Prelab Exercise 5 – DIGESTIVE TRACT

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DIGESTIVE TRACT From the lower esophagus to the lower rectum four fundamental layers comprise the wall of the digestive tube: mucosa, submucosa, muscularis propria (externa), and adventitia or serosa (see figure at right). However, this basic plan is modified to varying degrees in the different segments of the digestive tube in keeping with the performance of special functions. After learning the basic plan of the tube, complete understanding involves the systematic analysis of these modifications segment by segment.

ESOPHAGUS

The esophagus is a transition organ connecting the pharynx (voluntary skeletal muscle) with the stomach (involuntary smooth muscle). Not surprisingly this transition is reflected in the histology of the organ. For example, in the human esophagus, the upper third contains mostly skeletal muscle; the middle third, a mixture of skeletal and smooth muscle; the lower third, only smooth muscle. Slide #126 [Esophagus, monkey, PTS]. This is a very thin cross section of the upper third of the organ, stained with an H&E-like stain. Try to identify the layers of the esophagus. Note nonkeratinized stratified squamous epithelium lining the lumen. In most sections, the lamina propria and the muscularis mucosa are poorly developed at this level. The submucosa is well developed. What type of tissue comprises the submucosa? Can you spot any mast cells here? Mucous type glands may be found in the submucosa of the human esophagus. The muscularis externa consists of skeletal muscle in ill-defined inner and outer layers. (Good chance to review the morphology of skeletal muscle cut in various planes). Is there an adventitia or serosal layer here? Slide #127 [Esophagus, human, H&E]. You will have a section cut from either the middle or the lower third of the organ. As in the previous slide, identify the four basic layers. Note the distinct muscularis mucosa (smooth muscle cut mostly transversely). It separates a delicate lamina propria from the more robust submucosa. The muscularis

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externa consists entirely of smooth muscle (lower third) or a mix of skeletal and smooth muscle (middle third). Slide #128 [Esophageal-stomach jct., dog, H&E, mixed set]. This well-prepared longitudinal section nicely illustrates the transition between lower (subdiaphragmatic esophagus) and the cardiac portion of the stomach. Again define the four basic layers. Note the rather abrupt transition in the type of lining epithelium. The cardiac region of the stomach is lined by simple columnar epithelium that extends into the lamina propria forming the shallow, mucus and lysozyme-secreting cardiac glands. Just beyond, the deeper and more typical gastric glands are evident in the fundic portion of the stomach. Can you find the lamina propria and muscularis mucosa (smooth muscle cut longitudinally). In many of the sections, the submucosa of the lower esophagus has well developed mucous glands. Note also the large blood vessels in the submucosa at the junctional zone. What is the relationship to “esophageal varices”? Depending on your section, you may see a gradual transition from skeletal to smooth muscle in the muscularis externa, or the entire muscularis may be of smooth muscle composition. Is the outer layer of the esophagus and stomach serosal or adventitial in this region?

DRAWING OF THE STOMACH

STOMACH

Slide #129 [Stomach, human PTS]. This small piece of plastic embedded, thin-sectioned stomach shows the various layers of the stomach wall and is excellent for cytological detail. Identify the mucosa, submucosa, muscularis externa and serosa. The surface mucous cells of the mucosa are well shown. How do they differ from mucous secreting goblet cells? Identify gastric pits and gastric glands. Define the lamina

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propria. The parts of gastric glands are isthmus, neck, and base. What kinds of cells are typically found in each of these parts? Where are the “regenerative cells” of the gastric epithelium located? How frequently are the surface epithelial cells replaced? In this section, chief cells and parietal cells can be distinguished from each other since the chief cells show a cytoplasmic basophilia. Why? (To see the appearance of these cells in a more typical H&E stained section, study the gastric glands in the fundus region of stomach in Slide # 128. What is the secretion of each of these cell types? Review the E.M. characteristics of these cells in your text. Enteroendocrine cells are also present in the basal part of the glands, but can be positively identified only with the use of appropriate immunochemical markers. Be aware of the many endocrine/paracrine substances these cells secrete. Can you find any ganglion cells of the myenteric (Auerbach’s) plexus?

SMALL INTESTINE (duodenum, jejunum, ileum)

Slide #130 [Stomach-duodenal jct., dog or primate, H&E]. This slide shows the transition from pyloric region of stomach to duodenum (1st part). It is important to scan this slide with your 4x objective first. Try to find the exact point of transition, marked by distinct changes in mucosa and other morphological features. First study the pyloric region of stomach. Note that the gastric glands are different from those of the fundus/body of the stomach. They are much deeper and contain mostly mucous secreting cells. The muscularis of the stomach is usually described as three-layered: inner oblique, middle circular, outer longitudinal. However, in sections the distinction is often difficult to make. Which of these three muscle layers hypertrophies to form the pyloric sphincter? Now look at the duodenal portion of the slide. Identify the layers. What feature is the most reliable for identifying the duodenum from other parts of the small bowel? The villi of the initial part of the duodenum are short, broad and rather atypical. Slide #131 [Duodenum, monkey]. This is a mixed set of two slides, both good quality. Share so you can study both. Those of you having slide with ID. No. 643 in your box, have a typical H&E preparation. Although this section is rather thick, it shows the overall structure of the duodenum quite well. Starting with the mucosa, note the large villi which contain strands of smooth muscle, large central lymphatic vessels (lacteals) and blood capillaries. Note that the epithelial cells are of two types; most are absorptive cells and fewer are goblet cells. Define the intestinal glands (crypts of Lieberkühn). Note that they are confined to the lamina propria. What kinds of cells form the lining epithelium of the glands? Identify the abundant mitotic figures in the epithelial cells at the bases of the crypts. What is their significance? Which cell types of the intestinal epithelium do these dividing stem cells give rise to? What is the approximate renewal time for small intestinal epithelium? Note that the lamina propria is more abundant than in the stomach and that it contains diffuse aggregations of lymphocytes and plasma cells. In places, lymphocytes can be seen passing through the epithelium into the lumen of the gut. In other places, there are

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lymphoid nodules, some containing germinal centers. Some of these nodules extend through the muscularis mucosa into the submucosa. Identify duodenal (Brunner’s) glands in the submucosa. They open into the basal parts of the intestinal glands in the lamina propria after penetrating the muscularis mucosa. These glands are an important identifying feature of the duodenum. Note that the submucosa contains relatively few lymphocytes. Between the two layers of muscularis externa identify the myenteric plexus. This consists of nerve fibers, nerve cell bodies (what kind of nerves?) and nuclei of Schwann cells. There is a large mass of bluish tissue adherent to the muscularis externa on one side of the duodenum. What organ is this? Also note the fibroadipose tissue surrounding the duodenum which for the most part has an adventitia rather than a serosa. If you have a Slide #131 with ID No. 767 on its label, you will be looking at a plastic thin section stained by an H&E method. The quality of preservation and staining of this tissue makes it excellent for studying cytological detail. Look for the same duodenal features as described above. In addition, use your 40X or even 100X lens to study in detail the features of the villi. If you have never really seen microvillous brush borders, terminal bars and webs, now is your chance. Also look at the contents of the lamina propria core of the villi and intestinal glands. Note the many plasma cells, lymphocytes, occasional mast cells, etc. All show up remarkably well. Slide #132 [Jejunum, cat, H&E]. This is a classic H&E slide which shows well the general features of this small bowel segment. Note the long, tapered villi. Be able to distinguish villi from intestinal glands. Note absorptive and goblet cells in the epithelium and smooth muscle fibers in the lamina propria of the villi. You should be able to distinguish muscularis mucosa from submucosa. Can you find any ganglion cells in the submucosa (Meissner’s submucosal plexus)? Again, note the muscularis externa and the myenteric plexus. Locate the mesentery. Slide #62 [Jejunum, monkey, PTS]. This slide is useful mostly for a detailed cytological examination of the lining epithelium. This layer has been cut in various planes and, if you search, you can find places sectioned in a plane perpendicular to that of the epithelial surface. In such places, a brush border and other apical specializations can be clearly seen. Search for a place where the epithelium has been cut in a plane approximately parallel to its surface. Here the lateral cell outlines can be seen to form a hexagonal pattern. Identify the Paneth cells toward the base of the intestinal glands. What is their function? Paneth cells will also be seen in the next slide.

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Slide #134 [Ileum, primate, PTS]. This is an excellent plastic embedded thin section of this bowel segment. Notice: the increased number of goblet cells in the lining epithelium; the changed appearance of villi; as well as the increasing amounts of gut-associated lymphoid tissue (GALT). Look for lymph nodules, some with germinal centers, in the submucosa. Large aggregates of nodules are known as Peyer’s patches and in some places they may become so abundant that they become grossly visible. Note that the intestinal glands are shallower than in the jejunum. Examine the well-stained Paneth cells in the bases of the glands. Although present along the length of the small intestine and the ascending portion of the colon, they are most dramatically visualized in this slide.

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LARGE INTESTINE (cecum and appendix, colon, rectum)

H & E preparation of the mucosa of the large intestine. Note that the surface epithelium is continuous with the straight, unbranched, tubular intestinal glands. The openings of the glands at the intestinal surface are indicated by the arrows. Note the absence of villi.

Slide #135 (odd boxes) [Large intestine, monkey, mucus stain]. This section has been stained by the mucicarmine technique which stains mucus (and mucinogen) red. First of all note the complete lack of villi (typical of large bowel). Don’t mistake intestinal glands for villi. The epithelium again consists of absorptive and goblet cells. The goblet cells are even more abundant than in the small intestine. Looking at the material in the

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lumen one can get a good idea of the functional importance of goblet cell mucus secretion. Lymphoid nodules are present in the submucosa. Can you see any taeniae coli? Slide #136 [Colon, monkey. PTS). The finer details of the epithelial cells can be seen clearly in this section. Note the abundant mitotic figures in the bases of the crypts. What varieties of cells are abundant in the lamina propria? Slide #137 [Appendix, human, H&E or PTS]. Another mixed set of slides. You may have a standard paraffin-embedded H & E section or a plastic thin section. Both show general features well. Identify the layers and features typical of the large bowel. However, note that the appendix does not have taeniae coli. Dense aggregations of lymphoid tissue, nodular and non-nodular are characteristic of the appendix. Heaviest infiltrations extend from the lamina propria into the submucosa. In the PTS can you find any M cells? In the H&E section, note the many neutrophils (pus?) in the lumen. This appendix show signs of acute inflammation and was likely removed surgically.

CHECK LIST Understand the four parts of the digestive tract, i.e., esophagus, stomach, small bowel and large bowel, based on the morphology of the four fundamental layers of the tract:

-mucosa -epithelium -lamina propria (with large numbers of lymphoid cells)

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-muscularis mucosae -submucosa (extensive blood supply) -submucosal (Meissner’s) nerve plexus -muscularis externa (propria) -circular muscle (inner) -longitudinal muscle (outer) -oblique muscle (in stomach) -myenteric (Auerbach’s) nerve plexus -adventitia -serosa (visceral peritoneum)

Esophagus:

-submucosal mucous type glands -type of muscle in the muscularis propria - striated or smooth?

Stomach: -smooth lumen surface with gastric pits leading to gastric glands -mucous glands in the cardiac and pyloric parts of the stomach -gastric glands in the fundus and body containing -mucous neck cells -parietal (oxyntic) cells -chief (zymogenic) cells -enteroendocrine (argentaffin) cells

Small intestine: -plicae (seen in gross anatomy lab) and extensive villi -lacteals -intestinal glands (crypts of Lieberkühn) opening between the bases of the villi -Paneth cells in base of crypts, prominent distally

Duodenum -high ratio of epithelial absorptive cells to goblet cells -submucosal (Brunner’s) mucous glands

Jejunum -ratio of absorptive cells to goblet cells decreasing

Ileum -goblet cells abundant compared to absorptive cells -extensive lymphoid nodules (Peyer’s patches)

Large intestine (colon): smooth luminal surface with openings to glands -glandular goblets cells more abundant than in ileum.

Appendix: studied in lymphoid/immunology lab.