Shades of gray: The delineation of marker expression within the adult rodent subventricular zone.

The research field of adult neurogenesis is rapidly expanding with more and more information becoming available on the identity of the cells located within the subventricular zone (SVZ). Much of our understanding is based on rodent studies. The SVZ is comprised of several different cell types including B1 astrocytes, transit amplifying progenitor cells (C cells), and neuroblasts (A cells). B1 astrocytes are the quiescent neural stem cells that continue to divide throughout a lifespan. They give rise to a progenitor cell, termed a C cell, which in turn, generates neuroblasts destined for the olfactory bulb. There is much controversy over how to distinguish various SVZ cell types. This review summarizes the known markers for rodent SVZ cell types, with particular attention paid towards B1 astrocytes and C cells. Unfortunately, there is no perfect stem cell marker. B1 astrocytes, C cells, and neuroblasts gain and lose marker expression patterns throughout their lineage progression. These expression patterns often overlap at the transition from one cell type to another. The SVZ cell lineage must be seen as a continuum, rather than a static and inert system. This view will aid in understanding the mechanisms underlying marker expression and cellular behavior in the SVZ.