Periodic acid–Schiff is a staining method used to detect polysaccharides such as glycogen, and mucosubstances such as glycoproteins, glycolipids and mucins in tissues. The reaction of periodic acid oxidizes the vicinal diols in these sugars, usually breaking up the bond between two adjacent carbons not involved in the glycosidic linkage or ring closure in the ring of the monosaccharide units that are parts of the long polysaccharides, and creating a pair of aldehydes at the two free tips of each broken monosaccharide ring. The oxidation condition has to be sufficiently regulated so as to not oxidize the aldehydes further. These aldehydes then react with the Schiff reagent to give a purple-magenta color. A suitable basic stain is often used as a counterstain. • PAS diastase stain is PAS stain used in combination with diastase, an enzyme that breaks down glycogen. • Alcian blue/periodic acid–Schiff uses alcian blue before the PAS step.
Uses
PAS staining is mainly used for staining structures containing a high proportion of carbohydrate macromolecules, typically found in e.g. connective tissues, mucus, the glycocalyx, and basal laminae. PAS staining can be used to assist in the diagnosis of several medical conditions:
It is used to identify glycogen in lung biopsy specimens of infants with pulmonary interstitial glycogenosis.
It can be used to highlight super cross-linked lipids inclusions in ceroid lipofuscinosis.
Presence of glycogen can be confirmed on a section of tissue by using diastase to digest the glycogen from a section, then comparing a diastase digested PAS section with a normal PAS section. The diastase negative slide will show a magenta staining where glycogen is present within a section of tissue. The slide that has been treated with diastase will lack any positive PAS staining in those locations on the slide PAS staining is also used for staining cellulose. One example would be looking for implanted medical devicescomposed of nonoxidized cellulose. If the PAS stain will be performed on tissue, the recommended fixative is 10% neutral-buffered formalin or Bouin solution. For blood smears, the recommended fixative is methanol. Glutaraldehyde is not recommended because free aldehyde groups may be available to react with the Schiff reagent, which may result in false positive staining.
