Amoeboid microglia expressing GD3 ganglioside are concentrated in regions of oligodendrogenesis during development of the rat corpus callosum.

In recent study we demonstrated expression of the platelet-derived growth factor alpha receptor (PDGFR alpha) in cells of the early oligodendrocyte lineage that were identified as either GD3 ganglioside + oligodendrocyte progenitors or O4 sulfatide+ preoligodendrocytes. We also identified a subpopulation of GD3 immunoreactive cells that did not express mRNA for the PDGF receptor. The distinct large amoeboid morphology of these cells was characteristic of cells in the macrophage lineage rather than in the oligodendrocyte lineage. To determine if the GD3-positive but PDGFR alpha mRNA-negative cells were in the macrophage lineage, we compared the spatial and temporal expression patterns of GD3 ganglioside and ED1, a macrophage-specific antigen. Analysis prenatally indicated that at embryonic day 15, ED1+ and GD3+ cell populations resided in the subpial connective tissue. At embryonic day 21, these two populations were seen in a region extending from the lateral ventricle through the subventricular and intermediate zones. In this study we report that these large, round, GD3 immunoreactive cells have the same cell morphology and anatomical distribution as the ED1 immunoreactive cells. Both cell populations contained pyknotic nuclei within their cytoplasm. Furthermore, the GD3+/PDGFR alpha- cells appear to be involved in clearing cellular debris in regions of gliogenesis. These data suggest that this subpopulation of GD3 immunoreactive cells belongs to the microglia/macrophage lineage.