Mouth

Embryology

The oral cavity is demarcated as early as the fourteenth day of fetal development by the appearance of the prechordal plate. The endodermal thickening of the prechordal plate contributes to the oropharyngeal membrane, a temporary membrane that separates the ectoderm forming the mucosa of the mouth and the endoderm forming the pharynx. It sits at the depth of the stomodeum, a shallow depression in the centre of the developing face, which constitutes the primitive mouth. The stomodeum is surrounded by five prominences: the fronto­nasal process and the paired maxillary and mandibular prominences ( Fig. 11.1 ). Medial and lateral nasal processes form at the tip of the frontonasal process and fuse with each other and the maxillary prominence, forming the maxilla, lip, tip of the nose and primary palate. Failure of this fusion process produces clefts of the lip and premaxilla. The mandibular prominences are derived from the first branchial arch, merging to form the lower lip and mandible.

The primitive stomodeum deepens with the growth of the surrounding prominences; by the twenty-eighth day, the oropharyngeal membrane has usually disintegrated, creating a continuous passage between the mouth and pharynx. The frontonasal and maxillary prominences develop horizontal extensions into the stomodeum; these initially sit on either side of the tongue, which is developing from the floor of the cavity. In the eighth week, these shelves reorientate horizontally and then fuse, separating the oral and nasal cavities. Failure of reorientation or fusion of the palatal plates produces a cleft palate.

The three divisions of the trigeminal nerve provide sensory innerv­ation to the frontonasal, maxillary and mandibular parts of the face. The muscles of mastication, mylohyoid, anterior belly of digastric and tensor veli palatini are all innervated by the motor branch of the mandibular division of the trigeminal nerve. The anterior two-thirds of the tongue develop in the first (mandibular) pharyngeal arch from a small median elevation, the tuberculum impar or median tongue bud, and paired lateral lingual swellings distally. This part of the tongue is innerv­ated by the lingual branch of the mandibular nerve (the nerve of the first arch), and by the chorda tympani of the facial nerve. The posterior, pharyngeal part of the tongue is innervated by the glossopharyngeal nerve, the nerve of the third arch; this nerve also invades and innervates the tissue immediately distal to the sulcus terminalis, including the circumvallate papillae. The intrinsic and extrinsic muscles of the tongue arise from occipital somites; all except palatoglossus are innervated by the hypoglossal nerve. Palatoglossus is innervated by motor branches of the pharyngeal plexus.

The mandible forms from intramembranous bone lateral to Meckel's cartilage. A single ossification centre for each half of the mandible arises in the sixth week in utero in the region of the bifurcation of the inferior alveolar nerve and artery into their mental and incis­ive branches. At this stage, the tooth germs lie above the developing mandible; near the end of the second month of fetal life, the alveolar processes develop as a trough in response to the tooth buds and become superimposed on the basal bone of the maxillary and mandibular bodies. Odontogenic tissue can be identified as early as the twenty-eighth day in utero as ectodermal thickenings at the supero­lateral margins of the developing frontonasal, maxillary and mandibular prominences. Local proliferation of the dental lamina into the subjacent mesenchyme forms the primordia of the enamel organs. Ten tooth germs, representing the 10 deciduous teeth, form initially; the tooth germs of the permanent teeth form lingually to the primary tooth germs at a later stage.

Clinical anatomy

The mouth can be divided into two parts. The peripheral portion, the vestibule, is the narrow space between the lips and cheeks and the teeth and gingivae. The anterior boundary is formed by the lips. The inner oral cavity proper is bounded anteriorly and laterally by the teeth and gingivae ( Fig. 11.2 ). The roof is formed by the hard and soft palates, and the floor by the anterior two-thirds of the tongue and the floor of the mouth, on to which the submandibular and sublingual salivary ducts open. The posterior boundary of the oral cavity is the oropharyngeal isthmus, bounded by the tongue at the sulcus termin­alis (the boundary of the anterior two-thirds and posterior third of the tongue), the palatoglossal arch and the soft palate.

The dental alveolus and hard palate are covered by keratinized epithelium, which is tightly adherent to the underlying bone. The remainder of the oral cavity (soft palate, ventral surface of the tongue, floor of the mouth, alveolar processes (except the gingivae) and the internal surfaces of the lips and cheeks) is lined with a mucous membrane that has a non-keratinized stratified squamous epithelium that is more pliable and able to move with the underlying muscles of the cheek and tongue. The anterior surface of the hard palate is marked by folds of mucosa, the palatine rugae. In addition to the major salivary glands, many minor salivary glands open on to the non-keratinized lining of the oral cavity, providing lubrication. These are mostly serous, similar to the lingual glands, in contrast to the parotid and submandibular glands which produce more mucus saliva.

The mucous membrane of the superior aspect of the vestibule is innervated by the infraorbital branch of the maxillary division of the trigeminal nerve; the lower aspect is innervated by the buccal and mental branches of the mandibular nerve.

Lips and cheeks

The upper and lower lips form a muscular valve that maintains an oral seal. The bulk of the lips are formed by orbicularis oris, into which the elevator and depressor muscles of the lip and the buccinator muscle of the cheeks are inserted ( Fig. 11.3 ). The lips are covered externally by skin and internally by mucous membrane, which fuse on the exterior surface at the vermillion border. The vascular supply is from the superior and inferior labial branches of the facial artery and vein. Folds of mucosa in the midline of the upper and lower lips, labial frenulae, are attached to the anterior aspect of the maxillary and mandibular alveoli, respectively.

Cleft lips are the result of a separation of all, or part, of the upper lip and the maxillary alveolus either unilaterally or bilaterally, creating a communication between the oral and nasal cavities. The associated loss of orientation of the muscles of the lip affects speech and eating. Repair of a cleft lip requires dissection and reorientation of orbicularis oris and the muscles that insert into it in order to restore continuity of the lip and normal function.

The sensory supply to the upper lip is from the infraorbital branch of the maxillary division of the trigeminal nerve. The nerve emerges from the infraorbital foramen, approximately along a line from the canine tooth to the pupil. It is at risk from trauma following a fractured zygoma or maxilla, where the fracture line often passes through its foramen, or during surgical access procedures of the midface, which involve dissection of the mucoperiosteum from the anterior wall of the maxilla through an incision in the upper buccal sulcus.

The lower lip is supplied by the mental nerve, a terminal branch of the inferior alveolar nerve (a branch of the mandibular division of the trigeminal nerve). This nerve is vulnerable to injury as it passes through the mandible, from fractures or osteotomies that divide the bone between the lingula on the inner aspect of the ramus and the mental foramen. The nerve passes close to the roots of the teeth and so may also be injured during dentoalveolar surgery, particularly the extraction of impacted wisdom teeth, enucleation of mandibular cysts and dental implant placement. Access incisions for the mandible are often located immediately inferior to the mucosal–mucoperiosteal junction; the periosteum is dissected, taking care not to damage the mental nerve as it emerges from the mental foramen between the apices of the lower first and second premolar teeth.

Terminal branches of the facial nerve, predominantly the zygomatic, buccal and marginal mandibular branches, innervate the muscles of facial expression, mainly from their deep aspect. Injury to the facial nerve from trauma or surgery will produce weakness of the associated muscles; while this is most noticeable during function, compensatory action of the opposing muscles may also render weakness visible at rest.

Many minor salivary glands lie beneath the mucous membrane of the lips. They are most numerous near the labial sulcus and do not extend beyond the region where the lips come into contact passively, creating the wet–dry demarcation line of the lip.

The cheeks are formed largely by buccinator. The parotid duct passes through buccinator and emerges in the oral cavity opposite the maxillary second molar tooth. The inner surface of the cheeks is covered by mucous membrane that reflects at the upper and lower buccal sulci, passing loosely over the basal bone of the maxilla and mandible, respectively, before merging into the mucoperiosteum covering the alveolar components. The buccal branch of the mandibular division of the trigeminal nerve is sensory to the lower buccal gingiva, lower buccal sulcus and cheek mucosa, and may also provide some minor sensory innervation to the cutaneous surface of the cheek.

The maxilla and anterior mandible lie in the vascular bed of the ipsilateral facial artery. Blood supply is via both periosteal and intrabony vessels: care must be taken when accessing the maxilla – particularly for orthognathic surgery, where this intrabony supply will be interrupted – not to divide the periosteal blood supply completely; incisions are therefore usually limited up to the first molar teeth. The main arterial supply to the anterior mandible is the sublingual artery, which may be a branch of either the lingual or the submental artery. The dependence of the mandible on intrabony or periosteal supplies varies with age: the periosteal supply is said to be more important at older ages.