Localization of glial fibrillary acidic protein and glutamine synthetase in the human cerebral cortex and subcortical white matter--a double immunolabelling and electron microscopic study.

Astrocytes in the human cerebral cortex and subcortical white matter were labelled using antibodies to glial fibrillary acidic protein, or double labelled for glial fibrillary acidic protein and glutamine synthetase, and studied by light and electron microscopy. We described two types of astrocytic processes in an earlier study of human cortex: glial-filament-rich but mitochondria-poor, and filament-poor but mitochondria-rich. The question arose as to whether these were different segments of the same process, or whether the filament-rich processes belonged to a subpopulation of 'fibrous' astrocytes. In the present study no such fibrous astrocytes were found in histologically normal cortex, and all glial fibrillary acidic protein positive cell bodies in the cortex were also glutamine synthetase-positive, and had features of protoplasmic astrocytes. Since the processes of fibrous astrocytes in the white matter seldom extended more than 40-50 microns beyond the cell bodies, they were unlikely to account for the filament-rich astrocytic processes that were observed in the cortex by electron microscopy. The filament-rich but mitochondria-poor, and the filament-poor but mitochondria-rich processes are seen in continuity in the cortex and must therefore both stem from cell bodies with features of protoplasmic astrocytes.