A Nissl body, also known as Nissl substance and Nissl material, is a large granular body found in neurons. These granules are of rough endoplasmic reticulum (RER) with rosettes of free ribosomes, and are the site of protein synthesis. It was named after Franz Nissl, a German neuropathologist who invented the Nissl staining method.
Nissl bodies can be demonstrated by a method of selective staining developed by Nissl (Nissl staining), using an aniline stain to label extranuclear RNA granules. This staining method is useful to localize the cell body, as it can be seen in the soma and dendrites of neurons, though not in the axon or axon hillock. Due to RNA's basophilic ("base-loving") properties it is stained blue by this method.
Nissl bodies show changes under various physiological conditions and in pathological conditions such as axonotmesis they may dissolve and disappear (chromatolysis) in an attempt to upregulate protein synthesis for axonal repair and Nissl bodies would reappear after a successful repair. Chromatolysis means disappearance of Nissl bodies.
The ultrastructure of Nissl bodies suggests they are primarily concerned with the synthesis of proteins for intracellular use.