Amyloid-like green birefringence in cytoskeletal 10 nm filaments after staining with Congo red.

The commonly accepted markers for amyliod are a fibrillar ultrastructure and congophilia combined with green birefringence when viewed under polarized light. Although a number of cells have been implicated in amyloid formation the detailed events leading to accumulation of amyloid are still unknown. Previous studies have suggested that amyloid shares antigenic properties with cytoskeletal intermediate (10 nm) filaments and connective tissue microfibrils. In the present study we show that such filaments present in fibroblasts and lymphoid cells have an affinity for Congo red, exhibit the typical green birefringence and are ultrastructurally indistinguishable from amyloid fibrils. Intermediate filaments of cultured fibroblasts depleted of nutrient medium retained the Congo red birefringent property and typical ultrastructure despite cell damage. Our observations suggest that amyloid deposits may form locally through abnormal accumulation of bundles of cytoskeletal intermediate filaments under conditions characterized by increased cell proliferation and death.