Figure 1.

The intestinal epithelial-cell barrier. Simple columnar epithelial cells exhibit physical and biochemical adaptations to microbial colonization to maintain barrier integrity including actin-rich microvillar extensions (a), epithelial-cell tight junctions (b), apically attached and secreted mucins that form a glycocalyx (c) and the production of various anti microbial peptides (d). Specialized intestinal epithelial cells known as M (microfold) cells overlie Peyer’s patches and lymphoid follicles to facilitate luminal sampling. M cells exhibit reduced mucin secretion and have modified apical and basolateral surfaces (e) to promote uptake and transport of luminal contents to professional antigen-presenting cells that inhabit the subepithelial dome (SED) of the Peyer’s patches and lymphoid follicles (f). Specialized dendritic cell (CD) subsets can also extend dendrites between the tight junctions of intestinal epithelial cells to sample luminal contents (g). (Reproduced with permission from reference 16.)