Neurofibrillary tangles of paired helical filaments.

The abnormal aggregates of fibrillar material found in neurons in Alzheimer's disease, senile dementia, the Guam Parkinsonism-Dementia complex, Down's syndrome, and postencephalitic Parkinsonism were studied by means of tilt-stage electron microscopy and with X-ray images of scale models of a bifilar helix. The model fulfills the structural criteria established by electron microscopy. These studies showed that the "twisted tubule" which makes up the neurofibrillary tangle in many pathological situations is a bifilar helix made up of 130 A filaments.