Figure 1632

Photograph of the large intestine. This photograph shows the outer (serosal) surface (left) and internal (mucosal) surface (right) of the transverse colon. On the outer surface, note the characteristic features of the large intestine: a distinctive smooth muscle band representing one of the three teniae coli (TC); haustra coli (HC), the sacculations of the colon located between the teniae; and omental appendices (OA), small peritoneal projections filled with fat. The smooth mucosal surface shows semilunar folds (arrows) formed in response to contractions of the muscularis externa. Compare the mucosal surface as shown here with that of the small intestine (Fig. 16.17).

the microscopic level reveals the openings of the glands, which are arranged in an orderly pattern (Fig. 16.33b).

The principal functions of the large intestine are reabsorption of electrolytes and water and elimination of undigested food and waste

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 ente-rocytes 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 in testine (see Fig. 16.33a). 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. 16.33a). 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+/I<+-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.

Goblet cells may mature deep in the intestinal gland, even in the implicative zone (Fig. 16.34). 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

AN intestinal epithelial cells in the large intestine derive from a single stem cell population

As in the small intestine, all of the mucosal epithelial cells of the large intestine arise from stem cells located at the bottom of the intestinal gland. The lower third of the gland constitutes the normal implicative zone where newly generated cells undergo 2 to 3 more divisions as they begin their migration up to the luminal surface where they are shed about 5 days later. The intermediate cell types found in the lower third of the intestinal gland are identical to those seen in the small intestine.

The turnover times of the epithelial cells of the large intestine are similar to those of the small intestine, i.e., about 6 days for absorptive cells and goblet cells and up to 4 weeks for enteroendocrine cells. Senile epithelial cells that reach the mucosal surface are shed into the lumen at the midpoint between two adjacent intestinal glands.

Lamina Propria

Although the lamina propria of the large intestine contains the same basic components as the rest of the digestive

epithelium

-la miliar propria

- mucosa muscularis mucosae

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