Answer: The Nissl stain is an imaging technique performed on fixed tissue that stains genetic material.
One of the oldest methods of viewing neurons, the Nissl stain uses cresyl violet acetate or toluidine blue to permanently stain genetic material, changing the color of molecules of DNA and RNA. The DNA is within the nucleus and RNA is in the rough endoplasmic reticulum. The stain is most useful for identifying cytoarchitecture across brain structures (Improved method for combination of immunocytochemistry and Nissl staining).
Some regions of the brain, such as the cortex, are organized in an columnar manner. This organization in the cortex is described in terms of layers, one on top of the next. There are 6 layers of the cortex, and each layer has a unique cell composition. For example, layer 6 often contains the cell bodies of several pyramidal cells, while layer 1 contains several neurites (axons and dendrites). Because layer 6 contains the nuclei of several cells, it will appear significantly more dense in a Nissl stain compared to a layer 1.
The Nissl stain is also very helpful when identifying white matter versus gray matter. By definition, white matter is made up of the axons of neurons, and therefore do not take up the Nissl stain very much. However, gray matter is made up of the cell bodies of neurons, and stain very darkly.
Nissl stain versus Golgi stain
The major disadvantage of using a cresyl violet Nissl stain is that the detailed morphology of the cell is not stained. Axons, dendritic processes, and the majority of the cytoplasm is low in genetic material. Following a Nissl stain, the cell bodies of a densely branched Purkinje cell and a long and thin bipolar cell would appear similar: only the nucleus and rough endoplasmic reticulum would take up the stain. A Golgi stain will be better for visualizing the morphology of a cell, since the Golgi stain is taken up by every part of the neuron. Therefore, the Golgi stain can see the dendrites and axons of the neuron, while the Nissl stain will not.
Uses of the Nissl stain
With a cresyl violet Nissl stain, the nucleic acid appears dark purple-blue while the areas with low nucleic acid density appears light blue-gray.
The Nissl stain is often used when viewing the anatomical features of the brain. For example, the major white matter tract of the brain, the corpus callosum, will not take up much Nissl stain, and will therefore appear pale. This can be used to help identify where structures in the brain are located.
The Nissl stain is also one of the most histological tools for visualizing the spinal cord in cross section, especially because the spinal cord has very well defined gray matter versus white matter areas.
The dye itself is a base, which binds to nucleic acids.
The stain was named after the German neuropathologist Franz Nissl. “Nissl substance” is an older term for the endoplasmic reticulum, which is where the name comes from. “Nissl bodies,” on the other hand, refer to the free ribosomes.
