Transmission electron micrograph (TEM) showing an amyloid plaque (one from centre to lower centre) in a brain with Alzheimer's disease. Amyloid plaques are insoluble aggregates of beta-amyloid protein. They are believed to be linked to Alzheimer's disease by causing nerve cell death and affecting nerve cell signalling pathways. Insoluble fibres of the protein tau, known as 'paired helical filaments' (PHF, stacks of parallel lines, one at centre left) are shown. Tau protein is an abundant neural protein, aggregations of which are thought to play a role in Alzheimer's disease and other neural disorders. Other hallmarks of Alzheimer's disease such as degenerating neurites and amyloid fibrils are seen. Magnification: x3,600 when printed at 10 centimetres wide.