FIGURE 17.12 • Electron micrograph and diagrams of enteroendocrine cells. a. This electron micrograph shows an example of the “closed” enteroendocrine cell. Arrowheads mark the boundary between the enteroendocrine cell and the adjacent epithelial cells. At its base, the enteroendocrine cell rests on the basal lamina (BL). This cell does not extend to the epithelial or luminal surface. Numerous secretory vesicle (G) in the base of the cell are secreted in the direction of the arrows across the basal lamina and into the connective tissue (CT ). En, endothelium of capillary; M, mitochondria; rER, rough endoplasmic reticulum; sER, smooth endoplasmic reticulum. b. This diagram of an enteroendocrine “closed” cell is drawn to show that it does not reach the epithelial surface. The secretory vesicles are regularly lost during routine preparation. Because of the absence of other distinctive organelles, the nucleus appears to be surrounded by a small amount of clear cytoplasm in H&E–stained sections. c. The enteroendocrine “open” extend to the epithelial surface. Microvilli on the apical surface of these cells possess taste receptors and are able to detect sweet, bitter, and umami sensations. These cells serve as chemoreceptor cells, which monitor an environment on the surface of the epithelium. They are involved in a regulation of gastrointestinal hormones secretion.
“open” cells. It is now known that open cells serve as primary chemoreceptors that sample the contents of the gland lumen and release hormones based on the information obtained from those samples. The taste receptors, similar to those found in taste buds of the specialized oral mucosa (page 530), detect sweet, bitter, and umami sensations and have now been characterized on the free surface of the open enteroendocrine cells. They belong to the T1R and T2R families of G protein–coupled receptors described in Chapter 16 (page 531). Secretion from closed cells, however, is regulated by luminal content indirectly through neural and paracrine mechanisms.
Electron micrographs reveal small membrane-bound secretory vesicles throughout the cytoplasm; however, the vesicles are typically lost in H&E preparations, and the cytoplasm appears clear because of the lack of sufficient stainable material. Although these cells are often difficult to identify because of their small size and lack of distinctive staining, the clear cytoplasm of the cell sometimes stands out in contrast to adjacent chief or parietal cells, thus allowing their easy recognition.
The names given to the enteroendocrine cells in the older literature were based on their staining with salts of silver and chromium (i.e., enterochromaffin cells, argentaffin cells, and argyrophil cells). Such cells are currently identified and characterized by immunochemical staining for the more than 20 peptide and polypeptide hormones and hormone-like regulating agents that they secrete (a list of many of these agents and their actions is given in Fig. 17.13 and in Tables 17.1 and 17.2). With the aid of the TEM, at least 17 different types of enteroendocrine cells have been described on the basis of size, shape, and density of their secretory vesicles.
Cardiac Glands of the Gastric Mucosa
Cardiac glands are composed of mucus-secreting cells.