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Updated: 03/13/08 12:38 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: S. Shimoda, F. Silverstein, and D.R. Saunders
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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C. Colonic Histology
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No villi in the colon
Polys are not seen normally |
The wall of the colon consists of the usual four layers (mucosa, submucosa, muscularis externa, and serosa) but lacks circular folds or villi: The mucosal surface is flat although punctuated by numerous pits which are the mouths of the crypts of Lieberkuhn. (Figure 6). These crypts are like those of the small intestine except that they have a much larger ratio of goblet to absorptive cells and they lack Paneth cells in their extreme bases. Mitotic figures are found mainly in the basal half of the crypts, and they form new cells which renew most of the epithelium every five days. The surface columnar epithelial cells mostly resemble the intestinal absorptive cells but their microvillus brush border is much less well developed. The epithelium is supported by the connective tissue of the lamina propria which contains lymph, and blood vessels, unmyelinated nerve fibers, fibroblasts, lymphocytes, plasmacells and histiocytes. Polymorphonuclear leukocytes are not normally found in the lamina propria. Frequently lymphocytes aggregate in follicles with germinal centers; they are usually located in the lamina propria, but they may extend into the submucosa.
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Figure 6 |
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The mucosa is separated from the submucosa by a thin muscularis mucosae. The submucosa is composed of denser connective tissue with a rich network of small arteries, veins and lymphatics. The nerve plexus and ganglion cells of Meissner (submucosal plexus) are also found here. The external muscle coats vary in different parts of the large intestine. The inner circular muscle coat invests the entire length of the large intestine. Along the external surface of this muscle are found the clusters of ganglion cells and their ramifications which constitute the myenteric plexus of Auerbach. The outer longitudinal muscle, which forms the three taenia, becomes a continuous sheet in the rectum and appendix.
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| Most common site of carcinoid tumors |
The appendix is a blind sac with a small lumen. Its mucosa is similar to that of the colon except that the crypts contain many argentaffin cells which produce serotonin. Tumors composed of argentaffin cells are called carcinoids. The lamina propria also contains more lymphoid nodules than the rest of the colon.
The structure of the rectum is similar to the colon; but in the middle third of the anal canal the typical colonic epithelium becomes stratified squamous, and in the lower third of the anal canal it becomes keratinized stratified squamous epithelium (i.e., skin). |
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Next Section (D): Vasculature » |
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