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PAS; gallbladder

Periodic acid-Shiff (PAS): Chemistry

The PAS staining technique is the most widely used procedure for the demonstration of glycogen and certain glycoconjugates. The mechanism by which the PAS works is as follows:

  • Aldehyde groups are formed from carbohydrates (glycogen) within the tissue section via oxidation after exposure to periodic acid.
  • The tissue section is then incubated in  Schiff reagent, a colorless solution consisting of basic fuchsin, hydrochloric acid, and sodium metabisulfite.
  • Chemical bonding between the aldehyde groups and the Schiff reagent results in glycogen being colored  magenta (bright pinkish-red). 
  • The section is finally rinsed with running water, which excites chemical activity releasing additional chemical groups that intensifies the stain reaction.

Hematoxylin or fast green may be used as counterstains.