Lower Gastrointestinal Tract

OBJECTIVES: At the end of this laboratory you should be able to:

1. Identify and describe regional differences in the lower GI tract, from the beginning of the small intestine through the anal canal. Be able to relate structure with function.

2. Identify and describe the specialized epithelial cells and know their functions.

3. Identify and describe Meissner's (submucosal) and Auerbach's (myenteric) plexi and know their functional role.

4. Identify lymphatic tissue throughout the GI tract.

5. Identify and describe plicae circulares, villi, and microvilli.

SLIDES FOR THIS LABORATORY: 52, 53, 54, 55, 56, 57, and Supplemental Slide 122.

Slide 52 Duodenum.

Observe Brunner's glands in the submucosa. These glands are diagnostic of the duodenum. Examine the inner circular and outer longitudinal layers of smooth muscle in the muscularis externa . Find the peritoneal surface and note the delicate serosa . Observe the location of the muscularis mucosae , which is often disrupted by Brunner's glands. Note the large broad villi of the mucosa. The core of the villus is an extension of the lamina propria and also contains a blind ending lymphatic capillary, the lacteal . Study the simple columnar epithelium populated by goblet cells and the absorptive enterocytes with apical microvilli . The goblet cells increase in number from the proximal to distal small intestine. The cytological preservation in the crypts of Lieberkuhn is not very good and therefore it is difficult to observe the many mitotic figures that are normally present. Paneth cells may be observed at the base of the crypts but are not very well preserved. Recall that the crypts are longer in the duodenum than in the jejunum. Study the Auerbach's (myenteric) plexi between the layers of the muscularis externa.

Slide 54 Jejunum.

Notice the folding of the submucosa and the mucosa. The folds, containing a core of submucosa, are the plicae circulares (valves of Kerckring) which can be observed with your naked eye by holding the slide up to the light. The plicae circulares are most prominent in the distal duodenum and jejunum. Observe the layers of the wall of the jejunum . Note the absence of glands in the submucosa, which is a characteristic feature of the jejunum. Observe the crypts of Lieberkuhn . Paneth cells are found at the base of the crypt and are identified by their eosinophilic granules. Examine the villi on this slide with the core of lamina propria containing a lacteal . Observe the goblet cells and notice the greater number as compared to the duodenum. Study the muscularis externa and serosa .

Slide 53 Ileum.

The distinguishing feature of the ileum is the presence of large groups of lymphatic nodules in the lamina propria called Peyer's patches . The plicae circulares (valves of Kerckring) in this slide are not as prominent as those found in the duodenum and jejunum. Observe the villi and compare to the duodenum and jejunum. Here the villi are much more leaf-like in appearance when compared to the finger-like villi of the duodenum and jejunum. Note the muscularis mucosa and the absence of glands in the submucosa. Examine the muscularis externa and recall the inner circular and outer longitudinal organization. Study the crypts of Lieberkuhn and observe paneth cells . Observe the density of goblet cells covering the villi and lining the crypts of Lieberkuhn. Goblet cells are most numerous in the terminal ileum. Finally, observe the serosa .

Slide 57 Newborn human ileum.

Observe Peyer's patches in the lamina propria. Note the relationship of the patches to the mucosa and submucosa

Slide 56 Colon.

There are no villi or plicae circularis in the large intestine. The simple columnar epithelium has abundant goblet cells interspersed with absorptive cells . The crypts of Lieberkuhn are larger than in the small intestine. This slide best demonstrates the pale staining enteroendocrine cells (argentaffin or APUD cells) at the base of the crypts. Note the absence any glands in the submucosa . Examine the muscularis mucosae and the muscularis externa . Next, observe the serosa . Find Auerbach's plexi (between the layers of the muscularis externa) and Meissner's plexi (beneath the muscularis mucosae within the submucosa).

Supplemental Slide 122 Colon, newborn human.

Reinforce your ability to identify the large intestine by again finding the distinguishing features of this organ: crypts , simple columnar epithelium , goblet cells , muscularis mucosae , muscularis externa , and serosa . Taeniae coli may be present as longitudinal bands of the outer smooth muscle layer of the muscularis externa.

Slide 55 Appendix.

Note the absence of villi, few crypts, reduced surface area for absorption, and angular lumen. The lamina propria and submucosa are heavily infiltrated with lymphatic tissue; be able to locate lymphatic nodules . Note the absence of any glands in the submucosa. The muscularis mucosae is difficult to see. Examine the muscularis externa with inner circular and outer longitudinal layers. Observe the serosa .

Segments of the GI tract that are not represented in the loan collection are the stomach-duodenal junction, the rectum, and the rectal-anal junction.