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All mucosal surfaces are covered by a layer of mucus which hydrates, lubricates and protects the underlying epithelium. Endoscopists will be highly familiar with the shiny mucus layer covering the intestine. However, because of its intrinsic properties, particularly its high water content, mucus is difficult to preserve and measure, is lost in standard histological processing and is often overlooked in studies of intestinal pathophysiology.
The major macromolecular constituents of mucus are secreted mucin glycoproteins, and in the normal intestine this is, almost exclusively, MUC2. MUC2 is synthesised in intestinal goblet cells and packed into granules as large disulfide linked polymers. These granules form the distinctive goblet shaped thecae which is the defining morphological feature of goblet cells. On release and hydration, the mucin polymers swell 100–1000-fold, forming the viscous mucus barrier. While the mucins are largely responsible for the properties of mucus, mucus contains many other proteins, including antimicrobial molecules and antibodies. In the rodent colon, the mucus layer is around 700 µm thick, with an inner difficult to dislodge layer and an outer layer that is looser and easier to displace. Gunnar Hansson's group, from the University of Gothenburg, have previously shown that the inner mucus layer is devoid of microbes, effectively separating the luminal microbes from the epithelial cells.1 …