Embryology of the oral structures

The head and neck are formed from mesenchyme derived from paraxial and lateral plate mesoderm, neural crest cells, and ectodermal placodes. The paraxial mesoderm forms the floor of the brain case, the voluntary muscles of the craniofacial region, the dermis and connective tissue of the dorsal portion of the head, and the meninges caudal to the prosencephalon. The lateral plate mesoderm forms the lateral cartilages and connective tissue of the lateral region of the head and neck. The neural crest cells originate in the neuroectoderm of the forebrain, midbrain, and hindbrain regions and migrate ventrally into the branchial arches and rostrally into the facial region to form the tissue structures and the midfacial and branchial arch skeletal structures of that area. Together, the ectodermal placodes, thickened regions of ectoderm, and the neural crest cells form neurons from the fifth, seventh, ninth, and tenth cranial sensory ganglia.

The embryologic development of the head and neck is marked by the formation of the branchial (or pharyngeal) arches. The branchial arches appear in the fourth and fifth weeks of embryologic development. They contribute to the external appearance of the head and neck of the embryo. The branchial arches are formed from mesenchymal tissue and separated by deep clefts known as branchial clefts. Pharyngeal pouches, or outpocketings, form simultaneously with the development of branchial arches and branchial clefts. These pharyngeal pouches appear along the lateral walls of the internal pharyngeal gut, and they do not establish an open communication with the external branchial clefts (Figure 1).

The palatine tonsil is formed from the epithelial lining of the second pharyngeal pouch. The epithelial lining forms buds that penetrate into the surrounding mesenchyme. Then, the buds are invaded by mesodermal tissue. During the third and fifth months of development, the primordial palatine tonsil is infiltrated by lymphatic tissue. Part of the second pharyngeal pouch remains and is found in adults as the tonsillar fossa.

During the fourth week of embryologic development, the tongue develops in the form of 2 lateral lingual swellings and 1 medial swelling—the tuberculum impar (Figure 2). These 3 swellings arise from the second branchial arch. A second median swelling, the copula (or hypobranchial eminence), is formed from the mesoderm of the second, third, and fourth arches, and a third median swelling is formed from the posterior portion of the fourth branchial arch. The arrival of the third median swelling marks the development of the epiglottis. The lateral lingual swellings increase in size until they outgrow the medial tuberculum impar and merge to form the anterior two-thirds, or body, of the tongue. The body of the tongue is separated from the posterior third of the tongue by a v-shaped groove—the terminal sulcus. The posterior third, or root, of the tongue arises from the second, third, and fourth branchial arches.

Ankyloglossia results when the tongue is not freed from the floor of the mouth. Normally, apoptosis separates the tongue from the floor of the mouth, and the only tissue that remains to anchor the tongue to the mouth floor is the frenulum. However, in the most common form of ankyloglossia, the frenulum extends to the tip of the tongue.

Sensory and motor innervation of the tongue depends on the branchial arch from which that portion of the tongue is derived. Most of the muscles of the tongue are derived from myoblasts originating from occipital somites. Therefore, tongue musculature is innervated by the hypoglossal nerve. The sensory innervation of the body of the tongue is supplied by the mandibular branch of the trigeminal nerve because the mucosa of the body is derived from the first pharyngeal arch. The sensory innervation for the root of the tongue is supplied by the glossopharyngeal nerve because the tissue of the third branchial arch outgrows that of the second in the root of the tongue. Finally, the epiglottis and extreme posterior portion of the tongue are derived from the fourth arch; they are innervated by the superior laryngeal nerve. Special sensory innervation (taste) of the body of the tongue is provided by the chorda tympani branch of the facial nerve, whereas that of the root of the tongue is provided by the glossopharyngeal nerve.

At the end of the fourth week of embryologic development, facial prominences form from the neural crest–derived mesenchyme of the first pharyngeal arches. Maxillary prominences, mandibular prominences, and a frontonasal prominence all form. On both sides of the frontonasal prominence, nasal (or olfactory) placodes form under the inductive influence of the forebrain (Figure 3).

During the fifth week of embryologic development, the nasal placodes invaginate to form nasal pits with surrounding ridges known as the nasal prominences. The prominences on the outer edge of the pits are the lateral nasal prominences, and the prominences on the inner edge of the pits are the medial nasal prominences.

During the following 2 weeks, the maxillary prominences increase in size and grow medially, compressing the medial nasal prominences toward the midline. The clefts between the maxillary prominences and medial nasal prominences close as the maxillary and medial nasal prominences fuse to form the upper lip (Figure 4). The lower lip and jaw are formed from the fusion of the mandibular prominences.

Because of the medial growth of the 2 maxillary prominences, the medial nasal prominences merge not only at the surface but also at a deeper level to form the intermaxillary segment. The intermaxillary segment is composed of the labial component (which forms the philtrum of the upper lip), the upper jaw component (which carries 4 incisor teeth), and the palatal component (which from the triangular primary plate). In addition, the intermaxillary segment is continuous with the rostral portion of the nasal septum.

During the sixth week of development, the palantine shelves form as outgrowths from the maxillary prominences and are directed downward on each side of the tongue. Then, during the seventh week of development, these palantine shelves attain a horizontal position above the tongue and fuse to form the secondary palate. The incisive foramen is the midline of the fused palatine shelves. Anteriorly, the secondary palate fuses with the triangular primary palate (Figure 5). Simultaneously, the nasal septum grows down and joins with the cephalic aspect of the newly formed palate.

Cleft palate and cleft lip are common facial defects that result in impaired speech and esthetic abnormalities. Cleft deformities are generally divided into 2 groups: isolated cleft palate and cleft lip with or without cleft palate. The incisive foramen is considered the dividing landmark between the anterior and the posterior cleft deformities: anterior to the origin of the incisive foramen is considered an anterior deformity and lateral to or overlaying the incisive foramen is considered a posterior deformity. Anterior cleft deformities include lateral cleft lip, cleft upper jaw, and a cleft between the primary and the secondary palates. These defects are due to a lack of complete fusion of the maxillary prominence with the media nasal prominence on one or both sides of the oral cavity. Anterior cleft deformities may extend superiorly as far as the inferior portion of the nose and as deep as the maxilla; anterior clefts may split the maxilla between the lateral incisor and the canine tooth. Posterior cleft deformities include secondary cleft palate (more severe) and cleft uvula (less severe). Secondary cleft palate results from a lack of fusion of the palatine shelves, which may be due to a failure of the fusion process itself, a failure of the tongue to drop from between the shelves, or the smallness of the shelves. Oblique facial clefts are formed because of the failure of the maxillary prominence to merge with the lateral nasal prominence, and median cleft lip is formed because of the incomplete merging of the 2 medial nasal prominences. Submucous cleft palate is a subgroup of cleft palates with insufficient median fusion beneath the mucosal layer. A submucous cleft of the soft palate is characterized by a midline deficiency in muscular tissue and subsequent incorrect positioning of the muscles, whereas a submucous cleft of the hard palate is defined as a bony defect in the midline of the hard palate. Often, a submucous cleft palate is associated with a bifid or cleft uvula.

The prevalence of cleft lip is 1 every 1,000 live births. It is more common in male individuals (80%) than in female individuals, and its incidence rate increases slightly with maternal age. The incidence of cleft palate is much lower than that of cleft lip at 1 in every 2,500 live births. Cleft palate is more common in female individuals (67%) than in male individuals—possibly because, in female individuals, the palatal shelves fuse 1 week later than in male individuals. The incidence rate of cleft palate does not increase with maternal age; however, anticonvulsants, for example, phenobarbital and diphenylhydantoin, administered during pregnancy increase the risk of cleft palate.

During the sixth week of development, solid epithelial buds of ectodermal origin form the wall of the primitive mouth. A groove then develops within the surrounding mesenchyme, and the parotid gland forms posterior to the groove. Afterward, the groove becomes a duct, and the portion of the parotid gland immediately posterior to the duct becomes the secretory acini. The mesenchyme then forms the parotid capsule and connective parotid tissue. Eventually, the solid epithelial buds and acini become hollow. All the salivary glands have a similar embryogenesis.

Disruption or blockage of minor salivary ducts may lead to the formation of a congenital ranula. A ranula is a cystic malformation in the oral cavity. It may form following minor salivary duct disruption owing to extravasation of mucus in adjacent structures or following duct obstruction owing to proximal expansion and formation of mucous retention cyst. Ranulas may be simple or plunging. Simple ranulas only involve the sublingual space, whereas plunging ranulas extend posterior to the mylohyoid muscle and into the neck. Simple ranulas are mostly asymptomatic.

An epignathus is a teratoma composed of all the 3 germ cell layers. They are commonly benign and can arise from the upper jaw, palate, and sphenoid. It can protrude through the mouth to cause respiratory difficulties and death. Treatment first focuses on establishing an airway. This may require an ex utero intrapartum treatment procedure secondary to possible respiratory compromise. Surgical excision is often the definitive treatment performed later. The exact pathogenesis of an epignathus is unknown.

Procedures involving the oral cavity of pediatric patients require a thorough knowledge of the development of this area. The structures of the oral cavity in children can be quite different as they grow and mature into adulthood. In addition, knowledge of embryology can help surgeons understand how particular congenital anomalies come to be, which can be useful in their management.