Salivary Glands & Saliva – Comprehensive Study Notes

Saliva

  • Complex, multifunctional oral fluid produced by salivary glands.
  • Forms continuous film coating teeth & mucosa ➜ maintains healthy oral environment.
  • Flow rate
    • Low during sleep, high during sensory/mechanical stimulation (chewing, tasting, smelling, thinking of food).
    • Total daily output ≈ (750\;\text{–}\;1000\;\text{mL}).
  • pH range: 6.4\;\text{–}\;7.4 (slightly acidic to neutral).
  • Glandular contribution to total volume
    • Submandibular ≈ 60\%.
    • Parotid ≈ 30\%.
    • Sublingual ≤ 5\%.
    • Minor glands ≈ 1\%.

Composition

  • 99\% water; 1\% solids.
  • Inorganic ions: \text{Na}^+ , \text{K}^+ , \text{Ca}^{2+} , \text{Cl}^- , \text{HCO}3^- , \text{HPO}4^{2-}.
  • Organic / protein components
    • Enzymes: amylase, lysozyme, peroxidase, kallikrein.
    • Glycoproteins: mucins.
    • Antibacterials: lactoferrin, peroxidase–thiocyanate system.
  • Immunoglobulins: \text{IgA}, \text{IgG}, \text{IgM}.
  • Other: albumin, clotting factors, desquamated epithelial cells, gingival crevicular fluid, food remnants.

Functions

  1. Protection & lubrication
    • Lubricates mucosa ➜ facilitates speech, mastication, swallowing.
    • Mechanical cleansing (dilution & clearance of debris/bacteria).
    • Immunologic barrier (secretory \text{IgA}, \text{IgM}, \text{IgG} block microbial adherence).
    • Cushioning from mechanical / thermal insults.
  2. Anticariogenic
    • Rapid sugar clearance; buffering neutralises acids ➜ prevents enamel demineralisation.
  3. Digestion
    • Solubilises food; forms bolus; amylase initiates starch breakdown.
  4. Buffering systems (origin mainly in major glands)
    • \text{HCO}_3^-, phosphate, salivary proteins.
  5. Taste perception
    • Dissolves tastants; gustin protein required for taste-bud maturation.
  6. Antibacterial
    • Peroxidase + thiocyanate: bactericidal oxidants.
    • Lysozyme: hydrolyses bacterial cell-wall peptidoglycan.
    • Lactoferrin: sequesters \text{Fe}^{3+} → starves bacteria.
  7. Tooth integrity
    • Saturated \text{Ca}^{2+}/\text{HPO}_4^{2-} pool enables remineralisation.
  8. Tissue repair
    • Growth factors accelerate wound healing & epithelial regeneration.

Salivary Glands – General Features

  • Type: Exocrine, compound tubulo-acinar merocrine glands.
  • Compound = multiple secretory units drain into branching ductal tree.
  • Merocrine = secretion by exocytosis with no cytoplasmic loss.

Development (brief)

  • Oral epithelium proliferates → epithelial bud → invades ectomesenchyme.
  • Canalisation produces ducts; distal termini differentiate into acini; proximal segment forms excretory duct.

Classification

  1. By size
    • Major (paired): Parotid, Submandibular, Sublingual.
    • Minor (600–1000 glands): labial, buccal, glossopalatine, palatine, lingual, retromolar, minor sublingual.
  2. By nature of secretion
    • Serous ➜ watery, enzyme-rich (Adult parotid, von Ebner).
    • Mucous ➜ viscous, mucin-rich (glossopalatine, palatine, Weber, labial/buccal, anterior lingual, major sublingual).
    • Mixed ➜ both (submandibular, infant/aged parotid, posterior anterior-lingual; minor sublingual partly).
  3. By location
    • Vestibule: labial, buccal.
    • Oral cavity proper: floor of mouth (submandibular, sublingual, glossopalatine, retromolar), tongue (anterior, posterior serous, posterior mucous), palatine (hard, soft palate, uvula).

Histology & Micro-Anatomy

Parenchyma

  1. Secretory end pieces (acini)
    • Serous: pyramidal cells, narrow lumen, basophilic base (RER), apical zymogen granules, round nucleus; high enzymatic output.
    • Mucous: larger columnar cells, wide lumen, pale cytoplasm (lost mucin), flattened basal nucleus; secretion viscous, carb-rich, low enzyme activity.
    • Mixed: mucous acinus capped by serous demilunes (crescents of Gianuzzi).
  2. Duct system
    • Intralobular
      • Intercalated duct: low cuboidal epithelium, narrow; adds lysozyme & lactoferrin.
      • Striated duct: columnar, basal infoldings + mitochondria → “striations”; ion exchange converts primary isotonic saliva → hypotonic (reabsorbs \text{Na}^+, \text{Cl}^-; secretes \text{K}^+, \text{HCO}_3^-).
    • Interlobular / excretory ducts: pseudostratified columnar → stratified columnar cuboidal → main duct lined by stratified squamous epithelium before oral opening.
  3. Myoepithelial (basket) cells
    • Flattened contractile cells between acinar/ductal basal lamina & secretory cells; contain actin/myosin; expel saliva under autonomic control.

Stroma

  • Dense CT capsule extends septa → divides gland into lobes & lobules; conveys blood vessels & nerves.

Serous vs Mucous Acini (key contrasts)

  • Size: serous small; mucous larger.
  • Shape: serous pyramidal; mucous columnar-pyramidal.
  • Staining: serous dark (basophilic base, eosinophilic apex); mucous pale/“empty”.
  • Nucleus: serous round central-basal; mucous flat basal.
  • Lumen: serous narrow/obliterated; mucous wide.

Major Salivary Glands

Parotid

  • Largest; superficial to masseter & deep behind mandibular ramus.
  • Secretion: purely serous in adults; mixed but serous-predominant in infants.
  • Duct: Stensen’s duct — crosses masseter, pierces buccinator, opens opposite maxillary 2nd molar papilla.

Submandibular

  • Second largest; medial to mandibular body in submandibular triangle; ~½ weight of parotid.
  • Secretion: mixed, \approx90\% serous; mucous acini with serous demilunes.
  • Duct: Wharton’s duct (~5\;\text{cm}) opens at sublingual caruncle (papilla beside lingual frenum).

Sublingual

  • Smallest major gland; beneath mucosa of floor of mouth above mylohyoid muscle.
  • Many small lobules: one main gland + series of minor sub-lobules.
  • Secretion: mixed, predominantly mucous; mucous acini with serous demilunes.
  • Ducts: Bartholin’s duct (joins/opens near Wharton’s) + \approx8!–!20 ducts of Rivinus along sublingual fold.

Minor Salivary Glands

  • Scattered in submucosa throughout oral cavity except gingiva & anterior hard palate.
  • Each cluster has short duct, no capsule.
  • Types & secretions
    • Labial & Buccal: pure mucous.
    • Lingual:
      • Anterior (Blandin–Nuhn): anterior portion mucous; posterior portion mixed.
      • Posterior mucous (Weber) near foliate/vallate papillae.
      • Posterior serous (von Ebner) beneath vallate papillae — flushes taste buds.
    • Glossopalatine: pure mucous in glossopalatine arch.
    • Palatine: pure mucous in posterior hard & soft palate, uvula; duct openings = fovea palatinae.

Innervation & Reflex Control

  • Dual autonomic supply (parasympathetic & sympathetic).
    • Parasympathetic ➜ abundant watery saliva; originates from cranial nerves.
      • Parotid: Glossopharyngeal (CN IX) → otic ganglion → auriculotemporal nerve.
      • Submandibular & Sublingual: Facial (CN VII) → chorda tympani → submandibular ganglion.
    • Sympathetic ➜ viscous, protein-rich saliva; vasoconstriction.
  • Both influence myoepithelial cell contraction.
  • Salivary reflexes
    1. Simple/unconditioned: tactile/pressure receptors stimulated by food.
    2. Conditioned (Pavlovian): sight, smell, thought, sound associated with food.

Age-Related Changes

  1. Decline in number of secretory end pieces; terminal atrophy.
  2. Serous acini replaced by mucous cells (functional shift).
  3. Fatty degeneration (adipocyte infiltration).
  4. Loss of intercalated ducts; striations in striated ducts disappear.
  5. Increased fibrous tissue around ducts; stroma thickens.
  6. Lymphocytic infiltration.
  7. Oncocytic metaplasia of duct epithelium → predisposition to neoplasms.

Functional Summary of Ducts

  • Transport primary isotonic saliva from acini ➜ oral cavity.
  • Modify electrolytes: reabsorb \text{Na}^+, \text{Cl}^-; secrete \text{K}^+, \text{HCO}_3^- → final saliva hypotonic.
  • Intercalated ducts secrete lysozyme & lactoferrin (antimicrobials).

Clinical / Practical Connections

  • Buffering & remineralisation underpin caries prevention strategies.
  • Flow reduction (xerostomia) increases caries, mucosal infections, speech/swallowing difficulty.
  • Salivary gland tumours often arise where serous ➜ mucous conversion or oncocytic change (age-related).
  • Sialography & scintigraphy exploit duct anatomy for diagnostics.