A Rosenthal fiber is a thick, elongated, worm-like eosinophilic (pink) bundle that is found on H&E staining of the brain in the presence of long standing gliosis, occasional tumors, and some metabolic disorders. Its presence is pathognomonic of either pilocytic astrocytoma (more common) or Alexander's disease (a rare leukodystrophy). The fibers are found in astrocytic processes and are thought to be clumped intermediate filament proteins.

Shown below is an image depicting Rosenthal fibers.

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• Isolation of a major protein component of Rosenthal fibers