High resolution analysis of paired helical filaments in Alzheimer's disease

Results of this comparative study indicated that electron microscopy of supra‐ultrathin sections provided the best resolution, revealing the subunit structures, globules (32 Å ± 4), and longitudinal bars (47 Å ± 6) of paired helical filaments (PHF). A three‐dimensional model of PHF substructure is therefore presented. The image of brightfield electron micrographs of isolated PHF positively stained with 2% phosphotungstic acid (PTA) also provided high resolution and revealed the presence of the beaded structures of these heavily stained filaments in the crossing‐over region of PHF. Noisy background of the negatively stained preparation of isolated PHF substantially reduced the resolution. Severe flattened morphology of this preparation further complicated the interpretation of image analysis. Darkfield electron micrographs offered high‐image contrast with low noise in the background clearly demonstrating the ropelike twisting configuration of PHF, but did not reveal the substructures of PHF. The tilting (±45°) analysis of these PHF prepared by uranyl acetate staining and rotary‐shadowing and unidirectional shadowing methods with platinum showed that these heavy metals were deposited on the top filaments (away from the carbon film on the grid) in the crossing‐over region of PHF.