Special Stains
Grouping by chemistry
Special stains demonstrate a specific substance and must be validated with a known positive control. Organize them by mechanism:
- Periodic acid-Schiff (PAS): periodic acid oxidizes vicinal glycols to aldehydes that react with Schiff reagent, staining glycogen, basement membranes, mucin and fungi magenta. A diastase-digested companion section is required to prove glycogen, because diastase digests glycogen and the staining disappears.
- Silver stains: reticulin (black fibers, with gold chloride toning and oxalic acid or thiosulfate removal of unreduced silver), Grocott methenamine silver (GMS) for fungi after chromic acid oxidation, and Fontana-Masson for melanin.
- Trichromes: Masson stains collagen blue or green and muscle red; Gomori is comparable.
- Iron: Perls Prussian blue demonstrates hemosiderin; use liver or spleen as the control.
- Amyloid: Congo red with apple-green birefringence under polarized light.
- Microorganisms: Brown-Hopps Gram, Ziehl-Neelsen for acid-fast bacilli, and the Fite modification (peanut oil) for Mycobacterium leprae, which preserves the waxy capsule.
Traps
Omitting the diastase control; using an acid-decalcified control for a silver stain; forgetting that GMS requires oxidation, since no oxidation means no staining; reporting a stain without a positive control; confusing mucicarmine (epithelial mucin) with alcian blue (acid mucins, pH dependent).
Rule of thumb: No control, no result. Pair every positive special stain with a positive tissue control and, where relevant, a digestion or absorption negative control.