From human brain analyses and studies in model organisms such as rodents and birds, we know neurogenesis in the adult brain persists throughout the life of the organism and is essential for maintaining both brain structure and function. This article from The Guardian provides an excellent overview of the history of Neurogenesis research.

Stem cells are a major focus in this lesson. Watch this TED-Ed lesson: What are stem cells? Gain more insight into exactly what they are! In the adult mammalian brain, new neurons are generated from self-renewing and largely quiescent neural stem cells located mainly in the subventricular zone of the lateral ventricles and in the subgranular zone of the dentate gyrus in the hippocampus. (The hippocampus is an integral part of the human brain. Watch this TED-Ed lesson and learn “What happens when you remove the hippocampus?”) 

Neural stem cells in both neurogenic regions can divide asymmetrically generating a daughter stem cell and a transit-amplifying cell - a rapidly proliferating progenitor cell, which expands the pool of neural progenitors as needed. Transit-amplifying cells in turn differentiate into neuroblasts, which travel away from the neurogenic niche to the olfactory bulb or to the granule cell layer of the hippocampus. Apart from neural progenitors, neural stem cells give rise to a small number of glial cells as well - both astrocytes and oligodendrocytes. Interestingly, neural stem cells share many common features with brain astrocytes, including morphology and molecular signature. In the context of acute brain injury, astrocytes have been reported to acquire neurogenic capabilities similar to those of neural stem cells. Both cell types extend their processes, called end-feet, to establish contact with blood vessels in the brain - another important component of the neurogenic niche considered to provide a trough of nutrients and growth factors that help regulate the neurogenic process. The subventricular zone is separated from the lateral ventricles filled with cerebrospinal fluid (CSF) by a layer of ependymal cells. This boundary, however, is partially permeated by the stem cells, which squeeze in-between the ependymal cells to establish a contact with the CSF. This complexity of the neurogenic niche might be one of the reasons why neural stem cells and neuronal production persist only in discreet areas of the brain. 

Apart from the subventricular zone and the subgranular zone, a third area of neurogenic activity, the forebrain striatum, was only recently identified and it appears to be specific to the human brain. The discovery is explained in this Nature Reviews article. Striatal neurogenesis has been observed in mice as well but only following brain injury, such as after stroke. In humans, however, neuronal production in the striatum seems to be part of normal brain homeostasis and the process is disrupted specifically in patients suffering from Huntington’s disease - a genetically inherited condition resulting in progressive neurodegeneration. Whether the human striatum contains unique neural stem cells or another cell type with neurogenic abilities is still unclear. 

Similar to the brain, most adult tissues contain such rare slow-proliferating stem cells that maintain the stem cell pool and give rise to progenitors committed to a certain cell-fate. In self-renewing tissues like the adult forebrain and skin, progenitor cells are being continuously produced, whereas in other organs the quiescent stem cells are activated in response to disease onset or tissue injury. Usually, injury-triggered stem cell activation is mean to initiate tissue repair or minimize damages. In some cases, however, the local stem cells mobilized in response to the injury can go rogue, co-opting the healing process to tumor formation, as is the case with some skin cancers. Understanding the molecular mechanisms governing adult tissue-specific stem cell function can help scientists sway the outcome of adult stem cells’ response to disease and injury in order to promote healing and regeneration. 

Interested in learning all you can about the human brain? Start at TED Ed with these lessons that will stimulate those neural pathways and potentially answer questions you might already have! Still interested? Listen to some of the numerous “Brainy” TED Talks available at this link.

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