Transformation of degenerating neurofibrils into amyloid substance in Alzheimer's disease: histochemical and immunohistochemical studies.

Degenerating neurofibrils (DNF), which are composed of paired helical filaments (PHF) and amyloid fibrils (AF), are the 2 characteristic pathological fibrillar deposits in Alzheimer cortex. These fibrils were simultaneously studied by 2 techniques: The immunolabelling with a specific antiserum raised against PHF and elective thioflavine S staining of AF. In neuronal perikaryons, neurofibrillary tangles (NFT) consist of 3 populations: firstly, strongly immunolabelled tangles were weakly thioflavine-stained. Secondly, less dense tangles were weakly immunolabelled but strongly thioflavine-stained. Thirdly, ghost tangles which correspond to extracellular NFT were exclusively thioflavine-stained. Thus, it is likely that NFT are degraded to form extracellular AF. Around neuritic plaques and some vessels with amyloid angiopathy, immunolabelled neurites, thioflavine-stained neurites and transition figures were also observed. On the other hand, the central core of plaques and pathological vessel walls were strongly thioflavine-stained but were never immunoreactive. In conclusion, these observations favour catabolism of PHF bundles found in NFT and in degenerating neurites into an amyloid substance. This amyloid substance seems different from other amyloid deposits found in the central core of neuritic plaques and vessel walls.