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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.

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    $\begingroup$ First result on google for these reactants: when a colorless solution of silver nitrate is mixed with a yellow-orange solution of potassium dichromate, a reddish precipitate of silver dichromate is produced. AgNO3(aq)+K2Cr2O7(aq)→Ag2Cr2O7(s)+KNO3(aq) The black is result of contrast between brightfield backlight and opaque precipitate. $\endgroup$
    – S Pr
    Commented Nov 1, 2021 at 14:39

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Yes, according to Spacek (1992)

the Golgi deposit has been identified as silver chromate by Fregerslev et al. (1971) and Chan-Palay (1973).

I don't know why it's black when in neurons and not dark brown though...

Actually Spacek (p. 265) describes the crystals as dark brown.

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    $\begingroup$ It's more of a dark brown, it's not that easy to tell the difference in a microscope, especially looking against a brightfield - any dark color is going to throw a substantial shadow. $\endgroup$
    – Bryan Krause
    Commented Nov 1, 2021 at 14:25
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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.

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  • $\begingroup$ Out of curiosity, I spent a while trying to find a counterexample of this stain being used in non-nervous tissue. I cannot find anything. It seems to be so widely used for neurons/glia and for nothing else. Curious whether and why the chemistry has been restricted in utility to nervous tissue only... $\endgroup$
    – S Pr
    Commented Nov 1, 2021 at 14:49

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