Senile plaques are extracellular

Extracellular

In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...

 deposits of amyloid

Amyloid

Amyloids are insoluble fibrous protein aggregates sharing specific structural traits. Abnormal accumulation of amyloid in organs may lead to amyloidosis, and may play a role in various neurodegenerative diseases.-Definition:...

 in the gray matter

Gray Matter

"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...

 of the brain. The deposits are associated with degenerative neural structures and an abundance of microglia

Microglia

Microglia are a type of glial cell that are the resident macrophages of the brain and spinal cord, and thus act as the first and main form of active immune defense in the central nervous system . Microglia constitute 20% of the total glial cell population within the brain...

 and astrocytes. Large numbers of senile plaques and neurofibrillary tangles are characteristic features of Alzheimer’s disease, and some of the 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 the average 50 µm in size.