Anatomy

Salivary glands are exocrine organs responsible for the production and secretion of saliva. They comprise the three paired major glands, the parotid, sub-mandibular and sublingual, and the minor glands. The latter are numerous and are widely distributed throughout the mouth and oropharynx and similar glands are present in the upper respiratory and sinonasal tracts, and the paranasal sinuses.

Secretory acinus

The functional unit of salivary glands is the secretory acinus and related ducts, and myoepithelial cells. Acini may be serous, mucous or mixed. Serous acini form wedge-shaped secretory cells with basal nuclei. They surround a lumen that becomes the origin of the intercalated duct. The cytoplasm of serous cells contains densely basophilic, refractile zymogen granules that are periodic acid Schiff positive and diastase resistant. Their principle secretion is amylase. Mucous acinar cells also have basally placed nuclei and their cytoplasm is clear and contains vacuoles of sialomucin. The secretions of these cells pass through the intercalated ducts. These are often inconspicuous in routine histological sections. They are lined by what appears to be a single layer of cuboidal cells with relatively large, central nuclei. They are continuous with the much larger striated ducts. The intercalated ducts are lined by a single layer of cuboidal cells with relatively large, central nuclei and are linked to the much larger striated duct. The latter are lined by tall, columnar, eosinophilic cells that are rich in mitochondria. They have parallel infold-ings of the basal cytoplasm and are responsible for modifying the salivary secretions. The striated ducts join the interlobular excretory ducts, which are lined by pseudostratified columnar epithelium that often contains few mucous cells.

Myoepthelial cells

Myoepithelial, or basket cells, are contractile and are located between the basement membrane and the basal plasma membrane of the acinar cells. They are variable in morphology and are inconspicuous in H&E sections. They contain smooth muscle actin, myosin and intermediate filaments including keratin 14. Immunohistochemical stains for the proteins highlights their stellate shape. They have long dendritic processes that embrace the secretory acini. Myoepithelial cells also surround the intercalated ducts but their presence in striated ducts is not firmly established. Ultrastructurally, the cytoplasm of myoep-ithelial cells contains actomyosin microfilaments running parallel with the outer surface of the cell, glycogen granules and lipofuscin, and pinocytotic vesicles may also be a conspicuous feature.

Parotid gland

The parotid gland is almost purely serous and the parenchyma is divided into lobules by fibrous septa. There is abundant intralobular and extralobular adipose tissue which increases in relative volume with age. The parotid gland contains randomly distributed lymphoid aggregates and lymph nodes that range from one to more than 20 in number. Not infrequently the lymph nodes contain salivary gland ducts or occasionally acini (Neisse Nicholson rests). Sebaceous glands, either individually or in small groups, are commonly seen if the tissue is widely sampled.

Fig. 5.0 Parotid gland. Cytokeratin 14 staining highlights dendritic myoepithelial cells.

Submandibular gland The gland is mixed serous and mucous although the serous element predominates (~90%). In mixed acini the serous cells form caps, or demilunes, on the periphery of the mucous cells. The intercalated ducts are shorter and the striated ducts more conspicuous than those of the parotid gland.

Sublingual gland

The gland is also mixed but is predominantly mucous in type. The mucous acini form elongated tubules with peripheral serous demilunes.

Minor salivary glands

These are most numerous at the junction of the hard and soft palate, lips and buccal mucosa. The minor glands of the lateral aspects of the tongue, lips and buccal mucosa are seromucous whereas those in the ventral tongue, palate, glos-sopharyngeal area and retromolar pad are predominantly mucous. Salivary glands related to the circumvallate papillae (von Ebner's glands) are serous in type. The minor glands are not encapsult-ed, and those in the tongue and lip especially can be deeply located in the musculature.

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