Neuron in Alzheimer's disease, coloured transmission electron micrograph (TEM). This neuron (nerve cell) contains a neurofibrillary tangle (pink) in the cytoplasm (green) of the cell. It consists of abnormal aggregates and insoluble fibres of the protein tau, and is known as a 'paired helical filament' (PHF). Tau protein is an abundant neural protein, aggregations of which are thought to play a role in Alzheimer's disease and other neural disorders. The cell nucleus (yellow) is at centre.