Comparative study of the glial fibrillary acidic protein in vertebrates by PAP immunohistochemistry

Glial fibrillary acidic protein (GFA) has been visualized by direct peroxidase antiperoxidase (APA) immunohistochemistry in various vertebrates (cyclostomes, teleosts, amphibians, reptiles, birds, and several placental mammals). In this study GFA-immunoreactivity (GFA-I) was observed in all species examined except in cyclostomes and amphibians. Two types of immunoreactive elements were observed: astrocytes and long processes without visible somata. Astrocytic cells with GFA-I were first found in the snake, and more cells were in birds where the pattern of distribution was similar to that of mammals. Within mammals, few differences in the manner of localization were observed among different species, except in the corpus callosum and the ependymal and subependymal layers. Long straight processes were observed in the lower submammalians--the lamprey, carp, and turtle. They radiated through the neuropil from the ventricular wall and followed nerve fiber bundles in the white matter. An uncommon feature was observed in the turtle brain, which possessed very intense GFA-I within the ependymal layer. The presence of GFA-containing profiles in the ependyma of adult animals is discussed in relation to GFA-positive structures seen in the human brain during ontogeny or under certain pathological conditions.