Salivary Glands

Salivary Glands

• There are three bilateral pairs of major salivary glands: parotid glands, submandibular glands, and sublingual glands.
• The major salivary glands produce 90% of the total salivary volume.
• There are also numerous minor salivary glands.

Parotid

• Largest of the major glands.
• Produces 100% serous saliva.
• Responsible for 25% of the total saliva volume.
• Positioned below the external auditory meatus, between the mastoid process and the posterior border of the ramus.
• Its duct is Stensen’s (or parotid) duct, which is 5cm long, runs from the gland outside the masseter muscle parallel to, and 1cm below, the zygomatic arch, pierces the buccinator muscle opening into the oral cavity via the Stensen’s papillae.
• Innervation is via the glossopharyngeal nerve (autonomic), auriculotemporal nerve (sensory), and intimately associated with the Facial nerve.

Submandibular

• Mixed saliva secretions.
• Half the size of the parotid gland (walnut size).
• Produces 60-65% of total saliva volume.
• Positioned between the body of the mandible and the mylohyoid muscle, in the submandibular fossa.
• Its duct is Wharton's duct, which is 5cm long and opens under the anterior part of the tongue, lateral to the lingual fraenum at the sublingual caruncle.
• Innervation is via the chorda tympani and lingual branch of the inferior dental nerve.

Sublingual

• 60% mucous saliva.
• Smallest of the major salivary glands (almond size).
• Produces 5-10% of total saliva volume.
• Positioned in the floor of the mouth in the sublingual fossa.
• Its duct is Bartholin’s duct, with 10-20 smaller ducts (Rivinus ducts) opening along the sublingual fold.
• Innervation is the same as the submandibular gland.

Tubarial Glands

• Located in the nasopharynx.
• Contains a large number of seromucous acini.
• Plays a role in nasopharynx/oropharynx lubrication and swallowing.

Minor Salivary Glands

• Mixed saliva, predominantly mucous.
• Produce lots of salivary proteins.
• Responsible for >10% of the total saliva volume.
• Named according to their location, e.g., buccal or labial salivary glands.

Salivary Gland Structure

• Epithelium lines the ducts and produces the saliva.
• Connective tissue surrounds the epithelium and provides support and protection for the gland.
• The connective tissue is divided into the capsule and septa:
  • The capsule surrounds the entire outer portion of the gland.
  • The septum (plural, septa) divides the inner portion of the gland into lobes and smaller lobules.

Adenomeres

• The working part of a salivary gland.
• Surrounded by connective tissue.

Acini

• Secretory units within the adenomere made up of secretory cells.
• The base rests against the surrounding connective tissue, supplying nerves and blood to the cells.
• Classified into mucous acini, serous acini, or a mixture of both.

Serous Acini

• Made up of serous secretory cells.
• Produce a watery serous secretion (serous saliva).
• Functions include:
  • Lubricating food.
  • Beginning enzymatic digestion.
  • Removing epithelial debris.
  • Diluting food.

Mucous Acini

• Made up of mucous secretory cells.
• Wider lumen.
• Produce a viscous mucin-rich secretion (mucous saliva).
• Functions include:
  • Binding food into a bolus.
  • Protecting the oral cavity against frictional abrasion.
  • Lubrication.

Serous-Mucous Acini

• In a mixed serous-mucous acini, the serous secretory cell forms a serous demilune around a mucous secretory cell.

Myoepithelial Cells

• Embrace the acini secretory cells, contracting and squeezing to force the saliva out of the lumen and into the ducts.

Acinar Fluid

• Not yet saliva.
• Consists of:
  • Water.
  • Inorganic ions.
  • Small molecules and products synthesised by cells (mucoproteins and amylase).

Ducts

• Lie after the secretory end piece.
• Saliva travels through these ducts and is further modified via resorption.
• Types of ducts include intercalated, striated, and excretory.

Intercalated Ducts

• Lined by a single layer of cuboidal epithelial cells.

Striated Ducts

• Make up the bulk of the duct system.
• Lined with a single layer of columnar epithelial cells characterized by basal striations.
• Aids in the modification of the saliva.

Excretory (Terminal) Ducts

• Also known as the secretory duct.
• Saliva exits into the oral cavity via this duct.
• Lined by pseudostratified columnar epithelium, which then changes to stratified cuboidal and finally stratified squamous epithelium at its opening.

Blood Supply

• Supplied by the external carotid artery (plus facial and lingual artery), with vessels (and nerves) entering the gland at the hilum.
• There are two capillary networks: one for the secretory end piece and one for the ducts.

Control of salivation

• Salivary nuclei in the brain are stimulated by taste, smells, thoughts, etc.
• They trigger neurotransmitter release from nerve endings of salivary glands.

Disorders of the Salivary Glands

Hyposalivation

• Can be caused by:
  • Medications or tablets.
  • Radiotherapy.
  • Autoimmune diseases.
  • Diabetes.
  • Salivary stones (can cause severe pain when eating).

Obstruction

• Caniculi: Calcium deposits forming in the ducts.
  • Common in the submandibular glands.
  • Can lead to infection or inflammation of the gland.

Cysts

• Trauma to the salivary gland or duct causing an accumulation of saliva in the surrounding tissue.
  • Mucocele: minor glands.
  • Ranula: major glands in the floor of the mouth.

Irradiation

• Head and neck irradiation destroys the secretory cells (atrophy) and leads to xerostomia.

Functional Disorders

• Dry mouth caused by:
  • Emotional disturbances, e.g., anxiety.
  • Mouth breathing.
  • Smoking.
  • Drugs.