FIGURE 16.22 • Relationship of serous-secreting cells and mucus-secreting cells in the mixed acinus. a. This drawing indicates the relationship of the mucous and serous cells as observed in the electron microscope after the rapid-freezing method. The serous cells extend from the basal lamina to the lumen of the acinus. b. In this drawing, serous cells are shown occupying the periphery of the acinus to form the so-called serous demilune. This feature is visible in routine preparations using immersion fixation. The swollen mucous cells have forced out the serous cells, leaving small remnants of the cytoplasm between the mucous cells.
tus, and numerous spherical secretory granules (Fig. 16.23). As in most protein-secreting cells that store their secretions in zymogen granules, the granules are located in the apical cytoplasm. Most other organelles are located in the basal or perinuclear cytoplasm. In H&E sections, the basal cytoplasm of the serous cell stains with hematoxylin because of the rER and the free ribosomes, whereas the apical region stains with eosin, in large part because of the secretory granules.
When examined with the transmission electron microscope (TEM), the base of the serous cell may display infoldings of the plasma membrane and basolateral folds in the form of processes that interdigitate with similar processes of adjacent cells. The serous cells are joined near their apical surface by junctional complexes to neighboring cells of the acinus (see Fig. 16.23).
Mucous cells are mucin-secreting cells.
As in other mucus-secreting epithelia, the mucous cells of the mucous salivary acini undergo cyclic activity. During part of the cycle, mucus is synthesized and stored within the cell as mucinogen granules. When the product is discharged after hormonal or neural stimulation, the cell begins to resynthesize mucus. After discharge of most or all of the mucinogen granules, the cell is difficult to distinguish from an inactive serous cell. However, most mucous cells contain large numbers of mucinogen granules in their apical cytoplasm, and because the mucinogen is lost in H&E–stained paraffin sections, the apical portion of the cell usually appears empty. In TEM preparation, the rER, mitochondria, and other components are seen chiefly in the basal portion of the cell; this part of the cell also contains the nucleus, which is typically flattened against the base of the cell (Fig. 16.24). In rapid-freeze preparations (Fig. 16.25), cells are rounded and clearly isolated from each other. The nuclei are round and centrally located. The apical portion of the mucous cell contains numerous mucinogen granules and a large Golgi apparatus, in which large amounts of carbohydrate are added to a protein base to synthesize the glycoprotein of the mucin. Mucous cells possess apical junctional complexes, the same as those seen between serous cells.
Myoepithelial cells are contractile cells that embrace the basal aspect of the acinar secretory cells.
Myoepithelial cells are contractile cells with numerous processes. They lie between the basal plasma membrane of the epithelial cells and the basal lamina of the epithelium (Fig. 16.26). Myoepithelial cells also underlie the cells of the proximal portion of the duct system. In both locations, the myoepithelial cells are instrumental in moving secretory products toward the excretory duct. Myoepithelial cells are sometimes difficult to identify in H&E sections. The nucleus of the cell is often seen as a small round profile near the basement membrane. The contractile filaments stain with eosin and are sometimes recognized as a thin eosinophilic band adjacent to the basement membrane.