The various epithelia of the gastrointestinal tract separate the outside intraluminal environment that often contains aggressive microorganisms and bioactive chemicals (e.g., bile salts, enzymes, microbial toxins, microbial metabolites, and toxic chemicals, etc.) from the internal milieu (Figure 12.2). The secretion by goblet cells of specific glycoproteins, trefoil peptides, and phospholipids forms the hydrophobic mucus, a gel-like layer that strengthens the wall-barrier functions of these epithe-lia.16 The mucus is not only a physical barrier but also a biological barrier, because the oligosaccharides of these glycoproteins bind with the bacterial lectins.17 Moreover, it prevents the adherence and subsequent translocation of bacteria across the epithelial wall.18 The filamentous brush-border glycocalix, composed of membrane-anchored glycoproteins that are present at the top of the intestinal microvilli, also plays the role of a barrier because it is impermeant to macromolecules or microorganisms.1920
The wall-barrier functions of the gastrointestinal tract require not only integrity of the epithelial tissues but also production of the right type, the right combination, and the right quantity of various mucins, trefoil peptides, and phospholipids to form the most effective mucus layer. The hydrophobicity of the mucus layer is higher in the stomach and the colon than in various segments of the small intestine where most of the absorption processes take place.21
For most health-beneficial molecules (essentially nutrients, vitamins, minerals, and eventually phytochemicals), the transfer through the gastrointestinal epithelia in the small intestine and (at least partly) in the large bowel involves very specific and active transport processes, but passive diffusion also exists that is usually limited to water, some ions (e.g., Mg2+ in the colon, see Chapter 10, Section 10.3.1), small, mostly lipohilic, molecules, and very small oligomers.22 Such diffusion involves either a free paracellular passage through intercellular spaces or a transcellular process that often includes a metabolic transformation of the crossing molecules. The passage of toxic (mostly lipophilic) molecules is mostly due to passive diffusion, but, in some cases, those that are structurally similar to physiologically active molecules can make use of their specific transport systems. The passage of microorganisms (bacteria, fungi, and amoebae, etc.) requires a process of translocation that is not possible when the intestinal mucosa is normal and becomes possible only in the case of malnutrition or a diseased state.
In the large bowel, but most likely also in the small intestine, such functions are dependent on a balanced composition of the microflora that colonize these parts of the gastrointestinal tract (see Chapter 9, Section 9.1).
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