FIGURE 16.23 • Electron micrograph of the apical portion of parotid gland serous cells. The cells are polarized, with their product package within the secretory vesicles (SV ) near the lumen (L) of the acinus. The cells display rough endoplasmic reticulum (rER) and several profiles of the Golgi apparatus (G). Immature secretory vesicles (IV ) are present close to the Golgi apparatus. At the apical pole of the cells are junctional complexes (JC). The intercellular space (IC) is dilated, and profiles of sectioned lateral plications are seen. M, mitochondria. 15,000.
The lumen of the salivary acinus is continuous with that of a duct system that may have as many as three sequential segments:  Intercalated duct, which leads from the acinus  Striated duct, so-called because of the presence of “stria tions,” the infoldings of the basal plasma membrane of the columnar cells that form the duct  Excretory ducts, which are the larger ducts that empty into the oral cavity
The degree of development of the intercalated ducts and striated ducts varies, depending on the nature of the acinar secretion (see Fig. 16.21). Serous glands have well-developed intercalated ducts and striated ducts that modify the serous secretion by both absorption of specific components from the secretion and secretion of additional components to form the final product. Mucous glands, in which the secretion is not modified, have very poorly developed intercalated ducts that may not be recognizable in H&E sections. Moreover, they do not display striated ducts.
Intercalated ducts are located between a secretory acinus and a larger duct.
Intercalated ducts are lined by low cuboidal epithelial cells that usually lack any distinctive feature to suggest a function other than that of a conduit. However, the cells of intercalated ducts possess carbonic anhydrase activity. In serous-secreting glands and mixed glands, they have been shown to  secrete HCO3  into the acinar product.  absorb Cl from the acinar product.
As noted above, intercalated ducts are most prominent in those salivary glands that produce a watery serous secretion. In mucus-secreting salivary glands, the intercalated ducts, when present, are short and difficult to identify.
Striated duct cells have numerous infoldings of the basal plasma membrane.
FIGURE 16.24 • Low-magnification electron micrograph of a mucous acinus. The mucous cells contain numerous mucinogen granules. Many of the granules have coalesced to form larger irregular masses that will ultimately discharge into the lumen (L) of the acinus. Myoepithelial cell processes (MyC) are evident at the periphery of the acinus. 5,000.
Striated ducts are lined by a simple cuboidal epithelium that gradually becomes columnar as it approaches the excretory duct. The infoldings of the basal plasma membrane are seen in histologic sections as “striations.” Longitudinally oriented, elongated mitochondria are enclosed in the infoldings. Basal infoldings associated with elongated mitochondria are a morphologic specialization associated with reabsorption of fluid and electrolytes. The striated duct cells also have numerous basolateral folds that are interdigitated with those of adjacent cells. The nucleus typically occupies a central (rather than basal) location in the cell. Striated ducts are the sites of  reabsorption of Na from the primary secretion.  secretion of K and HCO3  into the secretion.
More Na is resorbed than K is secreted, so the secretion becomes hypotonic. When secretion is very rapid, more Na and less K appear in the final saliva because the reabsorption and secondary secretion systems cannot keep up with the rate of primary secretion. Thus, the saliva may become isotonic to hypertonic.