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Polymorphism of Aβ peptide fragments as observed in EM images. (A) Aβ1−28 fibrils, negatively stained, are periodically stained along some of their edges (indicated by arrows). Fibril ends appear to splay or unfurl. Reprinted with permission from ref (75). Copyright 1987, directly permission from Dennis J. Selkoe. (B) Aβ10−23 filaments prepared in vitro. The filaments are irregularly twisted around each other. Reprinted with permission from ref (79). Copyright 1991 Elsevier. (C) Aβ26−33 and Aβ34−42 fibrils. Aβ26−33 fibrils are thin and uniform as compared to those of Aβ34−42 fibrils and appears to be comprised of fibril pairs. At low magnification, Aβ34−42 shows two types of assemblies: twisted fibrils and untwisted narrow fibrils. At higher magnification, twisted fibrils and periodic deposition of enhanced staining (indicated by arrows) are observed. Reprinted with permission from ref (101). Copyright 1990 American Chemical Society. (D) Aβ17−40 form protofibrils (top) and a mature fibril of Aβ17−42 (bottom). Reprinted with permission from ref (103). Copyright 2003 American Society for Biochemistry and Molecular Biology.