Routine and Special Staining
Routine H&E
Hematoxylin and eosin is the workhorse stain. Hematoxylin is a basic dye that stains acidic structures (nuclei, nucleic acids) blue-purple; eosin is an acidic dye that stains basic proteins (cytoplasm, collagen) pink. Candidates should understand the roles of mordants (hematoxylin needs a mordant such as aluminum to bind), bluing (raising pH to convert hematoxylin to its blue form), differentiation (removing excess dye), and counterstaining.
Special stains and their logic
Special stains target specific substances: PAS for glycogen and basement membranes; trichrome for collagen versus muscle; reticulin for reticulin fibers; Prussian blue for iron; Congo red for amyloid (with apple-green birefringence under polarized light); and silver stains for organisms or fibers. The logic is chemical affinity-know which structure each stain highlights, not just the color.
Traps
Common traps: reversing the acid/base logic of hematoxylin and eosin; forgetting that bluing is a pH step; confusing PAS (glycogen/basement membrane) with trichrome (collagen); and assuming a special stain works without proper controls. Over-differentiation removes too much dye; under-differentiation leaves background.
Worked example
A question asks why nuclei look red rather than blue. The likely cause is inadequate bluing or over-differentiation, not the eosin step-eosin stains cytoplasm pink and would not turn nuclei red unless hematoxylin failed.