Figure 1. Intestinal crypt-villus axis.

Intestinal epithelium is a rapidly regenerating tissue which undergoes cycles of proliferation, migration, differentiation and apoptosis that maintain the integrity of the crypt-surface axis. Stem cells give rise to rapidly proliferating transit cells, located in the proliferating crypt compartment, which migrate up to the villus, the differentiated compartment, or down to the bottom of the crypt to become terminally differentiated cells. Transit cells give rise to four principal differentiated cell lines supporting digestive, absorptive and defense functions: absorptive enterocytes, goblet cells, enteroendocrine cells and Paneth. There is a gradient of metabolism, with glycolysis predominating in crypts and mitochondrial oxidative phosphorylation dominant in villi. Similarly, there is a gradient of cGMP production, lowest in crypts and greatest in villi. Adapted from [14]