|Atlas||Wheater’s, pgs. 274-287, Gastrointestinal tract|
|Text||Ross and Pawlina, Chapter 17 Digestive System II: Esophagus & Gastrointestinal Tract|
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I. SMALL AND LARGE INTESTINE
A. Small Intestine
The histology of the wall of the small intestine differs somewhat in the duodenum, jejunum, and ileum, but the changes occur gradually from one end of the intestine to the other.
Look at slide 162 first. Locate the duodenal portion in this slide and notice the presence of submucosal mucous glands (Brunner’s glands). Observe that the ducts of these glands (and, occasionally, some acini) penetrate the muscularis mucosae and open into a crypt of Lieberkühn. After viewing slide 162, move to slide 161 and try to find the duodenal region in this tissue section.
2. Jejunum and ileum
Slide 29 40x (jejunum, monkey, H&E) WebScope ImageScope
Slide 168 40x (ileum, H&E) WebScope ImageScope
Slide 169 40x (jejunum, H&E) WebScope ImageScope
Slide 170 40x (ileum, H&E) WebScope ImageScope
Slide 165 40x (ileum, PAS) WebScope ImageScope
Slide 171 20x (jejunum, vascular inj) WebScope ImageScope
View these sections with the low power objective and identify the mucosa, submucosa and the muscularis externa. Note that the mucosa consists of three sub-layers:
The mucosa, which is clearly demarcated from the submucosa by the prominent muscularis mucosae layer, frequently shows heavy lymphocytic infiltration in the lamina propria.
The appearance of the submucosa layer is a bit variable, but, in general, it’s best considered as irregular connective tissue: in slide 29 the submucosa appears more “loose” whereas in slides 168 and 170 it is more dense, and, in slide 169, here the submucosa is edematous and exhibits unusually dilated blood vessels. You can see the intestinal villi and intestinal glands (crypts of Lieberkühn). Examine the villi at a higher magnification and note that the lining epithelium consists of simple columnar cells (aka enterocytes) with a brush border and interspersed goblet cells, particularly well-demonstrated in slide 168. You can observe the distribution of goblet cells in the intestinal epithelium stained with PAS (slide 165).
The epithelium lining the villi continues into the intestinal glands. Examine several of these glands in slide 169 and note that goblet cells and enterocytes similar to those lining the villi, cover the upper portions of the gland. Also, notice that there are many mitotic figures [see example] . The cells which line the lower portions of the crypts are less well differentiated. You may be able to see the enteroendocrine cells [see example] in this region. These are the cells with spherical nuclei and clear cytoplasm –the secretory granules of these cells are not always stained very well, but, if they are, you should note that the granules are oriented basally.
Slides 246 WebScope ImageScope and 247 WebScope ImageScope from the UCSF collection have some excellent examples of enteroendocrine cells [see example]. Again, the enteroendocrine cells have a clear cytoplasm and, if visible, basally-oriented granules. Slide 247 in particular has been stained with ammonium silver nitrate to demonstrate so-called “argentaffin” cells [see example] (which, incidentally, are now known to be “S” or serotonin-secreting enteroendocrine cells –the serotonin in these cells reacts with the silver causing a black precipitate to form). Note that there are about 20 different types of enteroendocrine cell, and you are NOT expected to be able to identify a specific type of enteroendocrine cell (e.g. the “S” cells described above), but you should know the general histological characteristics and functions of enteroendocrine cells as a whole.
Paneth cells [see example] occupy the base of the intestinal cypts/crypts of Lieberkuhn. They are not well preserved in slide 169, somewhat better in slide 168, and quite good in slides 29 and 170. These cells are pyramidal shaped with round nuclei located near their base. They contain brightly eosinophilic (almost orange) secretory granules in the apical cytoplasm. In slide 168, the secretory granules in the Paneth cells stain a refractory brown or green.
Just under the mucosal epithelium is the lamina propria (or lamina propria mucosa), which consists of loose connective tissuethat fills the spaces between the intestinal glands and forms the cores of the intestinal villi. Within the core of each villus is a central lacteal, capillaries, and delicate wisps of smooth muscle that extend from the muscularis mucosae below. However, in some regions, the lamina propria may be so packed with a heavy infiltration of lymphoid cells that these finer structures may not be visible. You may hear the term “Peyer’s patches” used to describe such regions in the GI tract. However, technically, Peyer’s patches are found ONLY in the ileum and they are big enough to be visible with the naked eye.
The muscularis mucosae (or laminae muscularis mucosae) consists of smooth muscle fibers. Observe that strands of smooth muscle fibers from the muscularis mucosae extend into the cores of the intestinal villi along the central axis. Contractions of this muscle layer are controlled by ganglion cells and nerve fibers of the submucosal (Meissner’s) plexus [see example] located in the submucosa, (W pg 266, 14.4a). The muscularis externa consists of two layers of smooth muscle: inner circular and outer longitudinal. Observe the ganglion cells and nerve fibers of the myenteric (Auerbach’s) plexus [see example] located between the two muscle layers, (W pg 266, 14.4c).
B. Large Intestine
The mucosa of the large intestine does not have folds comparable to the plicae circularis, except in the rectum. Also, the intestinal villi are absent beyond the ileocecal valve.
The mucosa of the colon is lined by a simple columnar epithelium with a thin brush border and numerous goblet cells. Note that there are no plicae or villi. The crypts of Lieberkühn are straight and unbranched and lined largely with goblet cells. In many regions the mucus is partially preserved and stains with hematoxylin. At the base of the crypts, undifferentiated cells and endocrine cells are present; however, Paneth cells are not usually present. The appearance of the lamina propria is essentially the same as in the small intestine: Leukocytes are abundant and the isolated lymphoid nodules present in this tissue extend into the submucosal layer (survey the left lower area of slide 176). The muscularis mucosae is a bit more prominent compared to the small intestine, and consists of distinct inner circular and outer longitudinal layers. The submucosa of this specimen is particularly well fixed such that you may better appreciate the mixture of irregular connective and adipose tissue, numerous blood vessels, and several excellent examples of ganglion cells and nerves of the submucosal plexus. The muscularis externa of the large intestine is different from that of the small intestine in that the outer longitudinal layer of smooth muscle varies in thichness and forms three thick longitudinal bands, the taeniae coli (taenia = worm). This section happened to be cut such that a piece of one of these longitudinal bands may be seen.
Study this slide and note the following characteristics.
3. Recto-anal Junction
Slide 177 40x (recto-anal junct, monkey, H&E) WebScope ImageScope
Slide 177-2 20x (recto-anal junct, human, trichrome) WebScope ImageScope
Slide 177-3 20x (recto-anal junct, human, H&E) WebScope ImageScope
Look at these slides (especially slide 177-3) at low magnification first to locate the recto-anal junction. Here you will observe a narrow zone of transition from the simple columnar epithelium of the intestine to the keratinized stratified squamous epithelium of skin. Within the transition zone, you may find stratified columnar (or sometimes cuboidal) epithelium followed by nonkeratinized stratified squamous epithelium. While looking, move the image from the colon toward the direction of the recto-anal junction. Observe that the crypts become shorter and shorter, eventually disappearing near the junction. Also observe that the muscularis mucosae becomes tattered and disappears, allowing the lamina propria merge with the underlying submucosa in this area. Note the presence of a large number of submucosal veins [see example]. When these veins become dilated and varicose, they cause the mucosa to bulge and create the condition commonly known as hemorrhoids. Examine the skin lining the anal region and observe sebaceous and sweat glands, hair follicles, etc., particularly evident in slide 177-2 [see example]. Also, note the massive amount of smooth and skeletal muscle that form the internal [see example] and external anal sphincters [see example], respectively. The primate specimen (slide 177) shows these muscles quite well.