Carbohydrates are delivered to the alimentary canal as monosaccharides (e.g., glucose, fructose, and galactose), disaccharides (e.g., sucrose, lactose, and maltose), and polysaccharides (e.g., glycogen and starch). Enzymes involved in digestion of carbohydrates are classified as salivary and pancreatic amylases. Further digestion is performed at the striated border of the enterocytes by enzymes breaking down oligosaccharides and polysaccharides into three basic monosaccharides (glucose, galactose, and fructose). Glucose and galactose are absorbed by the enterocyte via an active transport using Na-dependent glucose transporters (SGLT1). These transporters are localized at the apical cell membrane (brown circles with G and Na labels). Fructose enters the cell via facilitated Na-independent transport using GLUT5 (gray circle with F label) and GLUT2 glucose transporters (orange octagon with G2 label). The three absorbed monosaccharides then pass through the basal membrane of the enterocyte, using GLUT2 glucose transporters, into the underlying capillaries of the portal circulation to reach their final destination in the liver.
eosinophils, macrophages, and a network of fenestrated blood capillaries located just beneath the epithelial basal lamina. In addition, the lamina propria of the villus contains a central, blind-ending lymphatic capillary, the lacteal (Fig. 17.19 and Plate 60, page 618). Smooth muscle cells derived from the muscularis mucosae extend into the villus and accompany the lacteal. These smooth muscle cells may account for reports that villi contract and shorten intermittently, an action that may force lymph from the lacteal into the lymphatic vessel network that surrounds the muscularis mucosae.
The intestinal glands, or crypts of Lieberkn, are simple tubular structures that extend from the muscularis mucosae through the thickness of the lamina propria, where they open onto the luminal surface of the intestine at the base of the villi (see Fig. 17.18). The glands are composed of a simple columnar epithelium that is continuous with the epithelium of the villi.
FIGURE 17.17 • Photograph of the mucosal surface of the small intestine. This photograph of a segment of a human jejunum shows the mucosal surface. The circular folds (plicae circulares) appear as a series of transversely oriented ridges that extend partially around the lumen. Consequently, some of the circular folds appear to end (or begin) at various sites along the luminal surface (arrows). The entire mucosa has a velvety appearance because of the presence of villi.
As in the stomach, the lamina propria surrounds the intestinal glands and contains numerous cells of the immune system (lymphocytes, plasma cells, mast cells, macrophages, and eosinophils), particularly in the villi. The lamina propria also contains numerous nodules of lymphatic tissue that represent a major component of the GALT. The nodules are particularly large and numerous in the ileum, where they are preferentially located on the side of the intestine opposite the mesenteric attachment (Fig. 17.20). These nodular aggregations are known as aggregated nodules or Peyer’s patches. In gross specimens, they appear as aggregates of white specks.
The muscularis mucosae consists of two thin layers of smooth muscle cells, an inner circular and an outer longitudinal layer. As noted above, strands of smooth muscle cells extend from the muscularis mucosae into the lamina propria of the villi.
At least five types of cells are found in intestinal mucosal epithelium.