The primary function of the columnar absorptive cells is reabsorption of water and electrolytes. The morphology of absorptive cells is essentially identical to that of the enterocytes of the small intestine. Reabsorption is accomplished by the same Na/K-activated ATPase-driven transport system as described for the small intestine.
Elimination of semisolid to solid waste materials is facilitated by the large amounts of mucus secreted by the numerous goblet cells of the intestinal glands. Goblet cells are more numerous in the large intestine than in the small intestine (see Fig. 17.28a and Plate 62, page 622). They produce mucin that is secreted continuously to lubricate the bowel, facilitating the passage of the increasingly solid contents.
The mucosal epithelium of the large intestine contains the same cell types as the small intestine except Paneth cells, which are normally absent in humans.
Columnar absorptive cells predominate (4:1) over goblet cells in most of the colon, although this is not always apparent in histologic sections (see Fig. 17.28a). The ratio decreases, however, approaching 1:1, near the rectum, where the number of goblet cells increases. Although the absorptive cells secrete glycocalyx at a rapid rate (turnover time is 16 to 24 hours in humans), this layer has not been shown to contain digestive enzymes in the colon. As in the small intestine, however, Na/K-ATPase is abundant and is localized in the lateral plasma membranes of the absorptive cells. The intercellular space is often dilated, indicating active transport of fluid.
FIGURE 17.28 • Mucosa of the large intestine. a. This photomicrograph of an H&E preparation shows the mucosa and part of the submucosa. The surface epithelium is continuous with the straight, unbranched, tubular intestinal glands (crypts of Lieberkn). The openings of the glands at the intestinal surface are identified (arrows). The epithelial cells consist principally of absorptive and goblet cells. As the absorptive cells are followed into the glands, they become fewer in number, whereas the goblet cells increase in number. The highly cellular lamina propria contains numerous lymphocytes and other cells of the immune system. b. Scanning electron micrograph of the human mucosal surface of the large intestine. The surface is divided into territories by clefts (arrows). Each territory contains 25 to 100 gland openings. 140. (Reprinted with permission from Fenoglio CM, Richart RM, Kaye GI. Comparative electron-microscopic features of normal, hyperplastic, and adenomatous human colonic epithelium. II. Variations in surface architecture found by scanning electron microscopy. Gastroenterology 1975;69:100–109.)
Goblet cells may mature deep in the intestinal gland, even in the replicative zone (Fig. 17.29). They secrete mucus continuously, even to the point where they reach the luminal surface. Here, at the surface, the secretion rate exceeds the synthesis rate, and “exhausted” goblet cells appear in the epithelium. These cells are tall and thin and have a small number of mucinogen granules in the central apical cytoplasm. An infrequently observed cell type, the caveolated “tuft” cell, has also been described in the colonic epithelium; however, this cell may be a form of exhausted goblet cell.
Epithelial Cell Renewal in the Large Intestine
All intestinal epithelial cells in the large intestine derive from a single stem cell population.