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The gastrointestinal epithelium is unique in that cell proliferation, differentiation, and apoptosis occur in an orderly fashion along the crypt-villus axis. The intestinal crypt is mainly a proliferative compartment, is monoclonal and is maintained by stem cells. The villus represents the differentiated compartment, and is polyclonal as it receives cells from multiple crypts. In the small intestine, cell migration begins near the base of the crypt, and cells migrate from here emerging onto the villi. The basal crypt cells at position 5 are candidate stem cells. As the function of stem cells is to maintain the integrity of the intestinal epithelium, it must self-renew, proliferate, and differentiate within a protective niche. This niche is made up of proliferating and differentiating epithelial cells and surrounding mesenchymal cells. These mesenchymal cells promote the epithelial-mesenchymal crosstalk required to maintain the niche. A …