SCANNING ELECTRON-MICROSCOPIC STUDY OF THE NEUROFIBRILLARY TANGLES OFALZHEIMERS-DISEASE

Neurofibrillary tangles (NFTs) have been ultrastructually studied by v arious methods, leading to several three-dimensional models of paired helical filaments (PHFs). In this study, we present the scanning elect ron microscopic findings of NFTs in an autopsy case of Alzheimer's dis ease and clarify the three-dimensional structures of NFTs. NFTs were c learly defined in freeze-cracked nerve cells and consisted of two type s of filamentous structures, straight and helical filaments. Straight filaments measured from 20 to 25 nm in diameter and had a smooth surfa ce. They were slightly bent but mostly straight with no constrictions. One type of straight filaments ran in a bundle in the same direction, another was intertwined to each other. Most of the helical profiles o f filaments usually measured about 28 nm in diameter, with a distance of 100 nm between periodic constrictions. They seemed to consist of a pair of isodiametric filaments of 10 nm in diameter. In addition, two unusual types of helical filaments were occasionally observed. One com prised thick filaments of about 38 nm in diameter, with a distance of 100 nm between constrictions; these helical filaments appeared to cons ist of two or more strands. The other comprised thin helical filaments of about 20 nm in diameter and regularly constricted at an interval o f 50 nm. All types of the helical filaments examined in this case were leotropic. This result supports a protofilament model of PHFs. Scanni ng electron microscopy using the freeze-cracked and maceration method is a useful and simple method for three-dimensional observation of the filamentous structures in NFTs.