Figure 1618

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 Lieberkuhn). x800. b. This three-dimensional diagram of the intestinal villi shows the continuity of the epithelium covering the villi

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. 16.18 and 16.21). 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 mi-

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.

crofilaments 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.

0 0

Post a comment