Why do neurons turn black in Golgi staining? I am learning about the Golgi staining of neurons. I know that to this day it is unknown why some but not all neurons in a tissue get stained. I have read online that to harden the neural tissue, Golgi submersed samples for up to 45 days in a 2.5% potassium dichromate solution. He then soaked the samples in a 0.5-1% silver nitrate solution for different amounts of time to dye some of the cells black. I have read a number of online sources about Golgi staining but none of them explain why neurons turn black. Is it a chemical reaction between potassium dichromate and silver nitrate that turns the neurons black? Any insights are appreciated.

The black/brown color is because silver chromate is in the cells.

As you say, the precise mechanism is unknown but this is more about why exactly silver preferentially precipitates within neurons by this procedure, and not other cell types, and also importantly why it does so _so sparingly_ \- the key to the Golgi method is that only a minority of all neurons actually get labeled, and get labeled very strongly. If all neurons and processes would be labeled, you would not be able to see any of the details in a single cell.

Pasternak, J. F., & Woolsey, T. A. (1975). On the “selectivity” of the Golgi‐Cox method. Journal of Comparative Neurology, 160(3), 307-312.

Scheibel, M. E., & Scheibel, A. B. (1978). The methods of Golgi. Neuroanatomical research techniques, 89-114.

Špaček, J. (1992). Dynamics of Golgi impregnation in neurons. Microscopy research and technique, 23(4), 264-274.