During the 4th week of development, a portion of the head and neck region, known as the oropharyngeal region, begins to differentiate as a functional unit from the frontonasal process and pharyngeal arches.
The pharyngeal arches, which extend from the upper jaw to the level of the cricoid cartilage at vertebral level C6, differentiate to form the oropharyngeal region and includes the upper and lower jaw, palate, oral cavity, pharynx, and larynx.
The pharyngeal arches (PAs) initially develop as six bilateral tissue swelings that surround the developing pharynx. Early developmental regression of the fifth arch leads to numbering of the arches as PA 1,2,3, 4, and 6. The numbering system reflects the early developmental regression of the fifth arch. The PA derivatives present in the adult are listed in and
Each arch develops in association with an ectodermal lined cleft and an endodermal lined pouch. In the adult, only the first PA cleft persists and forms the external auditory meatus of the ear. The pouches give rise to several bilateral structures which are listed in ().
Each arch develops in association with a specific cranial nerve (CNs V, VII, IX, and X) and gives rise to a central core of tissue consisting of striated muscle, connective tissue, skeletal elements, and a blood vessel ().
Fig. 19.1 Pharyngeal arch development: Head and neck region of a 5-week-old embryo showing the pharyngeal (branchial) arches and clefts (left lateral view). The pharyngeal arches are instrumental in the development of the face, neck, larynx, and pharynx. Development of the pharyngeal arches begins in the 4th week of embryonic development as cells migrate from the neural crest to the future head and neck region. By the 5th week, a series of four bilateral tissue swellings (first through fourth pharyngeal arches) develop and become visible on the external surface. Each arch is separated externally by four deep grooves (pharyngeal clefts). The pharyngeal arches and clefts are prominent features of the embryo at this stage. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Fig. 19.2 (a) Schematic showing the internal structure of the pharyngeal arches (after Sadler). Anterior view. The pharyngeal arches are covered externally by ectoderm and internally by endoderm. Each pharyngeal arch contains an arch artery, an arch nerve, and a cartilaginous element, all of which are surrounded by mesodermal and muscular tissue. The external furrows are called the pharyngeal clefts, and the internal grooves are called the pharyngeal pouches. (b) High power view of boxed region shown in (a) (after Sadler). Oblique view showing the relationship of pharyngeal arch cartilage, artery, and nerve in the pharyngeal arches. The vessels, nerves, and cartilaginous precursors of each arch are surrounded by mesoderm and developing skeletal muscle tissue. Pharyngeal arches are covered externally by ectoderm (blue) and internally by endoderm (green). (c) Migration of the pharyngeal arch tissues (after Sadler). Anterior view. During embryonic development, endoderm of each pharyngeal pouch differentiates bilaterally from the lateral wall of the pharyngeal tube and gives rise to the epithelial cells associated with the auditory tube (PA 1), the palatine tonsils (PA 2), the thymus (PA 3), and the inferior and superior parathyroid glands (PA 3 and PA 4). The epithelium of thyroid gland also develops from endoderm, but it originates from the foramen cecum in the midline of the tongue. Calcitonin-producing C cells, or parafollicular cells, arise from neural crest cells and migrate with the thyroid gland. The thymus, parathyroid glands, and the thyroid gland migrate (arrows) from their sites of developmental origin. The superior and inferior parathyroid glands follow the migratory path of the thyroid glands and come to lie on the posterior surface at the superior and inferior poles of the thyroid. Each lobe of the thymus migrates toward the midline and then descends into the superior mediastinum. (a: Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b,c: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Fig. 19.3 (a–d) Schematic of pharyngeal arch derivatives (after Sadler and Drews). Each arch develops in association with a cranial nerve, skeletal muscle derivatives, and connective tissue derivatives. (a) Anlage of the embryonic pharyngeal arches with the associated pharyngeal arch nerves. (b) Definitive arrangement of the future cranial nerves V, VII, IX, and X. (c) Muscular derivatives of the pharyngeal arches. (d) Skeletal derivatives of the pharyngeal arches. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 1. © Thieme 2014. Illustrations by Markus Voll and Karl Wesker.)
Fig. 19.4 Innervation of the pharyngeal arches (left lateral view). Each of the pharyngeal arches is associated with a cranial nerve: First pharyngeal arch is associated with the trigeminal nerve (CNs V2 and V3); second pharyngeal arch, facial nerve (CN VII); third pharyngeal arch, glossopharyngeal nerve (CN IX); fourth and sixth pharyngeal arches, vagus nerve (CN X—superior and recurrent laryngeal nerves). The frontonasal region which contributes to the midline of the face is also associated with the trigeminal nerve, but it is CN V1 division that provides sensory innervation to this part of the face. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
During development, the proliferation of tissue in the frontonasal region and the maxillary and mandibular processes of PA 1 gives rise to the primordia of the face and demarcates the boundary of the future oral cavity known as the stomodeum () () .
In the adult, the palatoglossal folds which form the anterior tonsillar pillars represent the embryonic location of the ectodermal–endodermal boundary and serve as the posterior anatomical boundary of the oral cavity proper. The region posterior to the palatoglossal folds is the oropharynx.
Fig. 19.5 (a–e) Development of the face between 5 and 12 weeks (after Sadler). (a) Anterior view at 24 days. The surface ectoderm of the pharyngeal arch (PA) 1 invaginates to form the stomodeum, which is a depression between the forebrain and the pericardium in the embryo. It is the precursor of the mouth, oral cavity, and anterior pituitary gland. The stomodeum is surrounded by the frontonasal prominence and bilaterally by the maxillary and mandibular prominence of PA 1. These five prominences contribute to the development of the face. (b, c) Anterior view at 5th and 6th weeks. The nasal placodes, which are ectodermal thickenings, form on each side of the frontonasal prominence and become surrounded by medial and lateral nasal processes. (d, e) Anterior view at 7th to 12th weeks. The medial nasal processes merge to form the intermaxillary segment and the midline of the nose. The lateral nasal processes and maxillary prominences fuse to from side of the nose. The maxillary processes also fuse with the intermaxillary segment to form the upper lip. The mandibular processes and maxillary prominences merge to form the side of the cheek. The frontonasal prominence differentiates to form the nose, philtrum of the upper lip, and region of the forehead. The maxillary process forms the cheek, upper lip, secondary palate, and upper jaw. The mandibular process forms the chin and lower jaw. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
The secondary palate develops as bilateral outgrowths from the maxillary prominences. The two palatal shelves fuse together in the midline and fuse anteriorly with the primary palate along the incisive foramen to form the definitive palate, which separates the oral and nasal cavities.
Fig. 19.6 (a, b) Palatal development (after Sadler). Caudal view of palate at 7th weeks. (a) The medial nasal processes fuse together to give rise to the intermaxillary segment. The posterior part of the intermaxillary segment develops into bone tissue and gives rise to the primary palate. In the adult, the primary palate is known as the premaxilla or intermaxillary segment of the maxilla and represents the portion of bone containing the maxillary incisors. (b) Caudal view, adult. The philtrum, an area of soft tissue in the midline of the upper lip, arises from anterior part of intermaxillary segment. The secondary palate develops from the two lateral palatal shelves of the maxillary process. Midline fusion between the lateral shelves forms the secondary palate. Development of the definitive (complete) palate occurs when the tissue of the primary palate fuses with the secondary palate of the maxillary processes. Due to the complexity in palatal development, palatal clefts, which represent defects in the fusion between facial tissue components, can arise. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
As the oral cavity begins to develop, the pharyngeal arches begin to proliferate along the inferior boundary of the oral cavity in the region that becomes the floor of the mouth. The tongue and larynx develop as midline structures from the floor of the pharyngeal arch region.
Occipital somites migrate into the developing tongue along with the hypoglossal nerve (CN XII) and differentiate into most of the striated muscle associated with the extrinsic and intrinsic tongue musculature.
Fig. 19.7 (a, b) Development of the tongue. (a) Early tongue development at 4th week. (b) Late tongue development around week 8. The tongue mucosa develops as swellings from the floor of the pharynx in the region of 1st, 3rd, and 4th pharyngeal arches. The musculature of the tongue is derived from occipital somites. The two lateral lingual swellings from the 1st pharyngeal arch merge together and contribute to the lingual mucosa covering the anterior two-thirds of the tongue. The single midline swelling (the hypobranchial eminence), from the 3rd and 4th pharyngeal arches, contributes to the mucosa covering the posterior one-third of the tongue and epiglottis. A V-shaped terminal depression (sulcus terminalis) separates the anterior two-thirds of the tongue from the posterior one-third. At the apex of the sulcus lies in the foramen cecum which represents the site of origin of the thyroid glands. In the region distal to the hypobranchial eminence, the epiglottic and laryngeal (arytenoid) swellings develop from the 4th and 6th pharyngeal arches and contribute to the epiglottis and laryngeal mucosa. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
The muscles and mucosa lining the laryngeal region receive innervation from branches of the vagus nerve (CN X). The pattern of innervation reflects the developmental origin of the larynx. The vagal nerve branches which innervate the laryngeal region include the superior laryngeal, external laryngeal, internal laryngeal (PA 4), and recurrent (inferior) laryngeal branches of the vagus (PA 6) ( ) ().
The oral cavity or mouth extends from the lips and cheeks to the palatoglossal folds, which form the anterior tonsillar pillars. Posterior to the tonsillar pillars the oral cavity becomes continuous with the oropharynx.
The oral cavity consists of two functional regions, the oral vestibule and oral cavity proper, which are separated by the dental (alveolar) arches. The oral vestibule is external to the upper and lower dental arches, and the oral cavity proper lies internal to the dental arches.
Fig. 19.8 Region of the oral cavity; anterior view. The dental arches (with the alveolar processes of the maxilla and mandible) subdivide the oral cavity into two parts: Oral vestibule: The portion outside the dental arches, bounded on one side by the lips and cheeks and on the other side by the dental arches. Oral cavity proper: The region within the dental arches. The oral cavity proper includes the anterior two-thirds of the tongue and extends from the dental arches to palatoglossal folds (anterior tonsillar pillars). Posterior to folds lies the posterior one-third of the tongue in the region of the oropharynx. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
The three types of oral mucosa found in the region of the oropharyngeal region include: lining mucosa, masticatory mucosa, and specialized mucosa ( ). Regional variations in the type of oral mucosa reflect the functional properties of the specific regions and can be of clinical significance.
The oral mucosa contains a high density of sensory receptors that transmit gustatory input, proprioception, mechanoreception, nociceptive, and thermal stimuli (see Chapter 11 for review).
Transmission of somatosensory innervation of the oral mucosa covering the oral cavity, pharynx, and laryngeal region occurs through branches of the maxillary (V2) and mandibular (V3) divisions of the trigeminal nerve, the glossopharyngeal (CN IX), and the vagus (CN X) nerves, respectively.
Most of the taste buds are associated with the lingual papillae, which comprise the specialized mucosa covering the anterior two-thirds of the tongue. Additional taste buds are scattered on the palate, oropharynx, posterior one-third of the tongue, epiglottis, and laryngeal mucosa ().
Afferent input from low threshold mechanoreceptors and proprioceptors in the oral mucosa plays an integral role in the feedback, perception, and reflexive response concerning the size, texture, and position of objects (stereognosis) placed in the oral cavity, including food, liquids, and oral devices.
The ability to perceive nociceptive, proprioceptive, and stereognostic inputs is essential for monitoring, adapting, and integrating motor responses involved in occlusion, mastication, swallowing, and speaking, as well as regulating various reflexes (see Chapter 22).
Regions of the oral mucosa such as the lips, teeth, periodontal ligaments, anterior and midline regions of the tongue, and palate contain areas of high receptor density with small receptive fields which permits increased sensitivity and greater discrimination of tactile input and oral stereognosis (Clinical Correlation Box 19.2).
Fig. 19.9 (a–d) Regional differences in the sensory innervation of the oral cavity and nasopharynx. (a) The oral cavity proper receives innervation from the trigeminal nerve (CN V2 maxillary and CN V3 mandibular). (b) Glossopharyngeal (CN IX) carries sensory (GSA/GVA) innervation to the oropharynx. (c) The vagus (CN X) transmits sensory input in the region of the laryngopharynx and larynx. (d) Anterior view of the somatosensory innervation (left side) and taste innervation (right side) of the tongue. The tongue receives its somatosensory innervation (e.g., touch, pain, thermal sensation) from three cranial nerve branches: Lingual nerve (branch of mandibular nerve, CN V3), glossopharyngeal nerve (CN IX), and vagus nerve (CN X). The three cranial nerves also convey the taste fibers: CN VII (facial nerve, chorda tympani), CN IX (glossopharyngeal nerve), and CN X (internal laryngeal of vagus nerve). Thus, a disturbance of taste sensation involving the anterior two-thirds of the tongue indicates the presence of a facial nerve lesion, whereas a disturbance of tactile, pain, or thermal sensation indicates a trigeminal nerve lesion. GSA, general somatic afferent; GVA, general visceral afferent. (a,b,c: Reproduced with permission from Gilroy AM. Anatomy: An Essential Textbook. 2nd Edition. © Thieme 2017. Illustrations by Markus Voll and Karl Wesker. d: Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)