(a) TEM images of Aβ40 fibrils negatively stained with ammonium
molybdate (pH 7.4). (b) Five-fold magnification of the portion of
panel a outlined in a dotted line, with measurements of a fibril at
its widest (8 mm) and narrowest (4 mm) points. (c) MPL histogram (60
nm × 40 nm window, 685 measurements) of unstained TEM dark-field
images of seeded Aβ40 fibrils. Dotted red vertical lines and
integers represent the x-axis positions corresponding
to two and three filaments per fibril. With windows that are 40 nm
wide, there is a clear predominance of segments with either two or
three filaments. (d) MPL histogram of the same images examined in
panel c with a wider window (60 nm × 80 nm, 305 measurements).
With the wider window, the same average MPL is obtained, but the two-filament
and three-filament regions cannot be resolved. (e) Typical dark-field
image with TMV (bright line) that serves as an internal mass standard
for determining the MPL of a fibril (dim line).