Relationship between plaques, tangles, and dystrophic processes in Alzheimerʹs disease

AD is characterized by paired helical filaments (PHFs) in neurofibrillary tangles (NFTs) and dystrophic neuronal processes, and PHFs are formed from derivatized tau known as PHFtau. Amyloid is made from 39–43 amino acid long peptides (Aβ) and amyloid fibrils are the dominant structures in senile plaques (SPs), but Aβ fibrils in SPs are associated with PHFtau and other neuronal components. Although a dense mesh of PHFtau positive dystrophic processes permeates nearly all amyloid plaques in the AD neocortex, a similar mesh is not seen in the neocortex of elderly controls. Taken together with evidence that injections of PHFtau into rat brain induce Aβ deposits, we infer from these observations that the formation of neuritic amyloid plaques could involve interactions between Aβ and neuronal proteins released into the extracellular space of the AD neocortex. Thus, the conversion of soluble Aβ into insoluble amyloid fibrils and the formation of neuritic plaques may be a multi-step process involving interactions between soluble Aβ and pathologic co-factors some of which may be derived from degenerating neurons.