Special Stains
Choose the stain for the question
PAS stains glycogen, basement membranes, and fungi magenta; always run a diastase-digested companion, since glycogen is removed and anything still staining is non-glycogen. Masson or Gomori trichrome gives collagen blue or green with muscle and cytoplasm red - used for fibrosis and muscle. Reticulin (silver) demonstrates black argyrophilic fibers and demands chemically clean glassware with controlled, light-protected development. Elastic stains (Verhoeff–van Gieson: elastic black, collagen red, muscle yellow; or orcein) show elastosis and vascular architecture. Congo red demonstrates amyloid, confirmed by apple-green birefringence under polarized light. Prussian blue shows hemosiderin iron; Fontana–Masson shows melanin; Alcian blue at pH 2.5 shows acid mucins; mucicarmine stains epithelial mucin and the capsule of Cryptococcus. Brown–Hopps or Brown–Brenn for Gram, Ziehl–Neelsen or Fite for acid-fast bacilli, and Giemsa for Helicobacter and mast cells.
Traps
A stain result without a control is not a result: each special stain needs a positive control, plus a negative control when background is the question. Pigments are a classic pitfall - a brown deposit may be hemosiderin, melanin, lipofuscin, bile, or formalin pigment, and each requires a different stain. Strong acid decalcification often ruins silver stains, and over-staining can mimic true positivity.
Rule of thumb: if a question mentions polarization, think Congo red for amyloid (and occasionally formalin pigment).