Figure 1.

Schematic representation of mucosa, villi, crypts of Lieberkühn and cells of the small intestine. The intestinal lining of the lower intestine is highly folded to maximize absorption and contains finger-like mucosal projections that form structures called villi, in between the villi downward invaginations, called crypts of Lieberkuhn, extend down to the muscularis mucosae. Underlying the epithelium, the lamina propria, harbors dendritic cells, important antigen-presenting cells, which regulate humoral and cellular gut immunity. The muscularis externa layer contains two layers of smooth muscle that enable continuous peristaltic activity of the small intestine. On the left, the epithelium of a crypt and part of a villus are represented and different epithelial cells can be identified: enterocytes, tall columnar absorptive cells with ‘brush-like border’ on the apical surface, called microvilli; goblet cells, which secrete mucin, for lubrication of the intestinal contents and protection of the epithelium; enteroendocrine cells that secrete various gut hormones; stem cells that lie near the base of the crypt and give rise to the specialized epithelial cells; above the stem cells are transit amplifying cells; and Paneth cells, which have a defensive function secreting antimicrobial molecules into the lumen.