taining glycoproteins destined for the cell surface are located in the apical cytoplasm, just below the terminal web, and along the lateral plasma membrane. Histochemical or autoradiographic methods are needed, however, to distinguish these secretory vesicles from endocytotic vesicles or small lysosomes.
The small intestine also secretes water and electrolytes. This activity occurs mainly in the cells within the intestinal
FIGURE 17.21 • Diagrams of an enterocyte in different phases of absorption. a. This cell has a striated border on its apical surface and junctional complexes that seal the lumen of the intestine from the lateral intercellular space. The characteristic complement of major organelles is depicted in the diagram. b. This cell shows the distribution of lipid during fat absorption as seen with the TEM. Initially, lipids are seen in association with the microvilli of the striated border. Lipids are internalized and seen in vesicles of the smooth endoplasmic reticulum (sER) in the apical portion of the cell. The membrane-bounded lipid can be traced to the center of the cell, where many of the lipid-containing vesicles fuse. The lipid is then discharged into the intercellular space. The extracellular lipids, recognized as chylomicrons, pass beyond the basal lamina for further transport into lymphatic (green) and/or blood vessels (red).
glands. The secretion that occurs in these glands is thought ination with the TEM reveals a large accumulation of mu-to assist the process of digestion and absorption by maintain-cinogen granules in the apical cytoplasm that distends the ing an appropriate liquid state of the intestinal chyme. apex of the cell and distorts the shape of neighboring cells Under normal conditions, the absorption of fluid by the vil-(Fig. 17.22). With the apex of the cell containing a large lus enterocyte is balanced by the secretion of fluid by the accumulation of mucinogen granules, the basal portion of the gland enterocyte. cell resembles a narrow stem. This basal portion is intensely basophilic in histologic preparations because it is occupied by
Goblet cells represent unicellular glands that are inter- a heterochromatic nucleus, extensive rER, and free ribo spersed among the other cells of the intestinal epithelium.
somes. Mitochondria are also concentrated in the basal cyto-As in other epithelia, goblet cells produce mucus. In the plasm. The characteristic shape, with the apical accumulation small intestine, goblet cells increase in number from the duo-of granules and the narrow basal stem, is responsible for the denum to the terminal part of the ileum. Also, as in other name of the cell, as in a glass “goblet.” An extensive array of epithelia, because water-soluble mucinogen is lost during flattened Golgi cisternae forms a wide cup around the newly preparation of routine H&E sections, the part of the cell that formed mucinogen granules adjacent to the basal part of the normally contains mucinogen granules appears empty. Exam-cell (see Fig. 17.22a). The microvilli of goblet cells are