Figure 1628

Electron micrograph of the myenteric (Auerbach s) plexus. The plexus is located between the two smooth muscle (SM) layers of the muscularis externa. It consists of nerve cell bodies (CB) and an exten sive network of nerve fibers (N). A satellite cell (SC), also referred to as an enteric glial cell, is seen in proximity to the neuron cell bodies. These cells have structural and chemical features in common with glial cells of the CNS. BV, Blood vessel, x 3,800.

sis, the second type of contraction, largely involves the longitudinal muscle layer and moves the intestinal contents distally.

Serosa

The serosa of the parts of the small intestine that are located intraperitoneally in the abdominal cavity corresponds to the general description at the beginning of the chapter.

Epithelial Cell Renewal in the Small Intestine

All of the mature cells of the intestinal epithelium are derived from a single stem cell population

Stem cells are located in the base of the intestinal gland. The zone of cell replication is restricted to the lower one half of the gland. A cell destined to become a goblet cell or absorptive cell usually undergoes several additional divisions after it leaves the pool of stem cells. The epithelial cells migrate upward in the intestinal gland onto the villus and are shed at the tip of the villus. Autoradiographic studies have shown that the renewal time for absorptive and goblet cells in the human small intestine is 5 to 6 days (see Fig. 4.32).

Enteroendocrine cells and Paneth cells are also derived from the stem cells at the base of the intestinal gland. Enteroendocrine cells appear to divide only once before differentiating. They migrate with the absorptive and goblet cells but at a slower rate. Paneth cells do not migrate; they remain in the base of the intestinal gland near the stem cells from which they are derived. They live for approximately 4 weeks and are then replaced by differentiation of a nearby "committed" cell in the intestinal gland. Cells that are recognizable as Paneth cells no longer divide.

^ large intestine

The large intestine comprises the cecum with its projecting vermiform appendix, the colon, the rectum and the anal canal. The colon is further subdivided on the basis of its anatomic location into ascending colon, transverse colon, descending colon, and sigmoid colon. The four layers characteristic of the alimentary canal are present throughout. However, several distinctive features exist at the gross level (Fig. 16.32):

• Except for the rectum, anal canal, and vermiform appendix, the outer longitudinal layer of the muscularis

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