1. Intestinal epithelial barrier.
Intestinal epithelial barrier. The intestinal epithelium consists of a single layer of five phenotypes of highly polarized epithelial cells located at the crypts and villi. The crypts contain single progenitor stem cells which divide and differentiate in each intestinal cell phenotype during the crypt-villous cell migration, which is responsible for epithelial cell renewal after cell death and shedding of mature epithelial cells at the tip of the villus. Enterocytes, which constitute the most abundant intestinal cell phenotype, express a dense brush border composed of well-ordered microvilli facing the luminal compartment, in which are located hydrolases and transporters involved in absorption and secretion transcellular transporters, whereas others transporters are specifically localized at the lateral or basal membrane domains. Goblet cells produce brush border membrane-bound mucins and secreted mucins intracellularly packaged with large vesicles (yellow vesicles) and which after exocytosis into the luminal compartment form a thick mucus layer overlying the epithelium. Paneth cells located at the crypt of intestinal villi contain vesicles containing antimicrobial peptides and proteins (red vesicles), which after secretion into the luminal compartment exert a bactericidal effect against enteric bacterial pathogens. Enteroendocrine cells express intracellular secretory granules containing hormones and peptides (black vesicles), which after exocytosis into the interstitial space at the basal cell domain exert paracrine and endocrine functions. Yellow, red, and black arrows indicate the vectorial exocytosis processes of vesicles or granules. In enterocytes, brown arrows indicate the F-actin- or microtubule-dependent routes of intracellular traffic of cargo vesicles containing the functional proteins which are specifically vectorized to the apical, lateral, and basal domains of the polarized intestinal epithelial cells.
Vanessa Liévin-Le Moal, and Alain L. Servin Microbiol. Mol. Biol. Rev. 2013; doi: /MMBR