Histochemical stains for carbohydrates and mucins identify glycogen, neutral and acidic mucins, glycoproteins, and proteoglycans. These stains are essential for diagnosing metabolic storage diseases, identifying tumor types, detecting fungal organisms, and evaluating gastrointestinal and respiratory pathology.

Periodic Acid-Schiff (PAS)

PAS is the most versatile carbohydrate stain. Periodic acid oxidizes vicinal hydroxyl groups (present in glycogen, glycoproteins, and mucins) to aldehydes, which then react with Schiff reagent to produce a magenta color. PAS-positive structures include glycogen (liver, cardiac and skeletal muscle), basement membranes (glomerular and tubular in kidney; vascular), mucus (gastric, intestinal, respiratory epithelium), fungal cell walls, and parasites (Echinococcus, Toxoplasma).

PAS with diastase (PAS-D) uses amylase digestion to remove glycogen before PAS staining. Diastase-sensitive PAS positivity confirms glycogen; diastase-resistant PAS positivity indicates glycoproteins, mucins, or basement membrane material. This distinction is critical — a diastase-sensitive PAS-positive cytoplasmic vacuole contains glycogen (e.g., clear cell renal cell carcinoma), while a diastase-resistant vacuole contains mucin (e.g., signet ring cell adenocarcinoma).

Alcian Blue

Alcian blue is a copper-containing phthalocyanine dye that binds to acidic mucopolysaccharides and sulfated glycosaminoglycans. At pH 2.5, Alcian blue stains both weakly acidic (sialomucins) and strongly acidic (sulfated) mucins blue. At pH 1.0, only strongly acidic (sulfated) mucins are stained. This pH-dependent selectivity helps characterize mucin types.

Alcian blue is used to identify intestinal metaplasia (gastric biopsies — goblet cells stain blue) and to classify mucinous tumors (ovarian, pancreatic, appendiceal). Combined Alcian blue-PAS (AB-PAS) distinguishes neutral (PAS-positive, magenta) from acidic (Alcian blue-positive, blue) mucins in a single section. In lung pathology, AB-PAS demonstrates mucin production in adenocarcinoma; in cartilage, Alcian blue stains proteoglycans.

Mucicarmine

Mucicarmine stains epithelial mucins deep rose to red. It uses an aluminum-carmine complex that binds to acidic mucopolysaccharides. Mucicarmine is most commonly used to demonstrate mucin production in adenocarcinoma and to identify the capsule of Cryptococcus neoformans, which stains intensely red (the organism itself is mucicarmine-negative). It is more specific for epithelial mucins than Alcian blue but less sensitive.

Colloidal Iron

Colloidal iron binds to acidic mucins at low pH and is visualized by Prussian Blue reaction (yellow-brown to blue). It is more sensitive than Alcian blue for detecting small amounts of acidic mucins but technically more difficult. Colloidal iron is used for detecting sialomucins in early intestinal metaplasia and acid mucins in ovarian tumors.

Best’s Carmine

Best’s carmine specifically stains glycogen red. It provides stronger glycogen staining than PAS (which stains many carbohydrate classes) but is used only for glycogen identification due to its limited color range. Best’s carmine is used for confirming glycogen-rich tumors (clear cell carcinoma, Ewing sarcoma, germ cell tumors) when PAS-D results are ambiguous.

Applications in Pathology

Gastrointestinal pathology uses PAS-Alcian blue to classify gastric and intestinal metaplasia, Barrett esophagus, and mucinous carcinomas. Renal pathology uses PAS to evaluate glomerular basement membrane thickening, mesangial expansion, and tubular basement membrane integrity. Liver pathology — PAS-D highlights alpha-1-antitrypsin globules (diastase-resistant, magenta cytoplasmic globules) and Kupffer cell storage material. Infectious disease — PAS and GMS are used together to detect fungal organisms; mucicarmine specifically identifies Cryptococcus. Tumor pathology — mucin stains classify adenocarcinomas (mucinous, signet ring, colloid types) and distinguish them from non-mucinous malignancies. Quality control for carbohydrate stains requires positive controls with known carbohydrate content (liver for glycogen, colon for mucins, umbilical cord for proteoglycans).