Mucins are a group of highly glycoslated proteins with lubricating and visco-elastic properties that function in mucosal defence and have roles in the inflammatory response. At least twenty four are known in man.
Broadly speaking, they can be divided into epithelial and stromal mucins.
Epithelial mucins, as the name suggests, are produced by epithelial surfaces, typically the GI and respiratory tracts. Histological stains can be used to further characterise epithelial mucins by their biochemical properties. 'Acid' mucins are Alcian blue positive, while 'neutral' mucins are PAS positive. Acid mucins can be further sub-divided according to composition into sialomucins and sulfomucins. Acidity and neutrality are determined by the presence of carboxylate or sulfonate residues attached to the protein core. At physiological pH, these side groups are ionised, imparting a negative charge. In contrast, neutral mucins have no net charge.
Stromal mucins are present in connective tissue, contributing a structural role. Some prefer to refer to these as proteoglycans and avoid the term 'mucin' for these. They differ biochemically in that they are composed mainly of hyaluronan. On microscopy, these have a pale blue appearance sometimes referred to as myxoid.