5.1 Salivary Gland Anatomy, Microarchitecture and Innervation

1. Functional Anatomy, Microarchitecture, and Innervation of Salivary Glands

Human salivary glands are categorized anatomically into three pairs of major salivary glands located external to the oral cavity with extended ductal systems, and hundreds of minor salivary glands distributed widely throughout the submucosa of the oral mucosa.

The Parotid Gland

• Anatomical Relations: The largest salivary gland (~15–30 g), situated in the retromandibular fossa anterior and inferior to the external acoustic meatus. It is encapsulated by the tough investing layer of deep cervical fascia. Crucial structures traversing the gland substance from deep to superficial include: the external carotid artery (which bifurcates into the superficial temporal and maxillary arteries), the retromandibular vein (formed by the union of the superficial temporal and maxillary veins), and the facial nerve (CN VII), which divides within the gland at the pes anserinus into its five terminal motor branches (temporal, zygomatic, buccal, marginal mandibular, cervical).
• Secretory Microarchitecture: Purely serous in adult humans. Acinar cells are pyramidal, enclosing a narrow lumen, with apical zymogen granules packed with salivary alpha-amylase and proline-rich proteins (PRPs).
• Ductal Pathway: Stensen's duct (~5 cm long) emerges from the anterior border of the gland, crosses the superficial surface of the masseter muscle, turns medially at its anterior border to pierce the buccinator muscle, and terminates at a mucosal papilla on the buccal mucosa opposite the crown of the maxillary second permanent molar.
• Secretomotor Innervation (CN IX):
  1. Preganglionic parasympathetic cell bodies originate in the inferior salivatory nucleus of the medulla oblongata.
  2. Fibres travel via the glossopharyngeal nerve (CN IX), exit through the jugular foramen, and branch into the tympanic nerve (Jacobson's nerve).
  3. Fibres form the tympanic plexus in the middle ear cavity and reform as the lesser petrosal nerve.
  4. The lesser petrosal nerve traverses the foramen ovale to synapse in the otic ganglion.
  5. Postganglionic parasympathetic fibres hitchhike along the auriculotemporal nerve (a branch of the mandibular division of the trigeminal nerve, CN V3) to supply the parotid parenchyma.
  6. Sympathetic supply: Originates from the T1–T2 spinal cord segments, ascends the sympathetic chain to synapse in the superior cervical ganglion, and postganglionic fibres travel as a plexus along the external carotid artery.

The Submandibular Gland

• Anatomical Relations: Intermediate in size (~7–10 g), located in the submandibular triangle, bounded by the anterior and posterior bellies of the digastric muscle and the inferior border of the mandible. The gland folds around the posterior free border of the mylohyoid muscle, dividing it into a large superficial lobe and a smaller deep lobe resting on the hyoglossus muscle.
• Secretory Microarchitecture: Mixed serous and mucous, with a strong serous predominance (~80% serous acini, ~20% mucous tubules capped by serous demilunes of Giannuzzi).
• Ductal Pathway: Wharton's duct (~5 cm long) emerges from the deep lobe, courses anteriorly between the mylohyoid and hyoglossus muscles, and opens into the oral cavity at the sublingual caruncle (papilla) located on the floor of the mouth immediately lateral to the base of the lingual frenulum. Crucially, the lingual nerve loops inferiorly and medially around Wharton's duct ("water under the bridge") as it travels anteriorly.
• Sialolithiasis Predilection: Approximately 80% to 90% of all salivary calculi (sialoliths) occur in the submandibular gland and Wharton's duct due to three distinct factors:
  1. Tortuous, anti-gravity ascending ductal path around the mylohyoid muscle.
  2. Higher viscosity resulting from higher mucin content compared to parotid saliva.
  3. High alkaline pH and elevated calcium and phosphate concentrations, promoting mineral precipitation.
• Secretomotor Innervation (CN VII):
  1. Preganglionic parasympathetic cell bodies reside in the superior salivatory nucleus in the pons.
  2. Fibres leave the brainstem via the nervus intermedius of the facial nerve (CN VII) and enter the internal acoustic meatus.
  3. In the temporal bone, fibres branch into the chorda tympani, which traverses the middle ear cavity across the medial neck of the malleus, exits the skull through the petrotympanic fissure, and merges into the lingual nerve (CN V3) in the infratemporal fossa.
  4. Fibres hitchhike with the lingual nerve and synapse in the submandibular ganglion, suspended from the lingual nerve above the gland.
  5. Postganglionic fibres enter the submandibular and sublingual glands directly.
  6. Sympathetic supply: Derives from the superior cervical ganglion via the plexus on the facial artery.

The Sublingual Gland

• Anatomical Relations: The smallest of the major glands (~2–4 g), situated beneath the sublingual mucosa in the sublingual fossa of the mandible, superior to the mylohyoid muscle.
• Secretory Microarchitecture: Mixed, but predominantly mucous (~60–70% mucous acini).
• Ductal System: Uniquely lacks a single large duct. It drains via 8 to 20 small ducts called the ducts of Rivinus, which open along the crest of the sublingual fold (plica sublingualis). Occasionally, several anterior ducts coalesce to form a main duct of Bartholin, which joins Wharton's duct.
• Innervation: Identical parasympathetic pathway to the submandibular gland (CN VII via chorda tympani, lingual nerve, and submandibular ganglion).

Minor Salivary Glands

• Between 600 and 1,000 unencapsulated minor salivary glands reside within the oral submucosa (labial, buccal, palatal, glossopalatine, and lingual).
• Von Ebner's Glands: Located exclusively in the troughs surrounding the circumvallate and foliate lingual papillae. Uniquely purely serous minor glands, they secrete lingual lipase (initiating lipid digestion) and a watery serous fluid that flushes tastants from the papillary trenches to enable rapid temporal taste perception.
• Glands of Blandin-Nuhn: Mixed glands located on the ventral tip of the tongue.
• Glands of Weber: Purely mucous glands located along the lateral border of the tongue and base of the tonsillar crypts.
• Physiological Importance: While minor glands contribute only ~7% to 10% of total daily salivary volume, they produce approximately 70% of total resting salivary mucins, providing a continuous, protective lubricating coating over the oral epithelium that persists independent of gustatory stimulation.

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