Oral Mucosa Development and Functions

Oral Mucosa

• Moist lining of the GI tract, nasal passages and other body cavities communicating with the external environment.
• Continuous with the skin (lips area) and with the rest of the gut (pharynx).
• Located between the skin and gastrointestinal (GI) tract.
• Shares similar components with skin and GI lining: a covering epithelium and underlying connective tissue.

Development of Oral Mucosa

• Primitive oral cavity (stomodeum) fuses to foregut after the rupture of the buccopharyngeal membrane (26 days of gestation).
• Structures that develop from branchial arches (tongue, epiglottis, pharynx) are covered by epithelium derived from endoderm.
• Palatal, buccal, gingival epithelium and epithelium covering the floor of the mouth and lips are ectodermal origin.

Functions of Oral Mucosa

• Protection:
  • Withstand compression, stretching, shearing, abrasion & bacterial influences.
  • Barrier to microorganisms, toxins and various antigens.
  • Immunological defense mechanisms: humoral and cell mediated.
• Sensation:
  • Temperature, pressure, pain, taste.
  • Reflex receptors for swallowing, gagging.
• Secretion:
  • Maintenance of moist surface, buffering, secretion of antibodies.
  • Major salivary glands produce saliva.
  • Minor salivary glands are present in the submucosa except gingiva and anterior part of the hard palate.
• Thermal regulation:
  • In animals like dogs, panting helps regulate body temperature.

Differences Between Oral Mucosa and Skin

• Oral mucosa:
  • Absence of skin appendages but has salivary glands.
  • Sometimes ectopic sebaceous glands (Fordyce’s granules).
  • Smoother with less wrinkles.
  • Deeply colored.
• Skin:
  • Hair follicles, sebaceous glands, sweat glands.

Fordyce Granules

• Common soft tissue variation seen in the oral cavity.
• Ectopic sebaceous glands.
• Characterized by yellowish-white papular lesions scattered throughout the oral mucous membrane.
• More common in the buccal mucosa.

Similarities Between Oral Mucosa and Skin

• Both have:
  • Covering epithelium
  • Underlying connective tissue

Organization of Oral Cavity

• Outer vestibule:
  • Bounded by lips & cheeks.
• Oral cavity proper:
  • Separated from vestibule by alveolus bearing teeth and gingiva.
  • Superior: Hard and soft palate.
  • Inferior: Floor of mouth & tongue base.
  • Posterior: Fauces & tonsils.

Regional Variation of Oral Mucosa

• Lining mucosa (60%):
  • Mobile and distensible.
  • Non-keratinized mucosa.
  • Found in the whole mouth except gingiva, hard palate, and dorsum of the tongue.
• Masticatory mucosa (25%):
  • Attached to bone (muco-periosteum).
  • Resists high levels of friction & compression.
  • Keratinized type of mucosa.
  • Found in gingiva, hard palate, and dorsum of the tongue.
• Specialized mucosa (15%):
  • Found in the dorsal surface of the tongue.

Lining Mucosa

• Composed of non-keratinized stratified squamous epithelium.
• Found in areas of the oral cavity where there is less wear and tear.
• Thinner and more flexible than masticatory mucosa.

Masticatory Mucosa

• Composed of keratinized stratified squamous epithelium.
• Found in areas of the oral cavity that are subject to more wear and tear, such as the hard palate and gingiva.
• Provides a protective barrier against abrasion and injury.

Composition of Oral Mucosa

• Overlying epithelium:
  • Epidermis in skin.
• Basement membrane:
  • Between epithelium & lamina propria.
• Underlying connective tissue (lamina propria):
  • Dermis in skin.
  • Supports and nourishes the epithelium.

Epithelium-Connective Tissue Interface

• Epithelium projections into underlying connective tissue: rete ridges.
• Larger interface with stronger binding.
• Interface for metabolic exchange (epithelium has no blood vessels).
• More numerous, longer, and thinner in masticatory mucosa than lining mucosa.

Basement Membrane

• Composed of two zones:
  • Lamina lucida: immediately underneath the epithelium.
  • Lamina densa: finely granular or filamentous material of type IV collagen.

Clinical Considerations

• Auto-immune diseases: antibodies directed against any element of the desmosomes, hemidesmosomes, basement membrane.
• Split between epithelium & underlying connective tissue - sloughing of epithelium.

Lamina Propria

• Connective tissue supporting the epithelium.
• Two layers:
  • Papillary LP between rete ridges.
  • Reticular (net-like) LP.
• Cells: fibroblasts, macrophages, mast cells, inflammatory cells.
• Fibers: collagen (type I & III, IV & VII in basal lamina), elastic fibres.
• Ground substance.

Vascular Supply of Mucosa

• Deep plexus of vessels in submucosa or deep reticular layer.
• Secondary plexus in LP when submucosa is absent.
• Necessary for metabolic needs of the avascular epithelium.

Regional Variations in Mucosal Structure

• Non-keratinized/ Lining/ Alveolar Mucosa:
  • Labial mucosa inside lips.
  • Cheek.
  • Soft palate.
  • Floor of mouth and ventral surface of the tongue.
• Keratinized/ Masticatory Mucosa:
  • Hard palate.
  • Gingiva.
  • Dorsum of the tongue.

Vermilion Zone

• Lacks appendages of skin.
• Sebaceous glands at the angles of the mouth.
• Lacks mucous glands, needs constant moistening with saliva.
• Keratinized translucent epithelium.
• Long and narrow connective tissue papillae with capillary loops.
• Junction between vermillion zone and labial mucosa is INTERMEDIATE ZONE - no granular layer, thick parakeratinised layer.
• Forms suckling pads in infants.

Turnover Times of Oral Epithelium

• Skin: 52-75 days.
• Gingiva: 41-57 days.
• Cheek: 25 days.
• Junctional epithelium: 5-6 days.
• Non-keratinized epithelium turns over faster than keratinized epithelium.

Age Changes of Oral Mucosa

• Mucosal surface is smoother, dryer, atrophic, and friable due to cumulative effect of systemic diseases and medication.
• Histologically: epithelium is thinner, smooth epithelium-connective tissue interface, flattening of epithelial ridges, lamina propria shows decreased cellularity and increased amount of cross-linked collagen.
• Dorsum of the tongue: glossy appearance, reduced number of filiform papillae, more prominent fungiform papillae.
• Declined cell-mediated immunity.
• Development of varicosities.
• Sebaceous glands of the lips and cheeks are more prominent, minor salivary glands show atrophic signs.
• Dryness of mouth, burning sensation, abnormal taste.