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Oral tissues


Introduction

The digestive process commences in the oral cavity with the ingestion, fragmentation and moistening of food but, in addition to its digestive role, the oral cavity is involved in speech, facial expression, sensory reception and breathing. The major structures of the oral cavity, the lips, teeth, tongue, oral mucosa and the associated salivary glands, participate in all these functions.

Mastication or chewing is the process by which ingested food is made suitable for swallowing. Chewing involves not only coordinated movements of the mandible and the cutting and grinding action of the teeth, but also activity of the lips and tongue, which continually redirect food between the occlusal surfaces of the teeth. The watery component of saliva moistens and lubricates the masticatory process, while salivary mucus helps to bind the food bolus ready for swallowing.

The entire oral cavity is lined by a protective mucous membrane, the oral mucosa , which contains many sensory receptors, including the taste receptors of the tongue. The epithelium of the oral mucosa is of the stratified squamous type, which tends to be keratinised in areas subject to considerable friction such as the palate. The oral epithelium is supported by dense collagenous tissue, the lamina propria . In highly mobile areas such as the soft palate and floor of the mouth, the lamina propria is connected to the underlying muscle by loose submucosal supporting tissue. In contrast, in areas where the oral mucosa overlies bone, such as the hard palate and tooth-bearing ridges, the lamina propria is tightly bound to the periosteum by a relatively dense fibrous submucosa. Throughout the oral mucosa, numerous small accessory salivary glands of both serous and mucous types are distributed in the submucosa.

Cancers of the mouth

Sunlight-induced damage and cancers such as squamous cell carcinoma are common on the exposed and unpigmented vermilion borders of the lips, particularly with increasing age.

Squamous cancers of the oral mucosa, including tongue, are strongly associated with tobacco use and with alcohol.

There are often preceding premalignant (dysplastic) changes in the epithelium. These are seen clinically as thickened white (leukoplakia) or red/white patches (erythroleukoplakia) in the mucosa.

G accessory salivary glands M skeletal muscle O oral mucosa S skin V vermilion border

B bone C cementum D dentine E enamel Ep epithelium L lamina propria P pulp cavity S submucosa

Tooth decay (caries)

Oral bacteria form plaque in crevices between teeth and indentations of the teeth. The bacteria metabolise sugar to organic acids which erode the adjacent enamel and then the dentine. Eventually, destruction extends into the pulp space, which becomes infected ( pulpitis) . There is pain and ultimately the tooth is lost.

A ameloblasts B bone D dentine DF dental follicle DL dental lamina DP dental papilla E enamel EO enamel organ HS epithelial sheath of Hertwig N neck of tooth O odontoblasts S space (enamel) SR stellate reticulum

B bone C cementum CE crevicular epithelium CEJ cemento-enamel junction Cp capillary D dentine Db organic debris DP dental pulp FG free gingiva M Malassez rest O odontoblasts P predentine PM periodontal membrane W Weil zone

Periodontitis and acquired odontogenic cysts

Bacterial colonisation of the crevice causes local inflammation called periodontitis. This damages the gingival attachment, which tends to migrate deeper into the alveolus (jaw), with permanent loss of gingiva and bone. This exposes dentine, causing tooth sensitivity and pain, especially upon temperature changes from cold drinks or ice. There is eventual loosening and tooth loss, a major cause of tooth loss in adults.

Inflammation can extend to the apex of the tooth. Here, the epithelial rests of Malassez can proliferate, becoming cystic and forming a periapical or radicular cyst .

B taste bud C cleft Cr crypt CV circumvallate papilla E epithelium F lymphoid follicle FG fungiform papilla FL filiform papilla L lymphoid tissue LP lamina propria M skeletal muscle MG mucous gland SG serous gland Sl skeletal muscle in longitudinal section St skeletal muscle in transverse section VE von Ebner gland

Oropharyngeal carcinoma

A specific type of squamous cell carcinoma arises in the epithelial crypts of the tongue base and tonsil. This tumour is driven by high risk human papillomavirus subtypes, most commonly HPV16 and HPV18. p16 immunohistochemistry is used as a surrogate marker for HPV status within the tumours. A p16 positive squamous cell carcinoma of the oropharynx will, in general, have a better prognosis than a p16 negative squamous cell carcinoma of the oropharynx. p16 positive squamous cell carcinomas of the oropharynx tend to be treated with chemotherapy and radiotherapy rather than surgery.

Salivary Glands

Saliva is produced by three pairs of major salivary glands, the parotid , submandibular and sublingual glands , and numerous minor accessory glands scattered throughout the oral mucosa. The minor salivary glands secrete continuously and are in general under local control, whereas the major glands mainly secrete in response to parasympathetic activity which is induced by physical, chemical and psychological stimuli. Daily saliva production in humans is 600 to 1500 mL.

Saliva is a hypotonic watery secretion containing variable amounts of mucus, enzymes (principally amylase and the antibacterial enzyme lysozyme ), antibodies and inorganic ions. Two types of secretory cells are found in the salivary glands: serous cells and mucous cells . The parotid glands consist almost exclusively of serous cells and produce a thin watery secretion rich in enzymes and antibodies. The sublingual glands have predominantly mucous secretory cells and produce a viscid secretion. The submandibular glands contain both serous and mucous secretory cells and produce a secretion of intermediate consistency. The overall composition of saliva varies according to the degree of activity of each of the major gland types.

Salivary gland pathology

Most neoplasms involving the parotid gland are benign. The commonest examples are pleomorphic adenoma and Warthin’s tumour. Most neoplasms involving the minor salivary glands are malignant. Examples include adenoid cystic carcinoma. In this tumour, the malignant cells are neurotropic , meaning that they tend to grow along nerve fibres. This makes excision of these tumours very challenging and recurrence is common. Another common tumour type, most often occurring in the minor salivary glands of the upper lip, is the canalicular adenoma.

A mucocoele is a collection of mucus that results from trauma to the minor salivary gland tissue, usually in the lip.

C capillary D striated duct ID intercalated duct E excretory duct L lobule M mucous cell nucleus My myoepithelial cell S septum SC serous cells

Review

A adipocyte D striated duct E excretory duct M mucous acinus S septum SD serous demilune

Fig. 13.1, Lip

E-fig. 13.1, Oral squamous cell carcinoma (LP)

E-fig. 13.2, Squamous dysplasia (MP)

Fig. 13.2, Palatal mucosa, monkey

Fig. 13.3, Lip and tooth

Fig. 13.4, Tooth structure

Fig. 13.5, Tooth development (caption opposite)

Fig. 13.6, Odontoblasts and dentine

Fig. 13.7, Dental pulp

Fig. 13.9, Gingival attachment

Fig. 13.10, Tongue, anterior two-thirds

Fig. 13.11, Filiform and fungiform papillae

Fig. 13.12, Circumvallate papillae

Fig. 13.13, Tongue, posterior third

E-fig. 13.4, p16 positive squamous cell carcinoma(a) H&E (MP) (b) Immunohistochemistry for p16 (MP)

Fig. 13.14, Salivary secretory unit

Fig. 13.15, Mucous acinus and myoepithelial cells

Fig. 13.16, Parotid gland

Fig. 13.17, Striated ducts

E-fig. 13.5, Pleomorphic adenoma (LP)

E-fig. 13.6, Warthin’s tumour (LP)

E-fig. 13.7, Adenoid cystic carcinoma (MP)

E-fig. 13.8, Canalicular adenoma (LP)

E-fig. 13.9, Mucocoele (LP)

Fig. 13.8, Periodontal membrane and cementum

Fig. 13.18, Sublingual gland

Fig. 13.19, Submandibular gland

E-fig. 13.3, Radicular cyst (LP)

Table 13.1
Review of oral tissues
Specialised tissue type Key features Functional attributes Figure
Oral mucosa Stratified squamous epithelium

  • Variable site-dependent keratinisation

  • Thicker keratin provides more protection

13.1 , 13.2
Teeth Enamel Surface layer of closely packed calcium hydroxyapatite crystals formed by an external ameloblast layer; destroyed by tooth eruption 13.3–13.5
Dentine Deeper zone of calcified tissue containing numerous fine parallel tubules radiating from odontoblasts which line the pulp cavity and form dentine 13.6
Pulp Central core of loose tissue with nerves and vessels supplying odontoblasts 13.7
Tongue

  • Stratified squamous epithelium with underlying skeletal muscle

  • Minor salivary glands

  • Filiform, fungiform and circumvallate papillae

  • Skeletal muscle bundles are orientated perpendicular to each other

  • Produce amylase and lubrication

  • Sensory organs of taste

13.10 , 13.13 13.10–13.12
Salivary glands Serous and/or mucinous glandsParotid is the largest glandOthers include submandibular, sublingual and minor salivary glands

  • Serous glands produce amylase

  • Mucinous glands produce mucin to lubricate

13.14–13.19
Tonsillar tissue

  • Palatine tonsil

  • Lingual tonsil

  • Located at the back of the mouth (fauces)

  • Located in the base of tongue

  • Lymphoid tissue part of mucosa associated lymphoid tissue (MALT)

11.15 13.13

Online questions

Chapter 13

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