Histochemical stains are used in pathology and histology to highlight specific structures, cellular components, or chemical substances in tissues. They play a vital role in diagnosis by differentiating between various tissue types, infections, and disease processes.
Hematoxylin and Eosin (H&E) Stain
H&E stain is the most commonly used stain in histopathology.
It provides an overview of tissue architecture and cellular morphology.
Structures Stained by H&E
| Component | Stain Used | Color | Structures Stained |
|---|---|---|---|
| Hematoxylin | Basic dye | Blue to purple | Nuclei, nucleoli, bacteria, and calcium |
| Eosin | Acidic dye | Pink to red | Cytoplasm, collagen, fibrin, RBCs, thyroid colloid |
Mechanism:
- Hematoxylin binds to negatively charged structures (basophilic).
- Eosin binds to positively charged structures (acidophilic).
Special Histochemical Stains
Certain diseases require special stains to highlight specific tissue components, pigments, or microorganisms not visible with routine H&E.
Important Special Stains and Their Diagnostic Use
| Stain | Target Substance | Color Reaction | Associated Diseases / Diagnostic Use |
|---|---|---|---|
| Prussian Blue | Iron (Fe³⁺) | Blue deposits | Hemochromatosis, Hemosiderosis |
| Congo Red | Amyloid | Apple-green birefringence under polarized light | Amyloidosis |
| Acid-Fast (Ziehl-Neelsen, Fite) | Mycolic acid in cell walls | Red rods on a blue background | Mycobacterium tuberculosis, M. avium complex infections |
| Periodic Acid–Schiff (PAS) | Carbohydrates (Glycogen, Mucopolysaccharides) | Magenta (pink) | Candida infection, α₁-antitrypsin deficiency (hepatocytes), glycogen storage diseases, diabetic nephropathy (Kimmelstiel–Wilson nodules) |
| Trichrome (Masson or Mallory) | Connective tissue, Type IV collagen | Blue or green fibers | Cirrhosis, crescentic glomerulonephritis |
| Reticulin (Silver stain) | Type III collagen (reticular fibers) | Black network | Cirrhosis, myelofibrosis |
Clinical Relevance of Special Stains
- Prussian Blue: Detects iron overload in tissues (liver, spleen, bone marrow).
- Congo Red: Essential for diagnosing amyloidosis—seen as apple-green birefringence under polarized light.
- Acid-Fast Stains: Identify acid-fast bacilli (AFB) in tuberculosis and atypical mycobacterial infections.
- PAS Stain: Highlights glycogen and basement membrane thickening (important in diabetes and fungal infections).
- Trichrome & Reticulin: Evaluate fibrosis and collagen deposition, commonly used in liver and bone marrow biopsies.
Learning Objective
By the end of this topic, the medical student should be able to:
- Identify key histochemical stains and their target structures.
- Recognize diseases associated with positive special stains.
- Correlate the histological appearance with the underlying pathology for diagnostic interpretation.
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