PAS Reaction – Histochemical Staining Method
The PAS reaction is a histochemical staining method used in pathology to visualize polysaccharides and glycoproteins in tissue sections under the microscope.
Things worth knowing about "PAS Reaction"
The PAS reaction is a histochemical staining method used in pathology to visualize polysaccharides and glycoproteins in tissue sections under the microscope.
What is the PAS Reaction?
The PAS reaction (Periodic Acid-Schiff reaction) is a classic histochemical staining technique widely used in medical diagnostics and pathology. It is designed to highlight specific carbohydrate-containing structures – including polysaccharides, glycoproteins, glycolipids, and mucopolysaccharides – in tissue sections viewed under a light microscope. Structures that react positively appear in a characteristic magenta to red-violet color, while cell nuclei are counterstained in blue or black.
Principle and Mechanism of Action
The PAS reaction is based on a two-step chemical process:
- Oxidation with periodic acid: In the first step, periodic acid (HIO₄) oxidizes vicinal diol groups (adjacent hydroxyl groups) present in carbohydrate molecules, converting them into aldehyde groups.
- Reaction with Schiff reagent: In the second step, the newly formed aldehyde groups react with Schiff reagent (fuchsin-sulfurous acid), producing a vivid red to violet color complex that makes carbohydrate-rich structures visible.
Structures lacking carbohydrate moieties – such as pure proteins or lipids without sugar components – do not react and remain unstained.
Diagnostic Applications
The PAS reaction is employed across numerous medical specialties to analyze tissue structures and diagnose various conditions:
Renal Pathology
In renal pathology, the PAS reaction is a standard staining technique. It highlights glomerular basement membranes, the mesangium, and tubular structures, supporting the diagnosis of conditions such as diabetic nephropathy, lupus nephritis, and various forms of glomerulonephritis.
Liver and Biliary Tract Diagnostics
In liver tissue, the PAS reaction is used to detect glycogen in hepatocytes and to identify PAS-positive inclusion bodies, as seen in alpha-1-antitrypsin deficiency. These inclusions appear as distinctly stained red granules within liver cells.
Mycology and Infectious Disease Diagnostics
The PAS reaction is highly effective for detecting fungi in tissue sections, as fungal cell walls are rich in polysaccharides (e.g., chitin, glucan). Fungal hyphae and spores stain strongly PAS-positive, facilitating the diagnosis of fungal infections such as aspergillosis, candidiasis, and histoplasmosis.
Gastrointestinal Pathology
In the gastrointestinal tract, the PAS reaction aids in visualizing mucus-secreting cells and goblet cells, and plays a key role in diagnosing conditions such as Whipple disease, characterized by PAS-positive macrophages in the small intestinal mucosa.
Pulmonary Diagnostics
In lung pathology, the PAS reaction is used to detect conditions such as pulmonary alveolar proteinosis, in which PAS-positive lipoproteinaceous material accumulates within the alveoli.
Staining Procedure
The staining is performed on paraffin-embedded or cryostat sections of fixed tissue. A typical protocol includes the following steps:
- Deparaffinization and rehydration of the tissue sections
- Oxidation in aqueous periodic acid solution (approximately 5–10 minutes)
- Rinsing in distilled water
- Incubation in Schiff reagent (approximately 15–30 minutes in the dark)
- Rinsing in running tap water (develops the color)
- Counterstaining of cell nuclei, e.g., with hematoxylin
- Dehydration and mounting
Interpretation of Results
When evaluating a PAS-stained tissue section, the following applies:
- PAS-positive (red-violet): Glycogen, glycoproteins, mucins, fungal structures, basement membranes, certain inclusion bodies
- PAS-negative (unstained): Pure proteins, lipids without carbohydrate components, collagen fibers
- Cell nuclei: Blue-violet (due to counterstaining)
A control reaction using diastase (amylase) prior to PAS staining allows differentiation between glycogen and other PAS-positive substances. Glycogen is digested by diastase and will no longer stain PAS-positive after diastase pretreatment, a method known as the D-PAS reaction.
Clinical Significance
The PAS reaction is one of the most frequently used special stains in histopathology. It provides important diagnostic information that cannot be obtained by conventional hematoxylin and eosin (H&E) staining alone. Due to its broad applicability and relatively straightforward execution, it remains an indispensable tool in modern tissue diagnostics.
References
- Carson F. L., Hladik C.: Histotechnology – A Self-Instructional Text, 3rd edition. American Society for Clinical Pathology Press, Chicago 2009.
- Kumar V., Abbas A. K., Aster J. C.: Robbins and Cotran Pathologic Basis of Disease, 10th edition. Elsevier, Philadelphia 2020.
- Lüllmann-Rauch R., Asan E.: Taschenlehrbuch Histologie, 6th edition. Thieme, Stuttgart 2019.
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