Most epithelia consist of a number of different cell types with different functions. One cell that is common in many epithelia is the mucosal cell, which secretes mucus. An epithelium containing mucosal cells is called a mucosal epithelium or simply mucosa.
Mucus has several functions. It restricts the penetration of large molecules, and prevents the tissue from dehydrating. It keeps the tissue surface clean by its continuous removal, and lubricates the passage of materials such as food through the gastrointestinal tract. Its most important property is its viscoelasticity, which enables it to act as a locally rigid mechanical barrier which can flow under the influence of peristalsis. The primary component of mucus is a large polysaccharide called mucin built up in subunits of 500,000 Daltons or larger. It consists of a protein backbone approximately 800 amino acids long, rich in serine and threonine, which are hydroxylated amino acids. Most of the hydroxy-residues are linked to oligosaccharide side chains which serve to stiffen the backbone, and which carry an extensive layer of water of hydration. The oligosaccharide chains are generally up to 18 residues in length and are composed of N-acetylgalactosamine, N-acetylglucosamine, galactose, fucose or N-acetylneuraminic acid.
Mucus is 95% water and so makes intimate contact with hydrophilic surfaces. Small particles less than ~600 pm (the average thickness of a mucus layer) may be buried in the surface and held securely due the stickiness of the mucus, but since the mucus is continually secreted the particles move further away from the mucosa and are ultimately sloughed off. Small molecules pass easily through mucus due to its high water content; larger molecules diffuse through the mucus more slowly and remain in contact for longer periods.
Many research groups have attempted to develop mucoadhesive materials, the idea being to bind a drug carrier to a mucous membrane in order to optimize drug delivery. However, mucus turnover can be rapid and there seems little point in attaching a drug to a surface which is to be sloughed off in a short time.
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