The mature cells of the intestinal epithelium are found both in the intestinal glands and on the surface of the villi. They include the following:  Enterocytes, whose primary function is absorption  Goblet cells, unicellular mucin-secreting glands  Paneth cells, whose primary function is to maintain substances  Enteroendocrine cells, which produce various paracrine and endocrine hormones  M cells (microfold cells), modified enterocytes that cover enlarged lymphatic nodules in the lamina propria
Enterocytes are absorptive cells specialized for the transport of substances from the lumen of the intestine to the circulatory system.
Enterocytes are tall columnar cells with a basally positioned nucleus (see Figs. 17.18 and 17.21). Microvilli increase the apical surface area as much as 600 times; they are recognized in the light microscope as forming a striated border on the luminal surface.
Each microvillus has a core of vertically oriented actin microfilaments that are anchored to villin located in the tip of the microvillus and that also attach to the microvillus plasma membrane by myosin I molecules. The actin microfilaments extend into the apical cytoplasm and insert into the terminal web, a network of horizontally oriented contractile microfilaments that form a layer in the most apical cytoplasm and attach to the intracellular density associated with the zonula adherens. Contraction of the terminal web causes the microvilli to spread apart, thus increasing the space between them to allow more surface area exposure for absorption to take place. In addition, contraction of the terminal web may aid in “closing” the holes left in the epithelial sheet by exfoliation of aging cells. Enterocytes are bound to one another and to the goblet, enteroendocrine, and other cells of the epithelium by junctional complexes.
Tight junctions establish a barrier between the intestinal lumen and the epithelial intercellular compartment.
The tight junctions between the intestinal lumen and the connective tissue compartment of the body allow selective retention of substances absorbed by the enterocytes. As noted in the section on occluding junctions, the “tightness” of these junctions can vary.
In relatively impermeable tight junctions, as in the ileum and colon, active transport is required to move solutes across the barrier. In simplest terms, active transport systems, e.g., sodium pumps (Na/K-ATPase), located in the lateral plasma membrane transiently reduce the cytoplasmic concentration of Na by transporting it across the lateral plasma membrane into the extracellular space below the tight junction. This transport of Na creates a high intercellular Na concentration, causing water from the cell to enter the intercellular space, reducing both the water and Na concentrations in the
FIGURE 17.18 • Intestinal villi in the small intestine. a. Scanning electron micrograph of the intestinal mucosa showing its villi. Note the openings (arrows) located between the bases of the villi that lead into the intestinal glands (crypts of Lieberkn). 800. b. This three-dimensional diagram of the intestinal villi shows the continuity of the epithelium covering the villi with the epithelium lining the intestinal glands. Note blood vessels and the blind-ending lymphatic capillary, called a lacteal, in the core of the villus. Between the bases of the villi, the openings of the intestinal glands can be seen (arrows). Also, the small openings on the surface of the villi indicate the location of discharged goblet cells.
cell. Consequently, water and Na enter the cell at its apical surface, passing through the cell, and exiting at the lateral plasma membrane as long as the sodium pump continues to function. Increased osmolarity in the intercellular space draws water into this space, establishing a hydrostatic pressure that drives Na and water across the basal lamina into the connective tissue.