Astrocytes have been classically described based on morphological characteristics as either protoplasmic or fibrous. However, the appearance of astrocytes at the light microscopic level following immunoreaction with glial filament acidic protein antisera or Cajal's gold-chloride method is markedly different from the morphology of Golgi-impregnated or horseradish peroxidase-filled astrocytes. The present study combines immunohistochemistry using glial fibrillary acidic protein antisera and a cellular suspension of structurally intact astrocytes isolated from the rat cerebral cortex to demonstrate that the distribution of glial filaments is limited to major astrocytic processes. Because the sheet-like processes which decorate the major branches of protoplasmic astrocytes do not contain glial filaments, the entire protoplasmic astrocyte is not visible when viewed in situ in the gray matter of the cerebral cortex. Thus staining techniques that have a propensity for the glial filaments such as antisera to glial fibrillary acidic protein and the gold-chloride method provide only a skeletal outline of the major processes of the protoplasmic astrocyte. On the other hand, fibrous astrocytes which do not have feathery decorations on major processes are visible in the cerebral cortex in their structural entirety. The present investigation also offers additional evidence of the specificity of glial fibrillary acidic protein antisera for astrocytes.
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