Figure 2.

Anatomy of the intestine. Epithelial proliferation is initiated in the stem cells and is restricted to the crypt. Epithelial cells migrate upwards to the villus (enterocytes, goblet cells, enteroendocrine cells) or downwards to the crypt base (Paneth cells) where they differentiate. The epithelium is lined by a mucus layer acting as the first line of defense against the greater than 100 trillion commensal bacteria that reside within the gut lumen. The gut has a complex immune system including intra-epithelial lymphocytes and elements of the gut-associated lymphatic tissue (GALT) present in Peyer’s patches.