Glutamine Synthetase as an Astrocytic Marker: Its Cell Type and Vesicle Localization

astrocytes, including all cells of the astroglial family (1), i.e., Bergmann glia, Müller cells (2), tanycytes (3), and ependymal cells. The star shaped morphology from classical silver impregnations relates to cortical and hippocampal astrocytes, which display a comparable pattern in material stained for GFAP. However, the dense population of GS stained astrocytes found in all diencephalic and mesencephalic regions, known to display faint GFAP-labeling (unpublished observations) indicated that while apparently all astrocytes contain GS they have GFAP-ir filaments only in a region-dependent pattern. This is complicated by the emerging view that “astrocytes” constitute a heterogeneous population even within a given region. In the rat hippocampus, combined immunostainings have revealed that the “classical” GFAP-ir astrocyte constitutes a subpopulation of GS-ir astrocytes, which can also lack GFAP staining [direct double staining (4)]. In view of several astroglial subtypes and/or glial precursors present in the adult rodent brain, anti-GS appears to be the most general astrocyte marker, covering all subtypes. In addition, GS has been found early on to label exclusively astrocytic cells and no other glial or neuronal cell types in situ or in culture [reviewed by (5)]. GS has, thus, been applied as a reliable astrocyte marker in very many studies since. “Complex cells” in rat hippocampus, initially assumed to be an astrocyte subtype (6) but now understood to belong to NG2 cells, a fourth glial type in the CNS (7), may display faint GS-ir in the soma but not its fine processes. Oligodendrocyte precursor cells, possibly also related to NG2 cells, were found to be devoid of GS-ir (8). The immunocytochemical profile and possible heterogeneity of NG2 cells is still under debate to date. Disputing the exclusion of non-astrocytic cells in GS staining, some authors have later reported GS+ oligodendrocytes, although this has not been investigated systematically. Reports on non-astrocytic GS will be discussed in detail here.