Figure 1638

Diagram of M cells in a lymphatic nodule of the intestine, a. This diagram shows the relationship of the M cells (microfold cells) and absorptive cells to the lymphatic nodule. The M cell is an epithelial cell that displays microfolds rather than microvilli on its apical surface. It has deep recesses within which lymphocytes come close to the lumen of the small intestine. M cells have MHC II molecules on their surface and are therefore considered antigen-presenting cells. Antigen from the intestinal lumen is presented to T lymphocytes residing within the recesses of the M cell. (Based on Owen

RL, Nemanic PC, eds. Scanning Electron Microscopy. Vol II. O'Hare, IL: AMF, 1978.) b. Scanning electron micrograph of a Peyer's patch lymphatic nodule bulging into the lumen of the ileum. Note that the area of the follicle covered by M cells is surrounded by the finger-like projections of the intestinal villi. The surface of the M cells has a smooth appearance. The absence of absorptive cells and mucus-producing goblet cells in the area covered by M cells facilitates immunoreactions to antigens. x80. (From Owen RL, Johns AL. Gastroenterology 1974;66.)

Hair follicles and sebaceous glands are also found at this site.

The submucosa of the anal columns contains the terminal ramifications of the superior rectal artery and the rectal venous plexus. Enlargements of these submucosal veins constitute internal hemorrhoids, which are related to elevated venous pressure in the portal circulation (portal hy pertension). There are no teniae coli at the level of the rectum; the longitudinal layer of the muscularis externa forms a uniform sheet. The muscularis mucosae disappears at about the level of the anal transitional zone (ATZ), where the circular layer of the muscularis externa thickens to form the internal anal sphincter. The external anal sphincter is formed by striated muscle of the pelvic floor.

absorptive cells cells lymphocytes lamina propria

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