The intestinal mucosa is composed of three layers: the innermost muscularis mucosae, lamina propria, and the epithelial layer which is exposed to the lumen. Besides being the structural support for the epithelium, the lamina propria contains elements important for immunological and absorptive functions. Thus in the lamina propria we find blood and lymph vessels, nerve varicosities, and a plethora of immune cells (plasma cells, lymphocytes, macrophages, eosinophils, mast cells, dendritic cells, etc).1 The lamina propria is densely innervated via both intrinsic (mainly from the submucosal plexus) and extrinsic nerve fibres. From the lamina propria, chemical messengers from nerves and immune cells reach and act on epithelial cell receptors.

The intestinal epithelium is a single layer of columnar cells. The most common cells are the enterocytes (absorptive cells). However, a rich variety of cells are also present, including goblet cells (secrete mucus), endocrine cells (secrete hormones and neuropeptides), Paneth cells, caveolated (tuft) and cup cells, M cells, stem cells (crypt), and intraepithelial lymphocytes. Epithelial cells are bound together by junctional complexes in the apical membrane1: the tight junction (zona occludens), the intermediate junction (zonula adherens), and the desmosomes (macula adherens). These structures define the paracellular space.

The epithelial lining serves several functions in the intestinal tract.2 The junctional complex acts as a physical barrier limiting the uptake of microbial and food derived antigens. However, in certain conditions macromolecules and even cells (mainly neutrophils) may be able to cross it to reach the lamina propria or the luminal surface, respectively. The presence of various enzymes on the apical surface and within epithelial cells contributes to the digestion and absorption of nutrients as well as to the degradation of immunogenic particles. Absorption and secretion of water and electrolytes is regulated by the epithelium. The epithelium maintains its own integrity …