Figure 1.

Model for the cell lineage in small intestinal crypts. The small intestinal epithelium consists of two main structures: villi (luminal protrusions) and crypts (invaginations). The villi are composed of fully differentiated cells that mediate absorption and secretion. Apoptosis of the cells at the tip of villi and the proliferation of crypt cells maintain the number of functional cells in the small intestinal epithelium. All cell types within an individual crypt are sustained by stem cells located at the bottom of the crypt. Two stem cell positions have been reported for crypts: One represents +4 position stem cells ^[20^] (shown between the dotted curve lines), and the other refers to CBC cells located between differentiated Paneth cells ^[21^]. Once TA cells divided from the stem cells reach the crypt–villus junction, they differentiate and migrate to the villi.