCSB Science Line. Available from http://scienceline.ucsb.edu/getkey.php?key=1710
I would like to propose an "answer" from a personal belief (no sources) based on an evolutionary approach to this question.
If neurons were readily capable of division, that is, healing, they would require a different form, thus mandating a different function; ultimately, they would be less efficient at what it is they are supposed to do (transfer electricity)--thus, not advantageous for the organism (during the evolutionary period); thus, did not occur.
Think of this for a moment: in physical space, what would happen to the other neurons if one were to divide? It would push into them, possibly 'disconnecting' a pathway, or impairing it's ability to fire (possibly below the activation threshold)--a whole new host of diseases could emerge.
Perhaps plausible, there was at one point something similar to what I've described above (a mitosing nervous system). The question is, though, would it be more efficient than a non-dividing nervous system? Would it have fewer diseases?