AB/PAS - Alcian Blue and PAS
Alcian Blue and PAS (Periodic Acid–Schiff) are two widely used histochemical stains for identifying carbohydrate-rich structures in tissue. They can be applied individually or combined, depending on whether the goal is to detect acidic mucins, neutral carbohydrates, or to differentiate between the two.
How it works
Alcian Blue is a basic dye that binds to negatively charged (acidic) carbohydrate components. The result is a blue staining pattern of acidic mucins, including sulfated and carboxylated glycosaminoglycans (GAGs), sialomucins, and sulfomucins, with staining intensity depending on tissue type and pH conditions.
The PAS (Periodic Acid–Schiff) reaction relies on two sequential chemical steps: oxidation by periodic acid followed by color development with the Schiff reagent. Periodic acid oxidizes carbohydrate structures to aldehydes, which then react with the Schiff reagent to produce the characteristic magenta/purple staining pattern. PAS highlights neutral carbohydrates such as glycoproteins, glycogen, neutral mucins, and basement membrane components.
When Alcian Blue and PAS are combined on the same section, they allow differentiation across a wide range of carbohydrate‑rich components. Typically, Alcian Blue identifies acidic mucins, while PAS highlights neutral mucins and glycogen. Together, they provide a powerful overview of mucin composition and distribution, supporting interpretation of mucus‑producing tissues across species and organ systems.
In AB–PAS stained intestine of Atlantic salmon, the brush border and basement membrane appear magenta due to PAS‑positive neutral carbohydrates, while mucous cells show differentiated coloration based on the charge of their glycans, acidic mucins staining blue with Alcian Blue and neutral mucins staining magenta with PAS.
Section through the buccal cavity of Atlantic salmon fry shows clear regional differences in mucous cell chemistry. Mucous cells in the developing oesophagus stain blue with Alcian Blue, indicating predominantly acidic mucins, while mucous cells in the stomach stain purple with PAS, reflecting neutral mucins and glycoproteins. The supporting cartilage stains bright blue, consistent with its acidic glycosaminoglycan content, whereas developing bone appears purple due to PAS‑positive carbohydrate components within the matrix.
Mammals
In mammals, AB/PAS is useful for detecting shifts in acidic vs neutral mucins across GI diseases such as colitis, intestinal metaplasia, Barrett's esophagus, and mucin‑producing tumors. These conditions often change the balance between blue acidic mucins and magenta neutral mucins long before architecture is visibly altered. AB/PAS therefore helps pathologists see early biochemical changes in goblet cells, epithelial stress, and secretory patterns that support diagnosis and disease classification.
Fish
In fish, AB/PAS is a sensitive indicator of mucosal health because mucous cells respond rapidly to diet, infection, parasites, and environmental stress. GI conditions like enteritis, parasitic infections, and feed intolerance often shift mucin profiles, while in Atlantic salmon the gills and skin show characteristic changes during AGD, sea lice exposure, bacterial dermatitis, and general epithelial stress. AB/PAS highlights these shifts clearly, showing where acidic mucins increase as a defense response or where neutral mucins decrease during damage.
Atlantic salmon gill with complex gill disease. The section shows a predominance of neutral mucous cells, indicating a shifted balance between neutral and acidic mucins. Neutral mucins are more hydrophobic, causing mucus to adhere to the tissue surface and remain during processing.
References:
Bancroft's Theory and Practice of Histological Techniques, 8th Edition.
Zaretsky JZ, Wreschner DH. Gel‑Forming and Soluble Mucins.
