Answer: The Golgi stain is a silver-based cell morphology visualization technique for nervous system cells.
The Golgi method was first described and published in 1873 by the Italian scientist Camillo Golgi. The Golgi staining method was used by the Spanish scientist Ramon y Cajal when he proposed the idea that neurons are not all physically connected, but rather distinct, separate units. This concept is called the neuron doctrine.
The major advantage of the Golgi stain is its ability to completely fill out the morphology of the cell. The cell body, axon, and small protrusions on the dendrites are fully labeled by the stain. This allows a morphologist to study the entirety of the structure of the cell. The Golgi stain is a high contrast imaging method, as the cell appears in dark brown or black, and stands out clearly against the background.
Another advantage of the Golgi stain is that it randomly stains a small population of cells, less than 1%. Because neurons are often densely packed in the nervous system, the sparse labeling of cells allows the scientist to pick out a single cell from among the field of cells, and study the shape of that cell.
Although immunostaining is a frequently used technique for analyzing cell populations, the Golgi stain does offer a unique advantage: no specific antibody is needed. With immunocytochemistry, antibodies must be available if a specific cell is to be labeled. The Golgi stain does not depend on any specific immunological tag for cell labeling.
Although sparse labeling of cells can be an advantage in some cases, other times it is less preferred. It can be difficult to get a large amount of meaningful information from Golgi stained samples about the overall composition of a particular brain region. Because only a small fraction of cells are filled, it is less useful for analyzing the composition of a brain region.
Another disadvantage of the Golgi technique is that the results are often difficult to reproduce. Because of the random nature of the Golgi stain, it becomes difficult when scientists try to extrapolate to a larger sample. Generalizations about cell morphology in the entire brain based on Golgi stain samples should be regarded with caution.
The Golgi stain involves exposing the preserved tissue to potassium nitrate and silver dichromate. The chemicals react, and the brownish-red silver chromate molecules form inside the cells. It is not completely understood as to what factors determine which cells take up the stain.
Recent changes in the Golgi staining protocol and a more complete history of the stain can be found here: Comprehensive Review of Golgi Staining Methods for Nervous Tissue