Alzheimer discovered the disorder that bears his name in 1906 when he reported the case of a 51-years-old woman (Auguste D.) with presenile dementia with characteristic changes in the cerebral cortex (Alzheimer, 1907, 1911). Alzheimer’s disease (AD), which is the most common cause of dementia, is characterized by a slow, progressive decline of cortical functions, particularly cognition and memory. Terry and Davies (1980) pointed out that the presenile form - with onset before age 65 - is identical to the most common form of senile dementia. Thus, the terms AD-type dementia, or probable AD are used for the clinical designation of  both, presenile and senile cases. Confocal image.

The pathological hallmarks of AD consist of a marked cortical atrophy, accumulation of senile plaques (known also as argyrophylic or neuritic plaques), neurofibrillary tangles and neuropil threads in the cerebral cortex. The occurrence of senile plaques was first reported by Blocq and Marinesco (1892) and the characteristic fibrillary changes in neuronal cells were first described and illustrated by Alzheimer (1907). Recently, particularly from the use of a Gallyas silver technique, the accumulation of neuropil threads or curly fibers was recognized as a characteristic cortical lesion in AD (Gallyas, 1971).

Fibrillary amyloid substance accumulates in senile plaques, but also in leptomeningeal and cortical vessel walls (Glenner and Wrong, 1984:, Kang et al., 1987). The major subunit of the amyloid fibrils is a small self-aggregating peptide. This 4.2-kDa peptide was designated beta amyloid peptide (Aβ) because of the beta-pleated sheet structure of fibrillary Aβ. Aβ is derived by proteolytic cleavage from a larger, transmembrane amyloid beta precursor protein (APP), which is expressed in a variety of tissues (Kang et al., 1987). APP contains features characteristic of glycosylated cell-surface receptors and revealed to be a proteoglycan core protein (Schubert, 1988).
Neurofibrillary tangles contain paired helical filaments (PHFs). The major constituants of these PHFs is the microtubule-associated protein tau. In AD, tau is hyperphosphorylated in PHFs, which abolishes its ability to bind microtubules and promote microtubule assembly. The pathomechanism of Aβ and tangle formation is still unclear.