When mercuric chloride is incorporated into fixatives one of the common effects is the presence of dark brown to black crystals in the tissue. These can be seen in the final sections and interfere with the appearance quite noticeably.
The crystals are likely a mercury salt and can be easily removed with a solution of iodine. This must be done with all tissues which have been fixed in a solution containing mercuric chloride. Although it is sometimes said that Heidenhain's SuSa does not produce mercury pigment, this cannot be relied upon.
The iodine may be applied during the dehydration step, but removal may only be partial and all sections should be treated with the iodine-thiosulphate sequence as a matter of course.
It has been suggested that Gram's iodine may remove some material from sections other than the mercury pigment, and that alcoholic iodine solutions should be used. This claim is unsubstantiated and is generally ignored, although alcoholic iodine works very well to remove the pigment.
Other than removing the mercury pigment, there is no affect on the tissue.
Removing mercury pigment
|Strong alcoholic iodine||0.5% alcoholic iodine|
|Absolute ethanol||1||L||70% ethanol||1||L|
|Iodine saturates at about 21.5% in ethanol.|
|Gram's iodine||Sodium thiosulphate|
Mix the iodine and iodide together.|
Add 25 mL of the water.
Dissolve the solids
Add the rest of the water.
Kiernan. J.A., (1999)
Histological and histochemical methods: Theory and practice, Ed. 3
Butterworth Heinemann, Oxford, UK.
Drury, R.A.B. and Wallington, E.A., (1980)
Carleton's histological technique Ed. 5
Oxford University Press, Oxford, UK.