The small intestine is the site where digestion is completed, using enzymes from the pancreas and bile, and where products of digestion are absorbed. The small intestine possesses several features that increase the surface area for digestion and absorption, including its long length, the presence of plicae circulares, numerous villi, and the simple columnar epithelium with microvilli (brush border). As seen in the image at the right, the four general layers of the GI tract (mucosa (M), submucosa (SM), muscularis externa (ME), and serosa/adventitia) are present in the small intestine. The three regions of the small intestine, the duodenum, jejunum, and ileum, are similar histologically with a few distinguishing features.

• Examine these two sections of the duodenum (sample 1, sample 2). Identify the major layers of the small intestine and observe the plicae circulares with tightly packed villi formed as projections of the mucosa layer. Study the villi, crypts (of Lieberkuhn), muscularis mucosa, submucosal Brunner’s glands (BG), inner circular and outer longitudinal layers of the muscularis externa, and adventitia.

• Examine two sections of jejunum (sample 1, sample 2) and ileum (sample 1, sample 2). For a comparison of all three regions, examine this small intestine composite specimen. Compare and contrast the features of each region in the slides and images below. Note that the duodenum contains Brunner’s glands (BG) and the ileum contains Peyer’s patches (PP), which are distinguishing features.

Now let's take a closer look at the small intestines.