MHS 247 Fundic Stomach

Periodic Acid - Schiff (PAS) Reaction

The Periodic Acid-Schiff (PAS) reaction is a histochemical staining technique developed in the 1940s that detects polysaccharides (such as glycogen), glycoproteins, and other carbohydrate-containing compounds in cells and tissues.

The stomach provides an ideal model for studying mucous cell secretions using PAS staining due to its specialized protective mechanisms and complex architecture designed to withstand the harsh acidic environment.

Protective Mucous Barrier

The stomach's interior surface forms prominent folds that maximize the surface area. These surfaces are lined with Surface Mucous Cells that continuously secrete mucus rich in glycoproteins and bicarbonate ions, creating a pH-neutral microenvironment at the epithelial surface. This alkaline, highly viscous barrier protects the gastric mucosa from both abrasive food particles and erosive gastric acid.

The following pages will examine the stomach stained using the PAS reaction to demonstrate glycoproteins.

Surface Mucous Cells

Surface Mucous Cells demonstrate the classic epithelial cell polarity, which is exquisitely revealed through PAS staining:

• Basal Region:
  • Nuclei are located near the base of the cells
  • Stain blue with hematoxylin used as a counterstain
• Apical Region:
  • Contains numerous secretory granules with the characteristic bright red-pink staining
  • Rich in mucins - large glycoproteins that give mucus its gel-like properties
    • Extensive carbohydrate side chains are attached to serine or threonine residues in the protein backbone
  • Secretes bicarbonate ions to create a pH gradient from acidic luminal contents to neutral conditions at the cell surface

This polarization is especially evident in cross-sections of the gastric pits.

Functional Correlation

The PAS staining technique provides several insights:

• Cell Identification: PAS-positive staining confirms the identity and functional status of the mucous cells
• Cellular Organization: Reveals secretory cell polarity and organization
• Glycoprotein Content: Demonstrates the high glycoprotein content of the protective mucus

Periodic Acid - Schiff (PAS) Reaction

While understanding the underlying mechanism isn't essential for basic histology practice, the staining process is detailed below for those interested in the biochemical principles.

• Periodic Acid-Induced Oxidation: Tissue sections are treated with periodic acid (HIO₄) at room temperature
  • Periodic acid selectively oxidizes compounds containing adjacent carbon atoms with hydroxyl groups (-OH)
  • Oxidation cleaves the carbon-carbon bond, creating aldehyde groups
• Schiff's Reagent Staining:
  • Tissue is treated with Schiff's Reagent, which reacts specifically with the newly formed aldehyde groups
    • Schiff's reagent is prepared by treating Fuchsin dye (a magenta dye) with sulfur dioxide gas (SO₂) or sodium bisulfite (NaHSO₃)
    • Fuchsin is reduced to form a colorless compound
  • Produces a characteristic magenta (purple-red) color wherever carbohydrates are present
    • Aldehyde groups react with the colorless Schiff's reagent, regenerating the colored form of fuchsin
  • Color intensity is directly proportional to the concentration of reactive aldehyde groups

This reaction is highly specific for polysaccharides and glycoproteins because the periodic acid oxidation requires adjacent carbon atoms with hydroxyl groups, which are present in all carbohydrates.