duct. Sometimes, a collection of draining ducts coalesce anteriorly to form a major duct (Bartholin's duct) which opens with the orifice of the submandibular duct at the sublingual papilla (Zhang et al. 2010).
Blood Supply, Innervation, and Lymphatic Drainage
The arterial supply is from the sublingual branch of the lingual artery and also the submental branch of the facial artery. Innervation is via the sublingual ganglion as described above. The lymphatics drain to the submental nodes.
Minor Salivary Glands
Minor salivary glands are distributed widely in the oral cavity and oropharynx. They are grouped as labial, buccal, palatoglossal, palatal, and lingual glands. The labial and buccal glands contain both mucous and serous acini whereas the palatoglossal glands are mucous secreting. The palatal glands which are also mucous secreting occur in both the hard and soft palates. The anterior and posterior lingual glands are mainly mucous secreting. The anterior glands are embedded within the muscle ventrally and they drain via four or five ducts near the lingual frenum. The posterior lingual glands are located at the root of the tongue. The deep posterior lingual glands are predominantly serous secreting. Additional serous glands (glands of von Ebner) occur around the circumvallate papillae on the dorsum of the tongue. Their watery secretion is thought to be important in spreading taste stimuli over the taste buds.
Tubarial Salivary Glands
In 2020, Valstar et al reported on the serendipitous presence of bilateral macroscopic salivary gland structures in the nasopharynx of humans. Their existence was visualized by positron emission tomography/computed tomography with prostate‐specific membrane antigen ligands (PSMA PET/CT). The presence of the PSMA‐positive nasopharyngeal regions was elucidated in a retrospective cohort of 100 consecutive patients with prostate or urethral gland cancer. The designated area of the posterior nasopharynx was also studied with hematoxylin and eosin (H&E) and PSMA and alpha‐amylase) immunohistochemistry in two human cadavers. All 100 patients (99 males, one female; median age 69.5, range 53–84) demonstrated a well‐demarcated bilateral PSMA‐positive region on PSMA PET/CT. This 3.9 cm cranio‐caudal structure (range 1.0–5.7 cm) extended from the skull base along the posterolateral pharyngeal wall on the pharyngeal aspect of the superior pharyngeal constrictor muscle with a PSMA‐positive structure located predominantly over the torus tubarius. The tracer uptake in these structures was similar to the uptake of the sublingual glands. The dissected structures from the two human cadavers demonstrated a large aggregate of mucous salivary gland tissue with multiple visible draining duct openings in the dorsolateral pharyngeal wall. The authors concluded that the human body contains a pair of once overlooked and clinically significant salivary glands in the posterior nasopharynx, and the authors proposed the name tubarial glands. Their particular interest in these structures was in gland sparing radiation protocols for head and neck cancer in the best interests of maintaining salivary function and the quality of life of patients.
Histology of the Salivary Glands
The salivary glands are composed of large numbers of secretory acini that may be tubular or globular in shape. Each acinus drains into a duct. These microscopic ducts coalesce to form lobular ducts. Each lobule has its own duct and these then merge to form the main ducts. The individual lobes and lobules are separated by dense connective tissue that is continuous with the gland capsule. The ducts, blood vessels, lymphatics, and nerves run through and are supported by this connective tissue.
The acini are the primary secretory organs but the saliva is modified as it passes through the intercalated, striated, and excretory ducts before being discharged into the mouth and oropharynx (Figure 1.14). The lobules also contain significant amounts of adipose tissue particularly in the parotid gland. The proportion of adipose tissue relative to excretory acinar cells increases with age.
In the human parotid, the excretory acini are almost entirely serous. In the submandibular gland, again, the secretory units are mostly serous but there are additional mucous tubules and acini. In some areas, the mucinous acini have crescentic “caps” of serous cells called serous demilunes. In the sublingual gland, the acini are almost entirely mucinous although there are occasional serous acini or demilunes.
The serous cells contain numerous proteinaceous secretory (zymogen) granules. These granules contain high levels of amylase. In addition, the secretory cells produce kallikrein, lactoferrin, and lysozyme. In mucous cells, the cytoplasm is packed with large pale secretory droplets.
Initially the secretory acini drain into intercalated ducts. These ducts function mainly to conduct the saliva but they may also modify the electrolyte content and secrete immunoglobulin A. The intercalated ducts drain into striated ducts that coalesce into intralobular and extralobular collecting ducts. The intercalated duct cells are very active metabolically, and they transport potassium and bicarbonate into saliva. They reabsorb sodium and chloride ions so that the resulting saliva is hypotonic. They also secrete immunoglobulin A, lysozyme, and kallikrein. The immunoglobulin is produced by plasma cells adjacent to the striated duct cells, and it is then transported through the epithelial lining into the saliva. The main collecting ducts are simple conduits for saliva and do not modify the composition of the saliva.
Myoepithelial cells are contractile cells closely related to the secretory acini and much of the duct system. The myoepithelial cells lie between the basal lamina and the epithelial cells. Numerous cytoplasmic processes arise from them and surround the serous acini as basket cells. Those associated with the duct cells are more fusiform and are aligned along the length of the ducts. The cytoplasm of the myoepithelial cells contains actin myofilaments that contract as a result of both parasympathetic and sympathetic activity. Thus, the myoepithelial cells “squeeze” the saliva out of the secretory acini and ducts and add to the salivary secretory pressure.
Although the parotid capsule is a continuous structure covering the superficial and deep aspects of the gland, it becomes very attenuated where the gland envelopes the branches of the facial nerve. This is of some significance in parotid surgery as when peeling the gland off the branches of the nerve there may be none or a very thin capsule separating the gland from the nerve.
Control of Salivation
There is a continuous low background saliva production that is stimulated by drying of the oral and pharyngeal mucosa. A rapid increase in the resting levels occurs as a reflex in response to masticatory stimuli including the mechanoreceptors and taste fibers. Other sensory modalities such as smell are also involved. The afferent input is via the salivatory centers that are themselves influenced by the higher centers. The higher centers may be facilitory or inhibitory depending on the circumstances. The efferent secretory drive to the salivary glands passes via the parasympathetic and sympathetic pathways. There are no peripheral inhibitory mechanisms.
Cholinergic nerves (parasympathetic) often accompany ducts and branch freely around the secretory endpieces (acini). Adrenergic nerves (sympathetic) usually enter the glands along the arteries and arterioles and ramify with them. Within the glands, the nerve fibers intermingle such that cholinergic and adrenergic axons frequently lie in adjacent invaginations of a single Schwann cell (Garrett and Kidd 1993). Secretion and vasoconstriction are mediated by separate sympathetic axons whereas a single parasympathetic axon may, through serial terminals, result in vasodilatation, secretion, and constriction of myoepithelial cells.
Figure 1.14. Diagram showing the histology of the major components of the salivary glands.
Secretory endpieces are the most densely innervated structures in the salivary glands. Individual acinar cells may have