Lower Gastrointestinal Tract

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. 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. Observe the crypts of Lieberkuhn, which are intestinal glands in the lamina propria. Paneth cells may be observed at the base of the crypts. The eosinophilic staining of the cytoplasm of Paneth cells are due to the many secretory granules containing the enzyme lysozyme, which breaks down bacterial cell walls. 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.

Jejunum
Notice the folding of the submucosa and the mucosa. The folds, containing a core of submucosa, are the plicae circulares which can be observed with your naked eye by holding the specimen 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 specimen 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 mucosae, muscularis externa, and serosa. Study the Auerbach's (myenteric) plexi between the layers of the muscularis externa.

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 in this specimen are not as prominent as those found in the duodenum and jejunum. Observe the villi in the mucosa 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. Find Meissner's plexi within the submucosa. Examine the muscularis externa and recall the inner circular and outer longitudinal organization. Study the Auerbach's (myenteric) plexus between the layers of the muscularis externa. Study the crypts of Lieberkuhn and observe a few Paneth cells, which are not obvious in this specimen because of the overall eosinophilic staining. Observe the density of goblet cells covering the villi and lining the crypts of Lieberkuhn. Goblet cells are most numerous in the terminal ileum. Observe the serosa, in which only the mesothelium can be discerned, since the underlying connective tissue is very sparse.

Colon, longitudinal section
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 specimen best demonstrates the pale-staining enteroendocrine cells (argentaffin or APUD cells) at the base of the crypts. Note the presence of lymphatic nodules (GALT) and the absence any glands in the submucosa. Examine the muscularis mucosae and the muscularis externa. Observe that the inner circular layer of the muscularis externa is considerably thicker than the outer longitudinal layer. Keep in mind that since this is a longitudinal section, the orientation of smooth muscle cells in the two layers of the muscularis externa are opposite of what would be seen in a cross section. The orientation of smooth muscle cells the inner circular layer and outer longitudinal layers of the muscularis mucosae are also opposite of what is seen on cross section. 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).

Colon, human, cross section
Reinforce your ability to identify the large intestine by again finding the distinguishing features of this organ in cross section: crypts of Lieberkuhn, absorptive cells, abundant goblet cells, muscularis mucosae, inner circular layer and outer longitudinal layer of the muscularis externa, myenteric plexus, submucosal plexus, and serosa. Note that the orientation of smooth muscle fibers in the two layers of the muscularis externa is the opposite of that seen in the previous longitudinal section.

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 not as obvious as in other regions of the digestive tract. Examine the muscularis externa with inner circular and outer longitudinal layers. Observe the serosa.