An electron micrograph of single astrocyte (blue nucleus) within the adult human SVZ astrocyte ribbon, which is extending a process of intermediate filaments (red) through the 'gap' region towards the ependyma.

The identification and characterization of neural stem cells within different regions of the adult brain, particularly in rodents, has revolutionized our understanding of adult neurogenesis and heightened hopes for treatments of traumatic damage and neurodegenerative disease. However, the degree to which this occurs in the adult human central nervous system has been less well characterized. Recently, Avlarez-Buylla and colleagues (Nature 427, 740–744 (2004)) identified some similarities, as well as some striking species-specific differences, for a pool of neural stem cells that reside in a specialized region of the human sub-ventricular zone (SVZ) – a part of the forebrain that lines the ventricular surface.

In mammals, the adult brain has two main areas that produce precursors for the generation of new neurons: the SVZ and the dentate gyrus of the hippocampus. In rodents, the SVZ produces a train of chain-migrating, newly formed neurons — the rostral migratory stream — that replace a population of inter-neurons in the olfactory bulb. Although human brains were known to contain neural precursors, the precise anatomical localization of these precursors and the degree to which they produce new neurons (particularly outside the dentate gyrus) remained relatively unclear.

Now, Alvarez-Buylla and colleagues use large numbers of cells from human biopsy and autopsy tissue samples to characterize the human neural stem cell pool. They find that the stem cells are localized to a ribbon-like layer of astrocytes lining the inner SVZ; this pronounced layer of the SVZ has not been identified in other mammalian species investigated. This population of astrocytes contained a small proportion (∼4%) of cells that could be clonally expanded to produce both neuronal and glial cell lineages. Thus, it is clear that the SVZ in humans contains potential neural stem cells. However, the authors find no strong evidence to suggest that that these cells are producing differentiated neurons in vivo. Most strikingly, they seem to completely lack the rostral migratory stream that is so prominent in rodents and that has even been identified in primate species.

An important conclusion that can be drawn from this work is that although humans clearly possess neural precursor cells that could potentially be used to replace areas of damaged neurons, the situation in adult humans is clearly different from that in other mammalian species. These differences have obvious implications for interpreting animal studies as a basis for human therapies. Uncovering the molecular underpinning of these differences will be necessary for deriving successful cell-replacement treatments for traumatic and neurodegenerative brain disorders.