Figure 1634

Electron micrograph of dividing goblet cells. This electron micrograph demonstrates that certain cells of the intestine continue to divide even after they have differentiated. Here, two goblet cells (GC) are shown in division. Typically, the dividing cells move away from the basal lamina toward the lumen. One of the goblet cells shows mu-cinogen granules (M) in its apical cytoplasm. The chromosomes (C) of the dividing cells are not surrounded by a nuclear membrane. Compare with the nuclei (IM) of the nondividing intestinal epithelial cells. The lumen of the gland (L) is on the right. CT, connective tissue; and E, eosinophil, x 5,000.

submucosa and as a network around the muscularis externa.

Muscularis Externa

As noted, in the cecum and colon (the ascending, transverse, descending and sigmoid colon), the outer layer of the muscularis externa is, in part, condensed into prominent longitudinal bands of muscle, called teniae coli, which may be seen at the gross level (see Fig. 16.32). Between these bands, the longitudinal layer forms an extremely thin sheet. In the rectum, anal canal, and vermiform appendix, the outer longitudinal layer of smooth muscle is a uniformly thick layer, as in the small intestine.

Bundles of muscle from the teniae coli penetrate the inner, circular layer of muscle at irregular intervals along the length and circumference of the colon. These apparent discontinuities in the muscularis externa allow segments of the colon to contract independently, leading to the formation of saccules (haustra) in the colon wall.

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