Hello Abhijay,

Thank you for your very insightful questions!
You ask, why don’t existing neurons divide to form more neurons when other neurons die or are injured? And also, could we use genetic approaches to induce differentiated neurons to begin dividing and produce new neurons?

First, although existing terminally differentiated neurons are incapable of dividing to form new neuronal populations, a process called neurogenesis produces new neurons using neural stem cells and progenitor cells. Neurogenesis is known to occur in the adult brain, particularly in the olfactory epithelium and bulb, the dentate gyrus of the hippocampus, and the striatum. Through the process of neurogenesis, neurons in these regions of the brain can be replenished. Much has been learned about the signaling pathways and mechanisms underlying the process of neurogenesis, but many questions remain. As you might imagine, researchers around the world are actively investigating this fascinating process with the hope of treating both neurodegenerative disease and nervous system injury.

Now let’s tell you a bit more about stem cells and about how researchers have discovered that the simultaneous expression of only three genes can convert differentiated adult somatic cells into stem cell-like derivatives. Unlike the differentiated cells that make up mature organs and tissues, stem cells are immature cells that have not yet specialized and have the capacity to divide and differentiate into a wide variety of tissue types. Stem cells naturally come in two different forms: embryonic and somatic. Embryonic stem (ES) cells are derived from a specific group of cells within an embryo — they’re immortal (capable of unrestricted cell division) and pluripotent (able to differentiate into nearly any cell type in the right environment). Somatic stem cells are derived from a specific group of cells that make up an adult tissue that help to renew the tissue population — they are more restricted in terms of the types of cells they can become when they divide.

As you likely know, much controversy surrounds the use of ES cells. One way around this problem would be to somehow modify adult somatic cells to make them both immortal and pluripotent. Recently, stem cell researchers have accomplished this feat: they have made induced pluripotent stem cells (iPSCs) that appear to be both immortal and pluripotent by expressing as few as three extra genes in adult skin cells, bone marrow-derived mesenchymal cells, and even fat cells. Researchers can then coax the iPSCs to become neurons by exposing them to specific growth factors and growth conditions. As you can see, researchers are indeed able to use genetic approaches to grow new neurons. How exciting!

We’ve provided you with a large collection of helpful links to help you learn more about neurogenesis, stem cells, and iPSCs. Happy reading!
To learn more about adult neurogenesis and neuronal differentiation, follow these links:
http://www.bioedonline.org/news/hot-topics/adult-neurogenesis/
http://www.ninds.nih.gov/disorders/brain_basics/ninds_neuron.htm
http://www.sciencedirect.com/science/article/pii/S0092867411005952

For a short list of primary research and review articles about neurogenesis, check out these links:
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002045
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312696/
http://www.jneurosci.org/content/16/6/2027.long
http://www.ncbi.nlm.nih.gov/pubmed/10521353
http://www.pnas.org/content/96/9/5263.long
http://www.molecularneurodegeneration.com/content/6/1/85
http://www.sciencedirect.com/science/article/pii/S0092867414001378
http://www.sciencedirect.com/science/article/pii/S0006899304009084
http://www.sciencedirect.com/science/article/pii/S0092867408001347
http://www.sciencedirect.com/science/article/pii/S0092867413005333

Lucky for you, Scitable offers a Spotlight page on Stem Cells that provides myriad resources about stem cells and issues surrounding their use:
http://www.nature.com/scitable/spotlight/stem-cells-6969855

Please check out the links below for additional general information about stem cells and iPSCs:
http://www.nature.com/scitable/topicpage/stem-cells-in-plants-and-animals-14164783
http://learn.genetics.utah.edu/content/tech/stemcells/
http://www.nlm.nih.gov/medlineplus/stemcells.html
http://www.nature.com/scitable/topicpage/turning-somatic-cells-into-pluripotent-stem-cells-14431451
http://www.nature.com/scitable/topicpage/controversies-in-treatment-approaches-gene-therapy-ivf-792
http://www.nature.com/scitable/topicpage/gene-based-therapeutic-approaches-3881
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4287204/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2869301/

To read about studies using iPSC-derived neurons, see these links:
http://www.cell.com/cell-reports/fulltext/S2211-1247(14)00877-8
http://www.sciencedirect.com/science/article/pii/S0092867411005952