The ileum is the principal site of water and electrolyte reabsorption in the small intestine. It has essentially the same histologic features as the jejunum. Some, however, are emphasized; namely, villi in the ileum are more frequently leaf-like, and lymphatic tissue in the lamina propria is organized into small and large nodes that are found in great number on the antime-senteric side of the ileum. The nodes fuse to form large accumulations of lymphatic tissue called Peyer's patches.
The surface epithelium of the small intestine renews itself every 5 to 6 days. The stem cells are restricted to the bottoms of the mucosal glands, and the zone of cell replication is restricted to the lower half of the glancl. The cells migrate onto the villus and are lost from its tip. All of the epithelial cells, absoiptive cells, and goblet cells, as well as enteroendocrine cells and Paneth cells, derive from the same stem cell population, but enteroendocrine cells migrate only slowly, and Paneth cells do not migrate.
Figure 1, ileum, monkey, H&E x20.
For purposes of orientation, the submucosa (SM) and muscularis externa (ME) have been marked in the cross section through the ileum shown here. Just internal to the submucosa is the mucosa; external to the muscularis externa is the serosa. The mucosa reveals several longitudinally sectioned villi (V), which have been labeled, and other unlabeled villi, which can be identified easily on the basis of their appearance as islands of tissue completely surrounded by the space of the lumen. They are, of course, not islands because this appearance is due to the plane of section that slices completely through some of the villi obliquely or in cross section, thereby isolating them from their base. Below the villi are the intestinal glands, many of which are obliquely or transversely sectioned and can be readily identified, as was done in the preceding plates, because they are totally surrounded by lamina propria.
There are about 8 to 10 projections of tissue into the intestinal lumen that are substantially larger than the villi. These are the plicae circulares. As noted above, plicae gen erally have circular orientation, but they may travel in a longitudinal direction for short distances and may branch. In addition, even if all the plicae are arranged in a circular manner, if the section is somewhat oblique, the plicae will be cut at an angle, as appears to be the case with several plicae in this figure. One of the distinctive features of the small intestine is the presence of single and aggregated lymph nodules in the intestinal wall. Isolated nodules of lymphatic tissue are common in the proximal end of the intestinal canal. As one proceeds distally through the intestines, however, the lymph nodules occur in increasingly larger numbers. In the ileum, large aggregates of lymph nodules are regularly seen; they are referred to as Peyer's patches. Several lymphatic nodules (LN) forming a Peyer's patch are shown in this figure. The nodules are partly within the mucosa of the ileum and extend into the submucosa. Although not evident in the figure, the nodules are characteristically located opposite where the mesentery connects to the intestinal tube.
Figure 2, ileum, monkey, H&E x40.
Sometimes, in a cross section through the intestine, a plica displays a clear cross-sectional profile such as that shown here. Note, again, that the submucosa (SM) constitutes the core of the plica. Although many of the villi in this figure present profiles (V) that would be expected if the vil lus were a finger-like projection, others clearly do not. In particular, one villus (marked with three asterisks) shows the broad profile of a longitudinally sectioned leaf-like villus. If this same villus were cut at a right angle to the plane shown here, it would appear as a finger-like villus.
Figure 3, ileum, monkey, H&E x100; inset x200.
Part of a lymphatic nodule and part of the overlying epithelium are shown here at higher magnification. The lymphocytes and related cells are so numerous that they virtually obscure the cells of the muscularis mucosae. Their location, however, can be estimated as being near the presumptive label (MM??), inasmuch as the muscularis mucosae is ordinarily adjacent to the base of the intestinal glands (Gl). Moreover, on examination of this area at higher magnification (inset), groups of smooth muscle cells (MM) can be seen separated by numerous lymphocytes close to the intestinal glands (Gl). Clearly, the lymphocytes of the nodule are on both sides of the muscularis mucosae and, thus, within both the mucosa and the submucosa.
In places, the lymph nodule is covered by the intestinal epithelium. Whereas the nature of the epithelium cannot be appreciated fully in the light microscope, electron micrographs (both scanning and transmission) have shown that among the epithelial cells are special cells, designated M cells, that sample the intestinal content (for antigen) and present this antigen to the lymphocytes in the epithelial layer.
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