In Gordon and Sweets (G&S), which solution covers aldehyde groups with Fe ions to prepare for silver binding to retic fibers?

• A. Ferric Ammonium Sulphate
• B. Potassium permanganate
• C. Sodium thiosulfate
• D. Citric acid

Answer: (A)

Aldehyde groups formed during formalin fixation can reduce silver nonselectively, so the staining method uses ferric ions to block or complex these sites before silver binding. Ferric ammonium sulfate provides Fe3+ that interacts with the aldehyde groups, creating ferric–aldehyde complexes and preventing nonspecific reduction. This prepares the tissue to receive silver specifically on the reticular fibers when the silver developer is applied, giving a clear reticulin pattern.

The other reagents serve different roles: potassium permanganate would oxidize tissue further and alter the staining; sodium thiosulfate is used to remove excess or unbound silver; citric acid acts as a buffer/mordant but does not block aldehyde groups.