Alzheimer's Disease; neurofibrillary tangle: The cell shown here demonstrates Alzheimer's neurofibrillary degeneration after silver impregnation. The coarse black fibrillar material within the neuron constitutes the "neurofibrillary tangle", which consists of abnormal cytoskeletal elements known as paired helical filaments. Ultrastructurally, these filaments have periodic twists about every 80 nm.