Abstract
The extent to which NG-2(+) cells form a distinct population separate from astrocytes is central to understanding whether this important cell class is wholly an oligodendrocyte precursor cell (OPC) or has additional functions akin to those classically ascribed to astrocytes. Early immuno-staining studies indicate that NG-2(+) cells do not express the astrocyte marker GFAP, but orthogonal reconstructions of double-labeled confocal image stacks here reveal a significant degree of co-expression in individual cells within post-natal day 10 (P10) and adult rat optic nerve (RON) and rat cortex. Extensive scanning of various antibody/fixation/embedding approaches identified a protocol for selective post-embedded immuno-gold labeling. This first ultrastructural characterization of identified NG-2(+) cells revealed populations of both OPCs and astrocytes in P10 RON. NG-2(+) astrocytes had classic features including the presence of glial filaments but low levels of glial filament expression were also found in OPCs and myelinating oligodendrocytes. P0 RONs contained few OPCs but positively identified astrocytes were observed to ensheath pre-myelinated axons in a fashion previously described as a definitive marker of the oligodendrocyte lineage. Astrocyte ensheathment was also apparent in P10 RONs, was absent from developing nodes of Ranvier and was never associated with compact myelin. Astrocyte processes were also shown to encapsulate some oligodendrocyte somata. The data indicate that common criteria for delineating astrocytes and oligodendroglia are insufficiently robust and that astrocyte features ascribed to OPCs may arise from misidentification.
Highlights
Glial cells expressing the NG-2 proteoglycan act as oligodendrocyte precursor cells (OPCs) and are retained in the mature CNS where they form a reservoir of progenitors that may be significant for the development of effective treatment strategies for common neurological diseases (Nishiyama et al, 2009)
Cell counting of stack projections in P 10 rat optic nerve (RON) showed 38.5 +/− 2.7% of cells were GFAP(+)/NG2(−), 28.5 +/− 2.6% were GFAP(−)/NG-2(+) and 33.0 +/− 5.3% were GFAP(+)/NG-2(+) (544 cells analyzed from 5 sections)
We examined P0 RON, a developmental point before the wide-spread arrival of OPC (Vaughn, 1969; Small et al, 1987; Barres et al, 1992) and a point when astrocyte production has peaked (Vaughn and Peters, 1967; Vaughn, 1969; Skoff et al, 1976; Skoff, 1990)
Summary
Glial cells expressing the NG-2 proteoglycan act as oligodendrocyte precursor cells (OPCs) and are retained in the mature CNS where they form a reservoir of progenitors that may be significant for the development of effective treatment strategies for common neurological diseases (Nishiyama et al, 2009). Cells with features of NG-2(+) cells have been variously described as “small glioblasts” (Vaughn, 1969), “3rd glial element” cells (Vaughn and Peters, 1968), “β-astrocytes” (Reyners et al, 1982),“oligodendrocyte/type 2 astrocytes” (O2A) (Raff et al, 1983), “smooth protoplasmic astrocytes” (Levine and Card, 1987), “OPCs” (Ong and Levine, 1999), “astons”. Phenotype overlap in glia (Matthias et al, 2003), “polydendrocytes” (Nishiyama et al, 2009) and “synantocytes” (Leoni et al, 2009). This diversity of nomenclature reflects the degree of uncertainty regarding the cells’ ontogeny and their physiological functions. The degree to which these cells may exhibit astrocyte-type behavior remains controversial, as does the extent to which astrocytes and NG-2(+) cells share a common cell fate
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