Membrane-bound cytoplasmic crystals, similar to those in alveolar soft part sarcoma, in a human muscle spindle.

Membrane-bound cytoplasmic crystals were found in the intrafusal fibers of a muscle spindle from a patient with neurogenic atrophy. The crystals have a periodicity of 10 nm and an intersecting axis angle of approximately 80 degrees. This makes the crystals similar to those described in alveolar soft part sarcoma (ASPS). Because the crystals in ASPS may not be quite as specific as previously believed, and because similar crystals have been described in various other neoplasms, the present findings should not be taken as evidence for a muscle spindle derivation for ASPS.