Each astrocyte forms an extensive network of fine processes within the surrounding neural tissue, interacting extensively with neighboring neurons and blood vessels. Fine glial processes surround synapses and probably modulate synaptic transmission. Glial endfeet on capillaries are responsible for transport of ions and metabolites and possibly control blood flow. Alterations in these fine structures may be of significance in brain function and disease. Glial fibrillary acidic protein (GFAP) is an intermediate filament found in astrocytes of the central nervous system. GFAP is commonly found in the perikarya and processes of protoplasmic and fibrous type astrocytes. Immunohistochemical labeling of GFAP is extensively used as a means of determining the location and shape of astrocytes. However, its labeling pattern varies with brain region (e.g. cortex vs. hippocampus), with cell state (natural vs. reactive astrocytes), and with the specific α- GFAP antibody used. Furthermore, Golgi-stained or dye-filled astrocytes show numerous small appendages or vellate structures that conform to the surrounding tissue and do not stain for GFAP.