III. THE INTESTINAL MUCUS BLANKET 1. Effects of Specific Bacteria on Mucus Secretion

Mucus is secreted throughout the gastro-intestinal tract by a variety of specialised cells located in glands or in the surface epithelium. The prime func­ tion of the mucus layer is to protect the delicate underlying mucosa from dam­ age by potentially harmful agents in the lumen (such as digestive enzymes and bacteria) and by the mechanical forces associated with digestion and the pas­ sage of solid material through the gut. Mucus exists in two distinct physical forms: 1. a thin layer of stable, water­ insoluble gel firmly adhering to the mu­ cosal surface and 2. a soluble layer mixed with luminal material that overlies the gel. The soluble mucus, although very viscous, can be removed from the mucosa by gentle washing. Both the adherent gel and the soluble mucus are composed of mucus glycoproteins (or mucins) which are responsible for the characteristic visco-elastic and gel­ forming properties of the secretion. The relationship between the soluble and gel forms of mucus has not yet been clarified but, since proteolytic enzymes are capable of solubilising mucus gel and degrading mucus glycoproteins, it is presently believed that the sol layer is likely derived from the gel by a com­ bination of enzymatic and mechanical erosion (Allen and Carroll, 1985). Therefore, to maintain an intact mucus barrier, erosion from the luminal surface must be balanced by secretion. In mucosal sections fixed and stained for light or electron microscopy, the ad­ herent mucus layer on the epithelial sur­ face is difficult to visualise and often appears discontinuous. This is because many of the fixatives used, such as ethanol and glutaraldehyde, dehydrate the mucus causing denaturation and shrinkage of the gel. However, gastric mucus can readily be observed in situ on unfixed mucosal sections mounted transversely under the light microscope. The gel appears as a thin but continuous translucent layer of varying thickness (50 to 450 μm in man) between the sur­ face epithelium and the luminal solution (Allen and Carroll, 1985). In the intes­ tine and colon, a mucus 'blanket' (30 to 400 μm thick, dependent on species and region of the gut) lines the crypts and covers the villi and surface mucosa (Sa­ kata and Engelhardt, 1981; Rozee et al., 1982). Observation of the continuity of the mucus layer in the intestine and co-