 Microvilli of the enterocytes provide the major amplification of the luminal surface. Each cell possesses several thousand closely packed microvilli. They are visible in the light microscope and give the apical region of the cell a striated appearance, the so-called striated border. Enterocytes and their microvilli are described below.
The villi and intestinal glands, along with the lamina propria, associated GALT, and muscularis mucosae, constitute the essential features of the small intestinal mucosa.
Villi, as noted, are projections of the mucosa. They consist of a core of loose connective tissue covered by a simple columnar epithelium. The core of the villus is an extension of the lamina propria, which contains numerous fibroblasts, smooth muscle cells, lymphocytes, plasma cells,  FOLDER 17.4 Functional Considerations: Digestive and Absorptive Functions of Enterocytes
Carbohydrate final digestion is brought about by enzymes bound to the microvilli of the enterocytes (Fig. 17.4.2). Galactose, glucose, and fructose are absorbed directly into venous capillaries and conveyed to the liver by the vessels of the hepatic portal system. Some infants and a larger percentage of adults cannot tolerate milk and unfermented milk products because of the absence of lactase, the disaccharidase that splits lactose into galactose and
The plasma membrane of the microvilli of the enterocyte plays a role in digestion as well as absorption. Digestive enzymes are anchored in the plasma membrane, and their functional groups extend outward to become part of the glycocalyx. This arrangement brings the end products of digestion close to their site of absorption. Included among the enzymes are peptidases and disaccharidases. The plasma membrane of the apical microvilli also contains the enzyme enteropeptidase (enterokinase), which is particularly important in the duodenum, where it converts trypsinogen into trypsin. Trypsin can then continue to convert additional trypsinogen into trypsin, and trypsin converts several other pancreatic zymogens into active enzymes (Fig. F17.4.1). A summary of digestion and absorption of the three major nutrients is outlined in the following paragraphs.
FIGURE F17.4.1 • Diagram showing events in the activation of the proteolytic enzymes of the pancreas. The majority of pancreatic enzymes (proteases) are secreted as inactive proenzymes. Their activation is triggered by the arrival of chyme into the duodenum. This stimulates the mucosal cells to release and to activate the enterokinase (blue box) within the glycocalyx. The enterokinase activates trypsinogen, converting it into its active form, trypsin (green box). In turn, trypsin activates other pancreatic proenzymes (red box) into their active forms (purple box). The active proteases hydrolyze peptide bonds of proteins or polypeptides and reduce them to small peptides and amino acids.
FIGURE F17.4.2 • Diagram showing the digestion and absorption of carbohydrates by an enterocyte.