Yet another view of Alzheimer's disease, this time as a consequence of increasing numbers of senescent microglia that no longer do their job of protecting brain cells: "In view of the recent work demonstrating presence of dystrophic (senescent) microglia in aged human brain, the purpose of this study was to investigate microglial cells in situ and at high resolution in the immediate vicinity of tau-positive structures in order to determine conclusively whether degenerating neuronal structures are associated with activated or with dystrophic microglia. ... We now report histopathological findings from 19 humans covering the spectrum from none to severe AD pathology, including patients with Down's syndrome, showing that degenerating neuronal structures positive for tau (neuropil threads, neurofibrillary tangles, neuritic plaques) are invariably colocalized with severely dystrophic [microglia] rather than with activated microglial cells. ... [The findings] support the idea that progressive, aging-related microglial degeneration and loss of microglial neuroprotection [contributes] to the onset of sporadic Alzheimer's disease."

Link: http://www.ncbi.nlm.nih.gov/pubmed/19513731