A 13 year-old male presents with a cerebellar mass. A biopsy is performed, and a representative section is shown here.

What are the red structures scattered throughout the biopsy? (Bonus points if you can figure out the diagnosis in this case.)

A. Blood vessels
B. Fungal organisms
C. Lewy bodies
D. Rosenthal fibers
E. Negri bodies

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The answer is D, Rosenthal fibers. Rosenthal fibers (or Rosenthal bodies) are eosinophilic structures present in great numbers in pilocytic astrocytomas. Pilocytic astrocytomas are indolent tumors that usually occur in childhood and often affect the cerebellum. Composed of long, thin, pilocytic (“hair-like”) astrocytes, these tumors tend to grow very slowly, even if a higher-grade histologic appearance is present.

Rosenthal fibers are present in pilocytic astrocytoma – but they are not specific for that tumor. They are also seen in areas of long-standing gliosis and in Alexander disease. They are weird little structures that contain two heat-shock proteins and ubiquitin (a protein that seems to be present everywhere! Hmm..ubiquitous…I guess that’s why they named it ubiquitin.).

As for the other answers: A is wrong; blood vessels should have red cells in them. These structures are more of a pinkish red, and they are solid, opaque, and stubby. B is incorrect because the structures don’t look like a typical fungus – they are rod-shaped, yes, but organisms with hyphae tend to have long, continuous hyphae (sometimes with septae), not short, stubby, disconnected ones. C is incorrect: Lewy bodies are light-pink, round inclusions that you see within neurons in Parkinson disease. And E is incorrect: Negri bodies are pink-purple round inclusions present in neurons infected with rabies.