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Senile plaques (also known as neuritic plaques) are extracellular deposits of amyloid beta in the grey matter of the brain. Degenerative neural structures and an abundance of microglia and astrocytes can be associated with senile plaque deposits. These deposits can also be a byproduct of senescence (ageing). However, large numbers of senile plaques and neurofibrillary tangles are characteristic features of Alzheimer's disease. Abnormal neurites in senile plaques are composed primarily of paired helical filaments, a component of neurofibrillary tangles. The plaques are variable in shape and size, but are on average 50 µm in size. In Alzheimer's disease they are primarily composed of amyloid beta peptides. These polypeptides tend to aggregate and are believed to be neurotoxic.
In 1892 Paul Blocq and Gheorghe Marinescu first described the presence of plaque deposits in grey matter. As a result of their similarity to actinomyces druses, they were called druse necrosis by Oskar Fischer during the early 20th century. The connection between plaques and dementia was discovered by Alois Alzheimer in 1906. By 1911 Max Bielschowsky proposed the amyloid-nature of plaque deposits. The same year Teofil Simchowicz introduced the term senile plaques. Wisniewski coined the term neuritic plaques in 1973. The second half of the 20th century saw proposed theories of immunological and genetic factors in plaque formation. Statistical investigations were performed by J.A.N. Corsellis and M. Franke in the 1970s. M. Franke showed that a demential disease is likely to occur when the number of senile plaques in the frontal cortex is more than 200/mm3. By 1985 beta amyloid formations were successfully identified through biochemical techniques, though many unsolved questions about the importance and formation of senile plaques remained.
Formation and description
Concentrated in the synapse of neurons is an amyloid precursor protein (APP) that extends across the plasma membrane. A portion of this protein contains an Aβ peptide sequence. This region comprises part of the DNA segment spanning APP that codes for proteins.
In the formation of senile plaques, Aβ is cleaved from the amyloid precursor protein. Although three enzymes can process APP, only β-(beta) and γ-(gamma) secretase are directly involved in the formation of senile plaques. β-secretase (BACE) is a protease enzyme that cleaves proteins and peptides. This protease cleaves APP to further expose the carboxyl terminal fragments of Aβ. After the bulk of APP is released by this process, γ-secretase cleaves the remaining carboxyl fragments on the transmembrane domain. The sequential actions of γ-secretase following BACE activity results in Aβ protein fragments (amyloid beta) that are released into the extracellular space. Eventually, the accumulation of amyloid beta outside of neuronal synapses forms an aggregate mass of plaque that pushes aside fine ramifications and connections of brain cells.
The formation of amyloid beta fragments is a normal occurrence in the human body. However, the accumulation of Aβ leading to the formation of plaque deposits is an unusual phenomenon. Although the cause of this abnormality is largely unknown, research has revealed certain species of amyloid beta that differ by a few amino acids in length that readily aggregate into plaques. These species are more pathogenic.
Senile plaques are visible in light microscopy using the staining techniques of silver, Congo red, Thioflavin, cresyl violet, and PAS-reaction. Plaque deposits are also visible through fluorescence and immunofluorescence microscopy. Bielschowsky silver staining was found to be the least efficient means of visualizing senile plaques whereas Campbell and Gallyas silver staining techniques showed improved identification and accuracy. Using Congo red, only one-fourth of senile plaques present were noticeably stained. Senile plaques may also be visualized with an immunostain directed against beta amyloid. The primary problem with the above staining methods is that sections of the neural tissue must be excised from the brain in order to accurately identify senile plaques. This is problematic because removing neural tissue can be as detrimental as the actual plaques themselves.
An important criterion of the neuropathological-histological verification of Alzheimer’s disease is the formation of senile plaques. Beta amyloid peptides associated with senile plaques have been found to play a central role in the risk, onset, and progression of Alzheimer's disease as well. Of particular importance is the longer species of beta amyloid known as beta amyloid 42. Elevated levels of beta amyloid, as well as an elevation in the ratio of beta amyloid 42 to the shorter major species of beta amyloid 40, has been identified as important in early events in the pathogenesis of Alzheimer's Disease.
Other factors in Alzheimer's disease verification include pathological neurofibrillaries, tangles, and atrophic brain with hydrocephalus. The formation and the distribution of the pathological neurofibrillaries have a regularity and allows one to determine the stage of the disease. In combination with the occurrence of a great number of plaques, Alzheimer’s disease can be diagnosed with high probability. A correlation between the presence of senile plaques and Down syndrome has also been verified.
Senile plaques can be found in human and animal brains (e.g. mammals and birds). From an age of 60 years (10%) to an age of 80 years (60%), the proportion of people with senile plaques increases linearly. A small proportion of plaques that form can be due to the physiological process of aging. Women are slightly more likely to have plaques than men. The plaques occur commonly in the amygdaloid nucleus and the sulci of the cortex of brain.
An increasing variety of compounds that reduce beta amyloid levels are being identified. Several of these compounds have beta amyloid 42 selectivity whereas others attempt to modulate amyloid precursor protein. Such compounds are now reaching clinical evaluation to determine how they may be of benefit in the treatment of Alzheimer's disease.
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