Major Salivary Glands

Overview

The major salivary glands are paired organs and include the parotid, submandibular, and sublingual glands.
They are compound tubuloacinar exocrine glands.
Each gland is surrounded by a thin capsule.
The capsule extends septa into the glandular parenchyma, dividing it into lobes and lobules.
Functions :
- Produce and secrete saliva into the oral cavity.
- Maintain hydration and protect the oral mucosa and teeth.
- Lubricate the oral cavity during speech, mastication, and swallowing.
- Begin carbohydrate digestion through salivary amylase.
- Contribute to oral immune defense through antimicrobial components, including secretory IgA.
Parotid Gland :
- The parotid glands are the largest major salivary glands, located in the parotid region, anterior and inferior to the ear.
- The main parotid duct is the Stensen duct. It opens into the oral cavity opposite the upper second molar.
- May contain intraparenchymal lymph nodes.
Submandibular Gland :
- The submandibular glands are also called submaxillary glands, located beneath the mandible, in the submandibular region, with a deep portion extending toward the floor of the mouth.
- The main submandibular duct is the Wharton duct. It opens at the sublingual caruncle, just lateral to the lingual frenulum.
Sublingual Gland :
- The sublingual glands are the smallest major salivary glands, located in the anterior floor of the mouth, beneath the oral mucosa.
- Numerous small ducts of Rivinus drain directly into the floor of the mouth along the plica sublingualis.
- A larger sublingual duct, the Bartholin duct, may join the submandibular duct near the sublingual caruncle.
Microscopically, the functional structure of a salivary gland consists of secretory acini and a duct system.

Acini

Each gland has a different acinar composition :
- The parotid gland is almost entirely serous.
- The submandibular gland is mixed, with serous predominance.
- The sublingual gland is mixed, with mucous predominance.
Serous Acini :
- Serous acini are usually rounded or spherical.
- They are composed of pyramidal serous cells with round nuclei, usually located basally.
- The basal cytoplasm is basophilic because of abundant rough endoplasmic reticulum.
- The apical cytoplasm contains eosinophilic zymogen granules.
Mucous Acini :
- Mucous acini are usually tubular or tubuloacinar.
- They are composed of pale mucous cells with flattened nuclei displaced toward the basal part of the cell.
- The cytoplasm contains mucin-rich secretory material.
Mixed Acini :
- Mixed acini contain both mucous cells and serous cells.
- The mucous cells usually form the main portion of the acinus.
- Serous cells may appear as crescent-shaped caps, known as serous demilunes or crescents of Giannuzzi, at the periphery of mucous acini.
Myoepithelial and Basal Cells :
- Myoepithelial cells are flattened contractile cells that are difficult to identify on routine H&E staining.
- They are located between the secretory epithelial cells and the basal lamina, mainly around acini and intercalated ducts.
- Their contraction helps expel secretory products into the duct system.
- Basal cells are noncontractile cells present mainly in larger ducts, especially excretory ducts.

The parotid gland is composed almost entirely of serous acini. Multiple striated ducts and larger interlobular ducts are visible in this section. — The parotid gland is composed almost entirely of . Multiple and larger are visible in this section.

This section shows numerous serous acini with dark-staining cytoplasm, together with an intercalated duct characterized by a narrow lumen. — This section shows numerous serous acini with dark-staining cytoplasm, together with an characterized by a narrow lumen.

Serous acinar cells show basally located round nuclei, basophilic basal cytoplasm, and apical secretory granules.

Intercalated ducts are closely associated with the acini and are lined by simple cuboidal epithelium, with small round nuclei and scant cytoplasm. — are closely associated with the acini and are lined by simple cuboidal epithelium, with small round nuclei and scant cytoplasm.

This section of the sublingual gland shows less prominent adipose tissue than the parotid gland, with clearly visible secretory acini. — This section of the sublingual gland shows less prominent than the parotid gland, with clearly visible secretory acini.

At higher magnification, the gland shows pale mucous acini and darker serous acini. The sublingual gland is predominantly mucous, with mixed acini showing serous demilunes. — At higher magnification, the gland shows pale and darker serous acini. The sublingual gland is predominantly mucous, with mixed acini showing .

In mixed acini, serous cells may form crescent-shaped caps at the periphery of mucous acini, known as serous demilunes. — In mixed acini, serous cells may form crescent-shaped caps at the periphery of mucous acini, known as .

The acini are surrounded by a thin layer of contractile myoepithelial cells, which help expel secretions into the ductal system. — The acini are surrounded by a thin layer of contractile , which help expel secretions into the ductal system.

This section of the submandibular salivary gland shows glandular parenchyma with less adipose tissue than the parotid gland.

The submandibular gland is a mixed seromucous gland, composed of both serous and mucous components, with predominance of serous acini.

This section shows mucous acini capped by characteristic serous demilunes, along with an adjacent intercalated duct. — This section shows mucous acini capped by characteristic , along with an adjacent .

The mucicarmine stain highlights mucous cells within the seromucous units, emphasizing their tubular architecture, in contrast to the more rounded appearance of serous acini.

Duct System

The duct system carries saliva from the acini to the oral cavity.
The duct system is branched and includes intercalated ducts, striated ducts, interlobular ducts, and main excretory ducts.
Intercalated Ducts :
- Intercalated ducts are intralobular ducts, located between the acini and the striated ducts.
- Lined by low cuboidal epithelium and have a usually narrow lumen.
Striated Ducts :
- Striated ducts are also intralobular ducts, located between the intercalated ducts and the larger excretory ducts.
- Lined by simple cuboidal to columnar epithelial cells with eosinophilic cytoplasm.
- Basal striations are visible at the base of the cells.
- These striations correspond to basal plasma membrane infoldings.
Interlobular and Main Excretory Ducts :
- They are larger than intralobular ducts and are located within the connective tissue septa.
- Their epithelium may be simple columnar, pseudostratified columnar, or stratified columnar.
- Near the oral cavity, the epithelium may become stratified squamous.
- Goblet cells may be present.

Striated ducts are lined by simple cuboidal to columnar epithelium and usually show a regular, rounded lumen. They receive secretions from intercalated ducts and drain into larger interlobular ducts.

The epithelial cells of striated ducts show basal striations, produced by basal plasma membrane infoldings. — The epithelial cells of striated ducts show , produced by basal plasma membrane infoldings.

This example shows a large-caliber interlobular duct lined by pseudostratified epithelium.

Large interlobular ducts are located within dense collagenous connective tissue septa and often show a wide, irregular lumen.

Large interlobular ducts are lined by tall epithelial cells with round to elongated nuclei and eosinophilic cytoplasm.

This section shows the buccal squamous mucosa overlying the sublingual gland. A large excretory duct is visible and courses toward the mucosal surface. — This section shows the buccal squamous mucosa overlying the . A is visible and courses toward the mucosal surface.

This example of the parotid gland shows thick nerve bundles and blood vessels traversing the glandular stroma. — This example of the parotid gland shows thick and traversing the glandular stroma.

Distinctive Features

Parotid Gland :
- The parotid gland is composed almost entirely of serous acini.
- It often contains abundant mature adipose tissue, especially in adults.
- Large nerve bundles, including branches of the facial nerve, may be seen within the gland.
- Intraparenchymal lymph nodes may be present and may contain benign salivary gland inclusions.
- Occasional sebaceous cells may be observed.
Submandibular Gland :
- The submandibular gland contains less adipose tissue than the parotid gland.
- It is a mixed seromucous gland composed mainly of serous acini, with fewer mucous acini.
- Striated ducts are usually prominent.
Sublingual Gland :
- The sublingual gland is predominantly mucous, and serous demilunes may be seen around mucous acini.
- Intercalated ducts and striated ducts are usually short and less conspicuous than in the parotid and submandibular glands.

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