A reexamination of ganglioside GD3-immunopositive glial cells in the forebrains of rats and mice less than 1 week of age.

Before examining remyelination in the brains of rats and mice in vivo, we evaluated the criteria for immature cells in both species that belong to the oligodendrocyte lineage. In the forebrains and cerebella of neonatal rats, ganglioside GD3 has been used as an early immunocytochemical marker for such cells. Using tissue sections from brains of perinatal rats and neonatal mice, we performed double immunofluorescence staining of GD3 with markers for brain macrophages and microglia (both termed "microglia" below) and for the early oligodendrocyte marker O4, respectively. In the rat brains, intense GD3 immunofluorescence was observed in cells also bearing the microglial marker ED1, but not in O4-bearing immature oligodendrocytes. In the rat corpus callosum there also were faintly GD3-positive longitudinal processes that did not co-stain with ED1 or BSI-B4. Some additional GD3-positive structures were observed, mostly in gray matter. In the brains of neonatal and 3-day-old mice, the most intensely GD3-positive structures were elongated longitudinal and radial processes, and the BSI-B4-positive microglia were GD3-negative. In both rat and mouse brains there were at least two additional types of GD3-positive structures, which were ovoid or had a diaphanous appearance, respectively. The present results suggest that there are GD3-positive microglia in immature rat brains and, in both rat and mouse brains, GD3-positive structures that have not yet been identified precisely but which may be radial glial or microglial processes. Accordingly, it is inappropriate to use GD3 as a specifically oligodendrocytic marker in vivo.