• Clinical science

Oral cavity

Summary

The mouth is the facial opening of the gastrointestinal tract. The oral cavity, which is bounded by the lips anteriorly, cheeks laterally, and the oropharynx posteriorly, encloses the tongue, palates, gums, and teeth. The mouth's primary function is the initiation of the digestion process, which involves ingestion, chewing to break down food (mastication), the release of digestive enzymes from the salivary glands into the oral cavity, and swallowing (deglutition). Secondary functions include taste (gustation), sound production and speech articulation, ventilation, facial expression, and touch. The oral cavity can be anatomically divided into the oral vestibule, the space between the teeth and the mucosa of the lips and cheeks, and the oral cavity proper, the space bounded anteriorly and laterally by the teeth and posteriorly by the oropharynx. The teeth are hard, calcified structures used for sound articulation as well as biting and masticating food. The oral cavity is separated from the nasal cavity by the palate, which is divided into the soft and the hard palate. The masticatory movements of the jaw are enabled by the muscles of mastication, and swallowing is facilitated by the palatine muscles. The tongue is the organ responsible for the gustatory sense, which occurs via stimulation of the taste buds situated in the lingual papillae. The oral cavity is, furthermore, a secondary respiratory channel, which communicates with the nasal cavity posteriorly.

Anatomy

Overview

The oral cavity is divided into two spaces: the oral vestibule and the oral cavity proper.

Anatomical boundaries

Anatomical structures

Structures of the oral cavity
Lips
Oral (buccal) mucosa
  • Mucous membrane lining the sides of the oral vestibule and the floor of the mouth to the gums
  • Nonkeratinized stratified squamous epithelium with no sebaceous glands or hair follicles
  • Composed of the following:
    • Lining mucosa, which covers the lips, the cheeks, the floor of mouth, and the ventral surface of the tongue
    • Masticatory mucosa, which covers the palate and gums
    • Specialized mucosa, which covers the dorsal surface and lateral borders of the tongue (carry nerve endings for sensory and taste perception)
Intermaxillary commissure
Teeth
  • Hard, calcified structures used to break down food
  • Embedded in the alveolar processes of the mandible and maxilla (see “Teeth” below)
Gingiva
  • Mucosal tissue covering the surfaces of the maxilla and mandible in the oral cavity.
  • It surrounds the alveolar surfaces of the maxilla and mandible and protects the roots of the teeth and the underlying bone (see “Gingiva” below)
Salivary glands
Tongue
Palate
Salivary gland

Teeth

Function

  • Food break-down into smaller particles, which facilitates digestion and absorption
  • Sound production and speech articulation

Types of teeth according to age

  • Primary teeth (also known as deciduous teeth or milk teeth)
    • Develop during embryogenesis, erupt during infancy, and shed by the age of 6–12 years
    • Five in each quadrant (right, left, upper, and lower) 20 in total
  • Permanent teeth
    • Usually completed by 13 years of age
    • Eight in each quadrant (right, left, superior, and inferior), 32 in total
    • Wisdom teeth (vestigial third molars)
      • Usually erupt by the age of 17–38 years
      • One in each quadrant (right, left, superior, and inferior), 4 in total
      • Partial eruption may cause inflammation and infection in the surrounding gingiva (pericoronitis), which are often indications for extraction.
    • Developmental alterations may affect the number, size, shape, and structure of permanent teeth.
      • Hypodontia: the congenital absence of one or more teeth
      • Hyperdontia: the condition of having one or more extra teeth (supernumerary teeth)

Types of teeth according to function

Type Characteristics
Molars
  • Primarily used for grinding of food
  • Contain 3–5 cusps
  • Upper molars have 3 roots and attach to the maxilla.
  • Lower molars have 2 roots and attach to the mandible.
Premolars
  • Primarily used for grinding of food
  • Contain 2 cusps
  • Contain a single root
  • The first upper premolar can be bifid.
Canines
  • Primarily used for tearing of food
  • Contain a single downward cone-shaped prominence
  • Contain a single root
Incisors
  • Primarily used for biting and cutting food
  • Contain a single root
  • The free end is shaped like a chisel.

Innervation

Structure

Part Characteristics
Crown
Neck
  • Located between the crown and the root
Root
  • Located in the alveolar surface of the mandible and/or maxilla
  • Connected to the bone by the periodontal ligament (a type of modified periosteal layer)
  • Covered with cementum
  • Mandibular molars have 2 roots.
  • Maxillary molars have 3 roots.

Composition

Structure Characteristics
Enamel
  • The outer layer of the tooth that covers the crown
  • Develops from the surface ectoderm
  • Produced by ameloblasts
  • Whitish, yellowish, or grayish in color
  • Hardest part of the tooth
  • Most highly mineralized substance in the human body
  • Primarily composed of hydroxyapatite
  • Does not contain collagen
  • Can be damaged and degraded by acids in food and drinks
  • Does not contain blood vessels or nerves
Dentin
Pulp
Cementum
  • Hard, bone-like structure that covers the root of the tooth
  • Composed of hydroxyapatite and collagen
  • Secreted by cementoblasts
  • Site of attachment for periodontal ligaments
  • Forms the cementoenamel junction

Gingiva

Structure

  • Definition: a mucosal tissue that covers the following:
  • Description: pinkish color (can be pigmented in dark-skinned individuals)
  • Types: can be divided into the following 2 types:
    • Attached gingiva
      • Keratinized tissue that is firmly attached to the bone structure
      • Fills the interdental spaces beneath each tooth to form the interdental papillae
    • Free gingiva: forms a collar around the tooth but is not directly attached to the tooth surface

Function

  • Protection of the root of the teeth and the underlying bone from mechanical insults and infections
  • Modulation of inflammatory response and tissue repair

Innervation

  • Buccal (outer, anterior) surface
    • Maxillary (superior) side: innervated by branches of the infraorbital nerve (anterosuperior, middle, and posterior branches)
    • Mandibular (inferior) side: innervated by the mental and buccal nerves
  • Lingual (inner, posterior) surface
    • Maxillary (superior) side: innervated by the nasopalatine and greater palatine nerves
    • Mandibular (inferior) side: innervated by lingual nerves

Tongue

Structure

  • Location: on the floor of the oral cavity
  • Parts
    • Body: anterior two-thirds
    • Root: posterior one-third
    • Apex: tip of the tongue
    • Body and root of the tongues are divided by the V-shaped sulcus terminalis
    • Left and right half are divided by the sulcus medianus
  • Attachments

Function

Histology

The tongue consist of the following layers:

  • Mucous membrane
    • Dorsum has a velvety appearance due to the presence of numerous papillae (see below)
    • Inferior surface appears smooth and thin
  • Epithelium
    • Keratinized surface
    • Nonkeratinized stratified squamous epithelium
  • Muscle layer (skeletal muscle)
  • Corium: connective tissue with mucous and serous glands
  • Adipose tissue

Lingual papillae

Lingual papillae type Characteristics Innervation
Filiform papillae
  • Most numerous type of the lingual papillae
  • Follow the V shape of the sulcus terminalis throughout the anterior two-thirds of the tongue, becoming more transverse at the tip of the tongue
  • Slender, small, and conical in shape
  • The only keratinized papillae
  • Responsible for
    • Sensation of touch
    • Increasing friction between the tongue and the food to facilitate mastication and digestion
  • The only papillae without taste buds
Fungiform papillae
  • Located on both sides of the body of the tongue and the apex
  • Mushroom-shaped
  • Deep red color is due to highly vascularized connective tissue core
  • Contain very few taste buds
Foliate papillae
  • Located in front of the palatoglossal arches and the lingual tonsils, on both sides of the tongue
  • Rudimentary in humans
  • Leaf-shaped
  • Contain abundant taste buds
Circumvallate papillae
  • The largest of the papillae
  • 8–12 in number
  • Follow the V shape of the sulcus terminalis anteriorly
  • Dome-shaped (narrower at the base and larger at the apex)
  • Surrounded by a slight circular mucosal elevation (vallum or wall) which is separated from the papilla by a circular sulcus
  • Covered by stratified squamous epithelium
  • Contain abundant taste buds

Lingual tonsils

Muscles of the tongue

Extrinsic muscles of the tongue

The extrinsic muscles of the tongue originate outside of the tongue.

Muscle Origin Insertion Innervation Function
Palatoglossus
  • Palatine aponeurosis (aponeurosis of the soft palate)
  • Posterolateral sides of the tongue
Genioglossus
  • Protrusion and depression of the tongue
Hyoglossus
Styloglossus

The genioglossus normally protrudes the tongue evenly in the midline, but if there is unilateral hypoglossal damage, the tongue will deviate towards the side of the damage.

The Genie propels out of the bottle (the genioglossus protrudes the tongue).

Intrinsic muscles of the tongue

The intrinsic muscles of the tongue originate and insert within the tongue.

Muscle Origin Insertion Innervation Function
Superior longitudinal muscle
Inferior longitudinal muscle
Vertical muscle
  • Flattens and broadens the tongue
Transverse muscle of the tongue
  • Narrows and elongates the tongue

Blood vessels and nerve supply of the tongue

Structure Characteristics
Arterial supply
Venous drainage
Innervation

All of the tongue muscles (intrinsic and extrinsic) are innervated by the hypoglossal nerve (CN XII), except for the palatoglossus (CN X).

Embryology

Structure Characteristics

Anterior two-thirds of the tongue

Posterior one-third of the tongue
Foramen cecum
  • Located at the intersection between the sulcus terminalis and sulcus medianus
  • Site of embryonic origin of the thyroglossal duct
Muscles of the tongue

Clinical significance

Macroglossia

Oral hairy leukoplakia

  • Definition: a benign condition characterized by painless, white plaques that cannot be scraped off.
  • Etiology
  • Clinical features
    • Painless, irregularly shaped white plaques with a feathery or hairy appearance
    • Most commonly located on the lateral tongue, but can also occur on the floor of the mouth, palate, or buccal mucosa
    • Patches cannot be scraped off (in contrast to oral thrush)
  • Treatment: treat the underlying condition (in contrast to leukoplakia)

Palate

Overview

  • The palate divides the oral cavity from the nasal cavity.
  • Constitutes the roof of the mouth and contributes to the floor of the nose

Embryology

  • 4th week of development
    • Downward proliferation of the mesoderm covering the forebrain → development of the frontonasal prominence
    • Local thickening of the surface ectoderm on both sides of the frontonasal prominence → formation of the nasal placodes
  • 5th week of development: invagination of the nasal placodes → formation of medial and lateral nasal prominences
  • 6th–7th week of development
    • Medial growth of the maxillary prominences and subsequent fusion with medial nasal prominences → formation of the intermaxillary segment, composed of:
    • Development of palatal shelves (outgrowths slanted obliquely downward from the maxillary prominences) and subsequent fusion of the two palatal shelves in the midline → formation of the secondary palate
  • 7th–8th week of development: fusion of the primary and secondary palates at the level of the incisive foramen → formation of the permanent palate
Structure Fused structures Developmental defects
Primary palate
  • Cleft lip: failed fusion of the palatine processes and medial nasal prominences
Secondary palate
  • Lateral palatine processes (palatal shelves) which derive from the maxillary prominences
Permanent palate
  • N/A

Anatomy of the definitive palate

Structure Characteristics
Hard palate
Soft palate

Muscles of the palate

Muscle Origin Insertion Innervation Function
Tensor veli palatini
  • Palatine aponeurosis (aponeurosis of the soft palate)

Levator veli palatini

Musculus uvulae
  • Palatine aponeurosis (aponeurosis of the soft palate)
  • Mucosa of the uvula
  • Closure of the nasopharynx
  • Uvular movement
Palatoglossus
  • Palatine aponeurosis (aponeurosis of the soft palate)
  • Posterolateral side of the tongue
Palatopharyngeus
  • Closure of the nasopharynx
  • Elevation of the pharynx

In vagus nerve (CN X) lesions, the uvula deviates away from the side of the lesion.

Salivary glands

In addition to the three larger paired glands (parotid, submandibular gland, and sublingual glands), there are several hundred small salivary glands in the oral cavity and throat. As secretory glands, they secrete up to 1.4 L of saliva per day. The primary functions of saliva include:

  • Digestion
  • Protection of the mucosa and teeth
  • Immunological defense
  • Transport of soluble flavors to the taste buds

Parotid gland

Submandibular gland

  • Located medial and caudal to the inner surface of the mandible above the mylohyoid and digastric muscles
  • The salivary duct (Wharton duct) ends in the sublingual caruncle
  • Produces seromucous fluid and the greater part (∼ 50%) of the secreted saliva

Sublingual gland

  • Located medial to the mandible above the mylohyoid muscle and below the sublingual fold
  • Its main salivary duct also ends in the Wharton duct
  • Produces mainly mucous fluid

Mastication

Muscles of mastication

Muscle Origin Insertion Innervation Function Embryology
Masseter
  • Zygomatic arch (lower and medial surface)
Temporalis
Lateral pterygoid
  • Bilateral contraction
  • Unilateral contraction: laterotrusion (sideways movement) of the mandible to the contralateral side
Medial pterygoid

Function

To remember the muscles of mastication that close the mouth: “Resist the TEMPtation to eat yuMMy food!” (TEMP = Temporalis; M = Masseter; M = Medial pterigoyd)

Deglutition

Phase Characteristics
Oral phase
Pharyngeal phase
Esophageal phase
  • Involuntary process
    1. Contraction of the pharyngeal constrictor muscles (superior, middle, and inferior) → propulsion of the bolus of food downward to the esophagus
    2. Esophageal peristalsis propulsion of the bolus of food towards the stomach
  • Affected by achalasia

Taste (gustation)

Overview

  • Definition: a chemosensory process triggered by ions and molecules in solution stimulating receptors located in the taste buds
  • Location: Taste buds are primarily located in the tongue, palate, and epiglottis.
  • Tastants: any molecule capable of eliciting taste

Anatomy of taste

Gustatory pathway

The nerves responsible for taste transmit information to the solitary nucleus in the medulla, which in turn projects to three different brain areas:

Taste papillae and taste buds

Physiology of taste

Types of taste Tastant Mechanism Function
Salty
  • Ions (e.g., Na+, NH4+, K+, SO42-, Cl)
  • Regulation of electrolyte intake
Sour
  • Hydrogen ions (H+)
  • Hydrogen ions enter the cell via transmembrane ionic channels → acidic intracellular environment → generation of action potentials in nerve fibers → brain perception of sour taste
  • Warning signal for toxins
Sweet
  • Increase appetite for calorie-dense foods
Bitter
  • Warning signal for toxins
Umami (savory)
  • Increase appetite for protein-rich foods

Clinical significance

Neurologic disorders

Inflammatory and infectious diseases

Immunologic disorders

Congenital diseases and dysmorphisms

Neoplastic disorders

Other disorders

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  • Daniel J Bell; Craig Hacking et al. Palate. https://radiopaedia.org/articles/palate?lang=us. Updated December 31, 2018. Accessed March 4, 2020.
  • Núñez-Martínez PM, García-Delgado C, Morán-Barroso VF, Jasso-Gutiérrez L. Congenital macroglossia: clinical features and therapeutic strategies in pediatric patients. Bol Med Hosp Infant Mex. 2016; 73(3): pp. 212–216. doi: 10.1016/j.bmhimx.2016.03.003.
last updated 10/26/2020
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