It is composed chiefly of mucins and inorganic salts suspended in water. Mucus-secreting cells are widely distributed through the body. Goblet cells are abundant in the epithelium of the gastrointestinal and respiratory tracts, mucous glands in these same organs deliver their products through ducts into the intestine and respiratory tree, and many of the acinar epithelial cells in salivary glands secrete mucus.
Mucins are a family of large, heavily glycosylated proteins.
Although some mucins are membrane bound due to the presence of a hydrophobic membrane-spanning domain that favors retention in the plasma membrane, the concentration here is on those mucins that are secreted on mucosal surfaces and saliva. A large central region formed of multiple tandem repeats of 10 to 80 residue sequences in which up to half of the amino acids are serine or threonine. This area becomes saturated with hundreds of O-linked oligosaccharides. N-linked oligosaccharides are also found on mucins, but much less abundantly. Mucins are secreted as massive aggregates with molecular masses of roughly 1 to 10 million Da.