Figure 22–2 The four main differentiated cell types found in the epithelial lining of the small intestine. all cells are oriented with the gut lumen at top. broad orange arrows indicate direction of secretion or uptake of materials for each type of cell. all of these cells are generated from undifferentiated multipotent stem cells living near the bottoms of the crypts (see Figure 22–1). Absorptive (brush-border) cells outnumber the other cell types in the epithelium by about 10:1 or more. the microvilli on their apical surface provide a 30-fold increase of surface area, not only for the import of nutrients but also for the anchorage of enzymes that perform the final stages of extracellular digestion, breaking down small peptides and disaccharides into monomers that can be transported across the cell membrane. Goblet cells secrete mucus; these are the commonest of the secretory cell types. Paneth cells secrete (along with some growth factors) cryptdins—proteins of the defensin family that kill bacteria. different subtypes of enteroendocrine cells secrete serotonin and peptide hormones into the gut wall (and thence the blood). Cholecystokinin is a hormone released from enteroendocrine cells in response to the presence of nutrients in the gut. it binds to receptors on nearby sensory nerve endings, which relay a signal to the brain to stop the feeling of hunger once one has eaten enough. (after t.l. lentz, Cell Fine Structure. philadelphia: Saunders, 1971;
R. krsti´c, illustrated encyclopedia  of human histology. berlin:  Springer-verlag, 1984.)
Stem cells are required wherever there is a recurring need to replace differentiated cells that cannot themselves divide. Although a stem cell must be able to divide, it does not necessarily have to divide rapidly; in fact, many stem cells divide at a relatively slow rate.
Stem cells are of many types, specialized for the genesis of different classes of terminally differentiated cells—intestinal stem cells for intestinal epithelium, epidermal stem cells for epidermis, hematopoietic stem cells for blood, and so on. Each stem-cell system nevertheless raises similar fundamental questions. What are the distinguishing features of the stem cell in molecular terms? What conditions serve to keep the stem cell in its proper place and to maintain its stem-cell character? What decides whether a given daughter cell commits to differentiation or remains a stem cell? In a tissue where several distinct types of differentiated cells must be produced, are they all derived from a single type of stem cell, or is there a distinct type of stem cell for each one?
wnt Signaling maintains the Gut Stem-Cell Compartment
For the gut, the beginnings of an answer to these questions came from studies of cancer of the colon and rectum (the lower end of the gut, also known as the large intestine). Some people have a hereditary predisposition to colorectal cancer and, in advance of the invasive disease, develop large numbers of small precancerous tumors (adenomas) in the lining of this part of the gut (Figure 22–4). The appearance of these tumors suggests that they have arisen from intestinal crypt cells that have failed to halt their proliferation in the normal way. As discussed in Chapter 20, the cause has been traced to mutations in the Apc (adenomatous polyposis coli) gene: the tumors arise from cells that have lost both gene copies. Because Apc codes for a protein that prevents inappropriate activation of the Wnt signaling pathway (see Figure 15–60), this loss of Apc is presumed to mimic the effect of continual exposure to a Wnt signal. The suggestion, therefore, is that Wnt signaling normally keeps crypt cells in a proliferative state, and that a cessation of exposure to Wnt signaling normally makes them stop dividing as they leave the crypt.
Stem Cells at the Crypt base are multipotent, Giving Rise to the Full Range of differentiated intestinal Cell types