Three dimensional analysis of abnormal filaments of Pick's disease by scanning electron microscopy

The three dimensional structure of filaments in Pick bodies and ballooned neurons in an autopsy case of Pick's disease was examined by scanning electron microscopy (SEM) and compared with the appearances on transmission electron microscopy (TEM). SEM revealed two types of filament in Pick bodies; one type was randomly orientated straight filaments of about 20 nm in width and the other was twisted helical filaments resembling paired helical filaments (PHFs). The maximum diameter of helical filaments was 44 nm with periodic constrictions of about 200 nm; the direction of the helix was left-handed. Ballooned neurons contained mainly randomly arranged filaments, 17-22 nm diameter. The three types of filament described here were greater in diameter in SEM images than in TEM; this may be due to the fixed increased thickness of 10 nm due to the platinum pallidium compound coating. However, SEM images are much more suitable than TEM profiles for three dimensional analysis of filamentous structures, especially those with helically twisted profiles; preparation for SEM did not change the periodicity of the twisted structures or the direction of the helix on these filaments even after metal coating. SEM has proved to be an effective and straightforward method for assessing the arrangement, direction and distribution of filaments and this technique could be applied to similar structures in other diseases.