I. Localization of Mu class glutathione-s-transferase in the forebrains of neonatal and young rats: Implications for astrocyte development

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Abstract
The Yb (Mu class) isoform of glutathione‐S‐transferase has recently been localized in ependymal cells, subependymal cells, and astrocytes in the forebrains of rats 3 weeks to adult in age. It was not known, however, at what age Mu might first be observed during postnatal development and whether the first cells in which it was found would be immature astrocytes or some less differentiated glial precursor cell, if the latter were present in vivo. Tissue sections from the forebrains of neonatal to 16 day old rats were immunostained with antibodies against Mu. In neonates Mu was observed in vimentin‐positive cells and their processes adjacent to the lateral ventricles, and in the corpus striatum. The colocalization with vimentin suggested that these were subependymal cells and radial glia. In the corpus striatum the radial glia, while still vimentin‐positive, rapidly lost Mu from their radial cell processes, whereas the cell‐bodies remained Mu‐positive. During the first postnatal week the Mu‐positive, glial‐fibrillary‐acidicprotein (GFAP)‐positive cell bodies of immature astrocytes appeared in the corpus striatum. The earliest Mu‐positive cells in the immature white matter of the corpus callosum were vimentin‐positive and had striking longitudinal processes that also were vimentin‐ and Mu‐positive, Like the processes of radial glia, the longitudinal processes lost their Muimmunoreactivity, only later and more gradually. Mu‐positive, GFAP‐positive cells appeared later in the corpus callosum than in the corpus striatum. The data suggest that (1) Mu is localized in the astrocyte line in the rat brain at all postnatal ages; (2) astrocytes in the white matter may have precursors corresponding to those of astrocytes in gray matter, but developing several days later; and (3) the loss of Mu from radial glial processes may be an early event in the retraction of those processes.