• Study the images below and this specimen of ileum at higher magnification. In the villi, identify the simple columnar absorptive cells or enterocytes with their brush border (microvilli or striated border), terminal web, and goblet cells. In the lamina propria, note the abundant lymphocytes, and locate the central lacteal. The central lacteals are lymphatics that function in immune response and transport of chylomicrons. Also in the lamina propria, note the presence of capillaries for transfer of absorbed molecules into the blood. Identify the crypts containing mitotic cells that renew the lining epithelium and Paneth cells with their distinctive, eosinophilic secretory granules (containing antimicrobial substances, e.g. lysozyme and defensins). Last, locate the muscularis mucosa.

• Next, in the images below, review the ultrastructural features of the enterocytes, including the brush or striated border (microvilli) with a glycocalyx that contains enzymes involved in digestion. Study the colorized TEM image of a goblet cell among the enterocytes. Note the unique shape of these cells and the abundant mucin secretory granules (colored blue) within the apical cytoplasm.

• Finally, in the image at the right and the specimen of duodenum on this slide, examine the muscularis externa, identifying the inner circular (IC) and outer longitudinal (OL) layers of smooth muscle. In between the layers, locate the myenteric (Auerbach) plexus.

Clinical note: A disorder called celiac disease (celiac sprue, gluten-induced enteropathy) is characterized by intolerance to gluten (a protein in many grains). It results in loss of enterocyte microvilli, flattening of the villi, and subsequent malabsorption of nutrients. Regeneration of these structures in the small bowel and return of normal nutrient absorption occur after a few weeks on a diet lacking gluten.

Next is large intestine.