The small intestine is the principal site for the digestion of food and absorption of the products of digestion. It is the longest component of the alimentary canal, measuring over 6 m, and is divided into three segments: duodenum (~25 cm); jejunum (~2.5 m); and ileum (~3.5 m). The first portion, the duodenum, receives a partially digested bolus of food (chyme) from the stomach, as well as secretions from the stomach, pancreas, liver, and gallbladder that contain digestive enzymes, enzyme precursors, and other products that aid digestion and absorption. The small intestine is characterized by plicae circulares, permanent transverse folds that contain a core of submucosa, and villi, fingerlike and leaflike projections of the mucosa that extend into the lumen. Microvilli, multiple finger like extensions of the apical surface of each intesti-nal epithelial cell (enterocyte), further increase the surface for absorption of metabolites. Mucosal glands extend into the lamina propria. They contain the stem cells and developing cells that will ultimately migrate to the surface of the villi. In the duodenum, submucosal glands (Brunner’s glands) secrete an alkaline mucus that helps to neutralize the acidic chyme. Enterocytes not only absorb metabolites digested in the intestinal lumen but also synthesize enzymes inserted into the membrane of the mi-crovilli for terminal digestion of disaccharides and dipeptides.
Duodenum, monkey, H&E ×120.
This figure shows a segment of the duodenal wall. As in the stomach, the layers of the wall, in order from the lumen, are the mucosa (Muc), the submucosa (SubM ), the muscularis externa (ME ), and the serosa (S ). Both longitudinal (L) and circular (C ) layers of the muscularis externa can be distinguished. Although plicae circulares are found in the wall of the small intestine, including the duodenum, none is included in this photomicrograph.
A distinctive feature of the intestinal mucosa is the presence of fingerlike and leaflike projections into the intestinal lumen, called villi. Most of the villi (V ) shown here display profiles that correspond to their description as fingerlike. One villus, however, displays the form of a leaflike villus (asterisk). The dashed line marks the boundary between the villi and the intestinal glands (also called crypts of Lieberkn). The latter extend as far as the muscularis mucosae (MM ).
Under the mucosa is the submucosa, containing the Brunner’s glands (BGl ). These are branched tubular or branched tubuloalveolar glands whose secretory components, shown at higher magnification in figure below, consist of columnar epithelium. A duct (D) through which the glands open into the lumen of the duodenum is shown here and, at higher magnification, in figure below, where it is marked by an arrow.
Mucosa, duodenum, monkey, H&E ×240.
The histologic features of the duodenal mucosa are shown at higher magnification here. Two kinds of cells can be recognized in the epithelial layer that forms the surface of the villus: enterocytes (absorptive cells) and goblet cells (GC ). Most of the cells are absorptive cells. They have a striated border that will be seen at higher magnification in Plate 60; their elongate nuclei are located in the basal half of the cell. Goblet cells are readily identified by the presence of the apical mucous cup, which appears empty. Most of the dark round nuclei also seen in the epithelial layer covering the villi belong to lymphocytes.