Unformatted text preview: e continuous but slow division of stem cells at the bottom of intestinal crypts (Figure 17.19). The stem cells give rise to a population of transit-amplifying cells, which divide rapidly and occupy about
two-thirds of the crypt. The transit-amplifying cells proliferate for three to
four cell divisions and then differentiate into the three cell types of the colon
surface epithelium: absorptive epithelial cells and two types of secretory
cells, called goblet cells and enteroendocrine cells. The small intestine also
contains a fourth cell type, Paneth cells, which secrete antibacterial agents.
Each crypt contains approximately six self-renewing stem cells, which can
give rise to all of the different types of cells in the intestinal epithelium.
Stem cells are also responsible for continuous renewal of the skin and
hair. Like the lining of the intestine, the skin and hair are exposed to a harsh
external environment—including ultraviolet radiation from sunlight—and
are continuously renewed throughout life. The skin consists of three major
cell lineages: the epidermis, hair follicles, and sebaceous glands, which
View Full Document