For a long time, the brain was thought to exclusively contain finally differentiated cells, such as neurons, astrocytes, and oligodendrocytes. These highly specialized cells are originated during embryonic development and their number decreases throughout adulthood.
However, from 1962, Joseph Altman and his colleagues challenged this dogma by demonstrating that the brain contained other types of cells that have the neuronal and glial potential .
They identified their presence in two regions of the brain (niches): the dentate gyrus (DG) in the hippocampus and the subventricular zone (SVZ) . Since then, these cells were named adult neural stem cells and are now a subject of intensive research in the neurobiology field .
Adult neural stem cells of the hippocampal dentate gyrus (DG)
The hippocampus is an essential organ that is involved in our short-term and long-term memories and the processing of spatial memory that controls navigation . Hippocampal neural stem cells have been identified as radial glia-like (RGL) cells that are able to self-renew and make intermediate proliferating progenitors (IPCs) and neuroblasts to generate hippocampal neurons named granule neurons of the dentate gyrus (DG).
These highly specialized cells are essential to memory function by decorrelating pattern separation .
Adult neural stem cells of the subventricular zone (SVZ)
During vertebrate neurodevelopment, the subventricular zones of the lateral ventricles are the source of embryonic neural stem cells that are essential for cerebral cortex formation.
Interestingly, after birth and during adulthood, the subventricular zone is also a source of adult neural stem cells that generate neuroblast precursors of interneurons that migrate to the olfactory bulb, a neural structure involved in receiving sensations of smell, and discriminating between different odors .
Could adult neural stem cells be used for regeneration and repair?
Several neurodegenerative diseases, such as Alzheimer’s or Parkinson’s disease, are accompanied by a significant loss of specialized neurons. Whether adult neural stem cells could be used to replace this loss is controversial.
The hippocampal dentate gyrus and subventricular zone stem cells are highly committed to specific neuronal lineages and are contained within niches that are controlled by specific environmental signals and cells  .
Regenerative and repair therapies using adult neural stem cells may not succeed due to the highlighted reasons above. However, further studies that focus on the molecular mechanisms of NSC regulation and niche responses may lead to breakthroughs in the use of these cells in the therapy of neurodegenerative diseases.
 GoodSmith, D., Chen, X., Wang, C., Kim, S.H., Song, H., Burgalossi, A., Christian, K.M. and Knierim, J.J., 2017. Spatial representations of granule cells and mossy cells of the dentate gyrus. Neuron, 93(3), pp.677-690.