The diameter of striated ducts often exceeds that of the secretory acinus. Striated ducts are located in the parenchyma of the glands (they are intralobular ducts) but may be surrounded by small amounts of connective tissue in which blood vessels and nerves can be seen running in parallel with the duct.
Excretory ducts travel in the interlobular and interlobar connective tissue.
Excretory ducts constitute the principal ducts of each of the major glands. They ultimately connect to the oral cavity. The epithelium of small excretory ducts is simple cuboidal. It gradually changes to pseudostratified columnar or stratified cuboidal. As the diameter of the duct increases, stratified columnar epithelium is often seen, and as the ducts approach the oral epithelium, stratified squamous epithelium may be present. The parotid duct (Stensen’s duct) and the submandibular duct (Wharton’s duct) travel in the connective tissue of the face and neck, respectively, for some distance from the gland before penetrating the oral mucosa.
The parotid glands are completely serous.
The paired serous parotid glands are the largest of the major salivary glands. The parotid duct travels from the gland,
FIGURE 16.25 • Electron micrographs of mixed acini. a. Low-magnification electron micrograph of the sublingual gland, prepared by the rapid freezing and freeze-substitution method, shows the arrangement of the cells within a single acinus. The mucous cells have well-preserved round mucinogen granules. The mucous and serous cells are aligned to surround the acinus lumen. Serous demilunes are not evident. 6,000. b. Electron micrograph of the sublingual gland prepared by traditional fixation in formaldehyde. Note the considerable expansion and coalescence of the mucinogen granules and the formation of a serous demilune. 15,000. (Courtesy of Dr. Shohei Yamashina.) which is located below and in front of the ear, to enter the oral cavity opposite the second upper molar tooth. The secretory units in the parotid are serous and surround numerous, long, narrow intercalated ducts. Striated ducts are large and conspicuous (Fig. 16.27a).
Large amounts of adipose tissue often occur in the parotid gland; this is one of its distinguishing features (Plate 52, page 565). The facial nerve (cranial nerve VII) passes through the parotid gland; large cross sections of this nerve may be encountered in routine H&E sections of the gland and are useful in identifying the parotid. Mumps, a viral infection of the parotid gland, can damage the facial nerve.
The submandibular glands are mixed glands that are mostly serous in humans.
The large, paired, mixed submandibular glands are located under either side of the floor of the mouth, close to the mandible. A duct from each of the two glands runs forward and medially to a papilla located on the floor of the mouth just lateral to the frenulum of the tongue. Some mucous acini capped by serous demilunes are generally found among the predominant serous acini. Intercalated ducts are less extensive than in the parotid gland (Fig. 16.27b and Plate 51, page 563).
The small sublingual glands are mixed glands that are mostly mucous secreting in humans.