Mouth (Oral Cavity)

• The oral cavity, or mouth, is the first part of the digestive tract and is concerned with ingestion, mastication, taste, and speech.
• It extends from the lips anteriorly to the oropharyngeal isthmus posteriorly, where it communicates with the oropharynx.

Divisions

The oral cavity is divided into two parts:

1. Vestibule: A narrow space between the lips and cheeks externally and the teeth and gums internally.
2. Oral cavity proper: The larger cavity situated within the dental arches.

VESTIBULE OF MOUTH

• The vestibule is a narrow, slit-like space between the lips and cheeks externally and the teeth and gingivae internally.
• It is lined by oral mucosa, except over the exposed surfaces of the teeth.
• Superiorly and inferiorly, it is bounded by reflections of the mucous membrane from the lips and cheeks onto the gingivae.

Communications

1. Oral fissure: It communicates with the exterior through the opening between the lips.
2. Oral cavity proper: It communicates freely with the oral cavity proper when the jaws are separated.
3. Retromolar region: When the teeth are occluded, communication persists through the space behind the last molar teeth.

Openings in the vestibule of mouth

1. Parotid Duct

– The parotid duct opens on a small elevation called the parotid papilla, opposite the upper second molar tooth.

2. Labial and Buccal Glands

• Numerous minor salivary glands lie in the submucosa of the lips and cheeks.
• Their ducts open into the vestibule and lubricate the oral mucosa.

3. Molar Glands

• These are minor mucous salivary glands situated near the posterior part of the cheek.
• Their ducts open into the vestibule near the molar teeth.

LIPS

• The lips are mobile musculofibrous folds surrounding the oral fissure.
• They are covered externally by skin and internally by oral mucosa.
• The upper and lower lips meet laterally at the angles of the mouth.

Vermilion Zone

• The vermilion is the reddish transitional zone between the skin and oral mucosa.
• Its characteristic colour results from the thin epithelium and underlying vascular connective tissue.
• The boundary between the vermilion and facial skin is called the vermilion border.

Layers of Lips

Each lip consists of five layers, arranged from superficial to deep:

1. Skin.
2. Superficial fascia.
3. Orbicularis oris muscle.
4. Submucosa containing labial glands and blood vessels.
5. Mucous membrane.

Important Features

1. Oral Fissure

– It is the opening between the upper and lower lips, bounded laterally by the angles of the mouth.

2. Labial Frenulum

– It is a median fold of mucous membrane connecting the inner surface of each lip to the corresponding gingiva.

3. Epithelium

• The external surface is covered by keratinized stratified squamous epithelium.
• The internal mucosa is lined by nonkeratinized stratified squamous epithelium.
• The mucosal epithelium has prominent rete ridges that strengthen its attachment to the underlying connective tissue.

Blood Supply, Lymphatics, Innervation of Lips

Blood supply

• The lips are supplied mainly by the superior and inferior labial arteries, branches of the facial artery.
• The labial arteries form vascular arcades through anastomoses with their counterparts.
• Venous blood drains mainly into the facial veins.

Lymphatic drainage

• Upper lip: Drains predominantly into the submandibular lymph nodes.
• Lateral parts of lower lip: Drain into the submandibular lymph nodes.
• Central part of lower lip: Drains mainly into the submental lymph nodes.

Innervation

Sensory Innervation

• Upper lip: Superior labial branches of the infraorbital nerve (V2).
• Lower lip: Mental nerve, a terminal branch of the inferior alveolar nerve (V3).

Motor Innervation

– The muscles of the lips are supplied by branches of the facial nerve (CN VII).

– The lips have a rich sensory supply and occupy a relatively large area of the somatosensory cortex.

Development of lips

Lower Lip

– The lower lip develops through fusion of the right and left mandibular prominences in the midline.

Upper Lip

• Central part (philtrum): It develops from the intermaxillary segment formed by fusion of the medial nasal prominences.
• Lateral parts: They develop from the right and left maxillary prominences.

Important Developmental Features

• The sensory supply of the upper lip is derived mainly from the maxillary division of the trigeminal nerve (V2).
• The muscles of facial expression develop from the second pharyngeal arch and are supplied by the facial nerve (CN VII).

CHEEKS (BUCCAE)

• The cheeks are fleshy muscular structures forming the lateral walls of the oral cavity.
• Each cheek is continuous anteriorly with the corresponding lip.
• The junction between the cheek and upper lip is marked by the nasolabial sulcus.
• The nasolabial sulcus extends from the side of the nose towards the angle of the mouth and becomes more prominent during smiling.

Layers of cheek

Each cheek consists of the following structures, arranged from superficial to deep:

1. Skin.
2. Superficial fascia containing facial muscles, blood vessels, and nerves.
3. Buccal fat pad.
4. Buccopharyngeal fascia.
5. Buccinator muscle.
6. Submucosa containing buccal glands.
7. Mucous membrane.

Important Features

1. Buccal Fat Pad

• It is a specialised mass of adipose tissue situated superficial to the buccinator.
• It is particularly well developed in infants.
• It supports the cheeks and facilitates suckling during breastfeeding.

2. Parotid Duct

• The parotid duct crosses the lateral surface of the masseter and turns medially.
• It pierces the buccinator muscle and opens on the parotid papilla opposite the upper second molar tooth.

3. Retromolar Space

• It is the region behind the last molar tooth.
• It provides communication between the vestibule and oral cavity proper when the teeth are occluded.

GUMS (GINGIVAE)

• The gingivae are firm fibrous tissues covering the alveolar processes of the maxilla and mandible.
• They surround the cervical portions of the teeth and contribute to their support and protection.
• The oral gingival surface is generally lined by keratinized or parakeratinized stratified squamous epithelium.

Parts of gum

1. Free Gingiva

• It forms a collar around the cervical portion of each tooth.
• It is not firmly attached to the underlying tooth surface and forms the outer wall of the gingival sulcus.

2. Attached Gingiva

• It is firmly bound to the underlying alveolar bone and periosteum.
• It provides a stable, protective covering around the teeth.

3. Interdental Gingiva

• It occupies the spaces between adjacent teeth.
• It forms interdental papillae that protect the underlying periodontal tissues.

Innervation

Upper Gingiva

A. Buccal and Labial Surfaces

• Anterior superior alveolar nerves.
• Middle superior alveolar nerves, when present.
• Posterior superior alveolar nerves.

B. Palatal Surface

• Nasopalatine nerve: It supplies the anterior palatal gingiva.
• Greater palatine nerve: It supplies the posterior palatal gingiva.

Lower Gingiva

A. Buccal and Labial Surfaces

• Mental nerve: It supplies the anterior labial gingiva.
• Long buccal nerve: It supplies the buccal gingiva of the mandibular molar region.

B. Lingual Surface

– The lingual nerve supplies the lingual gingiva of the mandible.

Lymphatic drainage

• Upper gingiva: Drains predominantly into the submandibular lymph nodes.
• Posterior lower gingiva: Drains mainly into the submandibular lymph nodes.
• Anterior lower gingiva: The region around the mandibular incisors drains principally into the submental lymph nodes.

TEETH

• The teeth are hard, mineralised structures embedded in the alveolar sockets of the maxilla and mandible.
• Humans are diphyodont, having two successive sets of teeth: deciduous and permanent.
• Human dentition is also heterodont, consisting of incisors, canines, premolars, and molars with different functions.

Functions of Teeth

1. Mastication: They cut, tear, crush, and grind food.
2. Phonation: They assist the tongue and lips in articulation of speech.
3. Facial appearance: They support the lips and cheeks and help maintain facial contours.
4. Protection: They can be used for biting in defence.

Number of Teeth

Deciduous Teeth

• The deciduous or milk dentition consists of 20 teeth, with 10 in each jaw.
• Each quadrant contains five teeth:
  1. Central incisor.
  2. Lateral incisor.
  3. Canine.
  4. First molar.
  5. Second molar.

Dental formula: \(\frac{2.1.0.2}{2.1.0.2}\times2=20\)

Permanent Teeth

• The complete permanent dentition consists of 32 teeth, with 16 in each jaw.
• Each quadrant contains eight teeth:
  1. Central incisor.
  2. Lateral incisor.
  3. Canine.
  4. First premolar.
  5. Second premolar.
  6. First molar.
  7. Second molar.
  8. Third molar.

Dental formula: \(\frac{2.1.2.3}{2.1.2.3}\times2=32\)

Important Features

• Succedaneous teeth: Permanent incisors, canines, and premolars replace corresponding deciduous teeth.
• Accessional teeth: Permanent molars erupt without replacing deciduous teeth.
• Anterior teeth: Incisors and canines.
• Posterior teeth: Premolars and molars.

Timings of Eruption of Temporary and Permanent Teeth

• Deciduous teeth generally begin erupting around six months of age, with most erupting by two to three years.
• Permanent teeth usually begin erupting around six years of age.
• Third molars generally erupt during late adolescence or early adulthood, although eruption may be delayed or absent.
• The eruption sequence and timing show individual variation.

Parts and Structures of Tooth

Parts of Tooth

Each tooth consists of three anatomical parts:

1. Crown: It is the portion covered by enamel, normally visible above the gingival margin.
2. Neck: It is the constricted region at the junction of the crown and root.
3. Root: It is the portion embedded within the alveolar socket and covered by cementum.

Structural Components

A tooth consists of four principal tissues:

1. Enamel: It covers the anatomical crown and is the hardest tissue in the body.
2. Dentin: It forms the major portion of the tooth beneath enamel and cementum.
3. Cementum: It covers the dentin of the root and provides attachment to the periodontal ligament.
4. Dental pulp: It is soft connective tissue occupying the pulp cavity and containing blood vessels and nerves.

– The periodontal ligament connects the cementum of the root to the alveolar bone and supports the tooth within its socket.

Enamel

• Enamel is the highly mineralised, outer covering of the anatomical crown.
• It is the hardest tissue in the human body.
• It contains approximately 96% inorganic material by weight, predominantly hydroxyapatite crystals.

Structure

• Enamel consists mainly of closely packed enamel rods or prisms.
• These prisms extend from the dentinoenamel junction towards the enamel surface.
• Each prism is surrounded by an interprismatic region, with an organic-rich prism boundary.

Ameloblasts

• Ameloblasts are specialised epithelial cells responsible for enamel formation.
• They differentiate from the inner enamel epithelium of the enamel organ.
• They are lost following completion of enamel formation and eruption, so mature enamel cannot regenerate by cellular deposition.

Histological Features

1. Striae of Retzius

• These are incremental growth lines produced by rhythmic enamel deposition.
• They appear as curved bands extending through the enamel.

2. Hunter–Schreger Bands

• These are alternating light and dark bands caused by changes in the orientation of enamel prisms.
• They extend from the dentinoenamel junction towards the enamel surface.

3. Neonatal Line

• It is a prominent incremental line marking the transition between enamel formed before and after birth.
• It is observed in deciduous teeth and first permanent molars.

4. Enamel Tufts

– These are short, hypomineralised structures extending from the dentinoenamel junction into enamel.

5. Enamel Lamellae

– These are thin, sheet-like defects extending from the enamel surface towards the dentinoenamel junction.

6. Enamel Spindles

– These are short extensions of odontoblastic processes that cross the dentinoenamel junction into enamel.

Dentin

• Dentin is a hard, mineralised connective tissue forming the bulk of the tooth.
• It surrounds the pulp cavity and lies beneath the enamel of the crown and cementum of the root.
• It is harder than bone but softer than enamel.
• It contains approximately 70% mineral, 20% organic matrix, and 10% water by weight.

PubMed Central (PMC)

Odontoblasts

• Odontoblasts are specialised cells responsible for dentin formation.
• Their cell bodies form a continuous layer along the peripheral surface of the dental pulp.
• Their cytoplasmic processes extend into the dentinal tubules.

Histological Features

1. Dentinal Tubules

• These are microscopic channels extending from the pulp cavity towards the outer surface of dentin.
• They contain odontoblastic processes and dentinal fluid.

2. Fibres of Tomes

– These are cytoplasmic processes of odontoblasts extending into the dentinal tubules.

3. Predentin

• It is the newly deposited, unmineralised dentin matrix adjacent to the odontoblast layer.
• It subsequently undergoes mineralisation to form mature dentin.

4. Incremental Lines of von Ebner

– These are growth lines reflecting the rhythmic deposition of dentin matrix.

5. Granular Layer of Tomes

– It is a granular-appearing zone of root dentin immediately adjacent to cementum.

6. Mantle Dentin

– It is the first-formed dentin, situated immediately beneath the dentinoenamel junction.

7. Circumpulpal Dentin

– It forms the major portion of dentin between the mantle dentin and predentin.

Cementum

• Cementum is a thin, mineralised, bone-like tissue covering the dentin of the tooth root.
• It provides attachment to the periodontal ligament.

Cells and Structural Features

1. Cementoblasts

– These are cells responsible for synthesising and depositing cementum.

2. Cementocytes

• These are mature cementum cells enclosed within lacunae of cellular cementum.
• Their processes extend through canaliculi and are involved in maintaining the surrounding matrix.

3. Sharpey’s Fibres

• These are collagen fibres of the periodontal ligament embedded within the cementum and alveolar bone.
• They provide firm attachment of the tooth to its socket.

Pulp

– Dental pulp is a specialised, vascular connective tissue occupying the pulp chamber and root canals.

Structure

• It contains fibroblasts, odontoblasts, immune cells, collagen fibres, and ground substance.
• It is richly supplied with blood vessels and sensory nerves.
• The odontoblast layer forms the peripheral boundary of the pulp.

Blood and Nerve Supply

• Blood vessels and nerves enter and leave the pulp through the apical foramen at the root apex.
• The neurovascular fibres form a plexus beneath the odontoblast layer.

Functions

1. Formative: Odontoblasts produce dentin.
2. Nutritive: Blood vessels maintain the viability of dental tissues.
3. Sensory: Nerve endings transmit pain and other sensations.
4. Defensive: Pulp cells participate in inflammatory and reparative responses.

Periodontal ligament

• The periodontal ligament is a specialised fibrous connective tissue occupying the space between the cementum and alveolar bone.
• Its collagen bundles insert into the cementum and alveolar bone as Sharpey’s fibres.

Functions

1. Anchors the tooth within the alveolar socket.
2. Absorbs and distributes forces generated during mastication.
3. Provides sensory information about tooth loading and pressure.
4. Contributes to the maintenance and repair of periodontal tissues.

Blood Supply, Lymphatics, Innervation of Teeth

Blood supply

Upper Teeth

• They are supplied by the anterior, middle, and posterior superior alveolar arteries.
• The posterior superior alveolar artery arises from the maxillary artery.
• The anterior and middle superior alveolar arteries usually arise from the infraorbital artery.

Lower Teeth

• They are supplied mainly by the inferior alveolar artery, a branch of the maxillary artery.
• It enters the mandibular canal through the mandibular foramen and supplies the mandibular teeth.

Lymphatic drainage

• Upper teeth: They drain predominantly into the submandibular lymph nodes.
• Lower teeth: Most drain into the submandibular lymph nodes.
• Lower incisor region: It drains mainly into the submental lymph nodes.

Nerve supply

Upper Teeth

• They receive sensory innervation through the anterior, middle, and posterior superior alveolar nerves.
• These nerves are branches of the maxillary nerve (V2) or its infraorbital branch.

Lower Teeth

• They are supplied by the inferior alveolar nerve, a branch of the mandibular nerve (V3).
• The incisive branch continues within the mandible to supply the anterior teeth.

Development of Teeth

• Tooth development, or odontogenesis, begins during the sixth week of intrauterine life.
• It is an example of epithelial–mesenchymal interaction between oral ectoderm and neural crest-derived ectomesenchyme.

Stages of teeth development

1. Dental Lamina Stage

• During the sixth week, the oral ectoderm thickens along the developing alveolar processes.
• These U-shaped epithelial thickenings form the dental laminae.
• The dental laminae give rise to tooth buds at specific locations.

2. Bud Stage

• Localised epithelial proliferations grow from the dental lamina into the underlying ectomesenchyme.
• Ten tooth buds develop in each jaw for the deciduous dentition.
• Each epithelial bud represents the early enamel organ.

3. Cap Stage

• The enamel organ enlarges and develops a concavity on its deep surface, becoming cap-shaped.
• Neural crest-derived ectomesenchymal cells condense within this concavity to form the dental papilla.
• The surrounding ectomesenchyme forms the dental follicle.

Components of Tooth Germ

1. Enamel organ: It gives rise to enamel.
2. Dental papilla: It forms dentin and dental pulp.
3. Dental follicle: It contributes to cementum, periodontal ligament, and alveolar supporting tissues.

4. Bell Stage

• The enamel organ becomes bell-shaped as its concavity deepens.
• Cells differentiate into distinct layers.

Layers of Enamel Organ

1. Outer enamel epithelium: It forms the outer boundary of the enamel organ.
2. Inner enamel epithelium: It gives rise to ameloblasts.
3. Stellate reticulum: It consists of loosely arranged star-shaped epithelial cells.
4. Stratum intermedium: It supports ameloblast differentiation and enamel formation.

Cell Differentiation

• Cells of the dental papilla adjoining the inner enamel epithelium differentiate into odontoblasts.
• The adjacent inner enamel epithelial cells differentiate into ameloblasts.
• The future shape and size of the dental crown become established during this stage.

5. Apposition and Mineralisation Stage

• Odontoblasts first deposit predentin, which subsequently mineralises into dentin.
• Dentin formation stimulates ameloblasts to begin depositing enamel matrix.
• Both enamel and dentin are progressively deposited and mineralised.

Formation of Dentin

• As odontoblasts move towards the developing pulp, their cytoplasmic processes remain within dentinal tubules.
• Continued dentin deposition gradually reduces the size of the pulp cavity.

Formation of Enamel

• Ameloblasts move away from the dentinoenamel junction while depositing enamel matrix.
• After enamel formation is completed, the enamel organ regresses.
• A temporary protective covering, called the reduced enamel epithelium, remains over the developing crown.

Formation of Root and Periodontium

• Root development begins under the influence of Hertwig’s epithelial root sheath.
• Ectomesenchymal cells differentiate into cementoblasts and form cementum.
• The dental follicle contributes to the periodontal ligament and supporting alveolar tissues.

6. Eruption Stage

• Tooth eruption is the movement of a developing tooth into its functional position in the oral cavity.
• It involves coordinated root development, bone remodelling, and changes in the periodontal tissues.
• Eruption of permanent successor teeth is associated with resorption of the roots of deciduous teeth.

Development of permanent teeth

• The permanent dentition consists of 32 teeth.
• Permanent incisors, canines, and premolars develop from successional extensions of the dental lamina associated with deciduous tooth germs.
• Their tooth buds develop during prenatal life and continue differentiating after birth.
• Permanent molars develop from posterior extensions of the dental lamina and do not replace deciduous teeth.
• During eruption, permanent successor teeth promote resorption of the corresponding deciduous roots, resulting in shedding of the deciduous teeth.

Important Developmental Features

1. Epithelial–Mesenchymal Interaction

• Tooth development depends on interactions between oral epithelial cells and neural crest-derived ectomesenchyme.
• These interactions regulate tooth shape, cell differentiation, and formation of dental tissues.

2. Embryological Derivatives

• Enamel: Derived from oral ectoderm.
• Dentin and pulp: Derived from neural crest-derived ectomesenchyme.
• Cementum and periodontal ligament: Derived mainly from dental follicle ectomesenchyme.

3. Enamel Knot

• The enamel knot is a specialised group of epithelial cells within the developing enamel organ.
• It acts as a signalling centre that regulates tooth crown morphology and cusp formation.

4. Hertwig’s Epithelial Root Sheath (HERS)

• It develops from the cervical loop of the enamel organ.
• It guides root formation and induces differentiation of root odontoblasts.
• It subsequently fragments, allowing dental follicle cells to participate in cementum formation.

5. Epithelial Rests of Malassez

• These are remnants of Hertwig’s epithelial root sheath persisting within the periodontal ligament.
• They may participate in periodontal tissue maintenance and can give rise to certain odontogenic cysts.

6. Diphyodont Dentition

• Humans normally develop two successive dentitions: deciduous and permanent.
• Animals with repeated tooth replacement throughout life are called polyphyodont.

7. Tooth Regeneration

– Research in stem cell biology and tissue engineering is exploring the possibility of regenerating dental tissues and whole teeth.

ORAL CAVITY PROPER

• The oral cavity proper is the larger part of the mouth situated within the upper and lower dental arches.
• It communicates posteriorly with the oropharynx through the oropharyngeal isthmus.

Boundaries of oral cavity proper

• Anteriorly and laterally: Teeth, gingivae, and alveolar processes of the maxilla and mandible.
• Superiorly: Hard and soft palates.
• Inferiorly: Tongue and floor of the mouth.
• Posteriorly: Oropharyngeal isthmus.

Oropharyngeal Isthmus

– The oropharyngeal isthmus is the opening connecting the oral cavity proper with the oropharynx.

Boundaries

1. Superiorly: Soft palate.
2. Inferiorly: Dorsum of the tongue.
3. Laterally: Palatoglossal arches.

Functional Significance

• The palatoglossal muscles help narrow the oropharyngeal isthmus during swallowing.
• This regulates the passage of the food bolus from the oral cavity into the oropharynx.

Floor of Mouth

• The floor of the mouth is a horseshoe-shaped region situated beneath the anterior two-thirds of the tongue.
• It lies above the oral diaphragm, formed primarily by the right and left mylohyoid muscles.
• It is covered by oral mucous membrane.

Sublingual Region

• The sublingual region forms the anterior part of the floor of the mouth.
• It is situated beneath the tongue and above the mylohyoid muscles.

Features of sublingual region

1. Frenulum Linguae

– It is a median fold of mucous membrane connecting the inferior surface of the tongue to the floor of the mouth.

2. Sublingual Papilla (Sublingual Caruncle)

• It is a small mucosal elevation situated on each side of the lower end of the frenulum linguae.
• The submandibular duct (Wharton’s duct) opens at its summit.
• The major sublingual duct, when present, may also open here.

3. Sublingual Fold

• It is an elongated mucosal elevation extending posterolaterally from the sublingual caruncle.
• It overlies the sublingual gland.
• Numerous small ducts of the sublingual gland open along this fold.

Roof of Mouth

– The roof of the oral cavity is formed by the palate and is divided into two parts.

1. Hard Palate

• It forms the anterior two-thirds of the roof of the mouth.
• It provides a rigid surface against which the tongue compresses food during mastication.

2. Soft Palate

• It forms the posterior one-third of the roof of the mouth.
• It is a mobile musculofibrous structure involved in swallowing and speech.
• During swallowing, it elevates to help separate the nasopharynx from the oropharynx.