Figure 1.

Structure of the intestinal epithelium. The intestinal epithelium is organized into units that consist of crypts and—in the small bowel—protrusions called villi. At the base of the intestinal crypt lies the stem cell compartment, where LGR5 intestinal stem cells (ISCs) divide and replicate, whereas +4 stem cells act as reserve stem cells. As ISCs divide, they advance along the crypt villus axis, first entering the transit-amplifying zone before differentiating into the specialized, terminally differentiated cells of the epithelium (eg, goblet, tuft, and enteroendocrine cells). Finally, at the tip cells undergo anoikis and shed into the lumen.