Embryology Chapter: Pharyngeal Arches

Development of the Pharyngeal Apparatus, Head, and Face

• The pharyngeal apparatus comprises pharyngeal arches, pouches, grooves, and membranes.
• These embryonic structures are crucial in forming the face and neck.
• Pharyngeal arches initially develop during the fourth week of gestation, migrating into the head and neck region as neural crest cells.
• The first pair of arches, the primordial jaws, appear as surface elevations flanking the developing pharynx.
• Subsequent arches form ridges on either side of the developing head and neck.
• By the end of the fourth week, four pairs of arches are visibly external.
• Fifth and sixth arches are rudimentary and not externally visible.
• The pharyngeal arches are demarcated by pharyngeal grooves (clefts).
• The first arch branches into maxillary and mandibular prominences, respectively, forming the maxilla and part of the vomer bone.
• The mandibular prominence develops into the mandible and squamous temporal bone.
• The second arch (hyoid arch) contributes to hyoid bone formation, alongside the third arch.
• The arches support the lateral walls of the primordial pharynx, derived from the cranial portion of the foregut.
• The stomodeum (primordial mouth) initially appears as a shallow depression in the surface ectoderm.
• Separating the stomodeum from the primordial pharynx is the bilaminar oropharyngeal membrane (ectoderm externally, endoderm internally).
• This membrane ruptures at around 26 days, allowing communication between the pharynx, foregut, and amniotic cavity.
• Failure of rupture can result in orofacial defects.
• Each arch (each pharyngeal arch) is structured by mesenchyme (connective tissue) covered by ectoderm externally and endoderm internally.
• Most mesenchyme originates during week three from mesoderm.
• During week four, most mesenchyme is derived from neural crest cells migrating into the arches.
• Each pharyngeal arch contains arteries, cartilage, nerves, and muscle components.
• An artery originating from the truncus arteriosus (primitive heart) passes around the primordial pharynx, entering the dorsal aorta.
• Cartilage forms the skeletal foundation of the arch.
• Muscle components develop into the head and neck muscles.
• Sensory and motor nerves from the neuroectoderm of the primordial brain supply the mucosa and muscles in each arch.
• The pharyngeal arches significantly contribute to the face, nasal cavities, mouth, larynx, pharynx, and neck regions.
• During the fifth week, the second arch enlarges, enveloping the third and fourth arches, while forming a cervical sinus.
• By the end of the seventh week, the second, third and fourth grooves, and the cervical sinus have disappeared, producing a smooth neck contour.
• The dorsal end of the first arch cartilage (Meckel's cartilage) is closely connected to the embryonic ear.
• During development, small nodules detach from the proximal cartilaginous portion and form the malleus and incus, middle ear bones, with perichondrium forming the anterior malleolar and sphenomandibular ligaments.
• The ventral part of the first arch cartilages creates the horseshoe-shaped mandible primordium.
• It guides early morphogenesis as it grows.
• The mandible forms laterally, close to each half of its cartilage.
• The first arch cartilage diminishes as the mandible develops by intramembranous ossification.
• The third arch cartilage, located ventrally, contributes to the formation of hyoid bone's greater and upper horn and thyroid cartilage.
• The fourth and sixth arches combine to create the laryngeal cartilages.
• The epiglottis cartilage originates from the hypopharyngeal eminence, derived from the third and fourth arches.
• Fifth arches, if existent, are rudimentary with no identifiable derivations.
• The muscular components of the arches originate from unsegmented paraxial mesoderm, including prechordal plate sources, contributing to numerous muscles.
• First arch musculature creates the masticatory muscles and other facial muscles.
• The second arch's musculature develops the stapedius, stylohyoid, posterior belly of the digastric, muscles of facial expression, and certain auricular muscles.
• The stylopharyngeus muscle arises from the third arch.
• The fourth arch musculature produces the cricothyroid, levator veli palatini, and pharyngeal constrictors.
• Intrinsic laryngeal muscles originate from the sixth arch.
• Each arch is supplied by a specific cranial nerve. The special visceral efferent fibers of these nerves supply the muscles derived from the respective arches.
• The trigeminal nerve (CN V) supplies the first arch structures.
• The facial nerve (CN VII) supplies the second arch structures.
• The glossopharyngeal nerve (CN IX) supplies the third arch structures.
• The superior laryngeal branch of the vagus nerve (CN X) supplies the fourth arch structures.
• The recurrent laryngeal branch of the vagus nerve (CN X) supplies the sixth arch structures.
• The sensory and motor functions provided by cranial nerves innervate head and neck structures, including bones, mucous membranes, and teeth.
• The first pharyngeal pouch develops the middle ear cavity and auditory tube.
• Internal parts of the two (second and third) pouches form the parathyroid glands.
• Fourth pouch produces the inferior parathyroid glands and thymus regions.
• The endoderm of the pouches merges with ectoderm in the pharyngeal grooves, forming pharyngeal membranes.

Pharyngeal Pouches

• The primordial pharynx emerges from the foregut, widening cranially and narrowing as it connects to the esophagus.
• Its lining endoderm forms the internal aspects of the arches and pouches.
• Pouches develop from outpouching endoderm in a craniocaudal sequence between the arches.
• The first pouch forms the cavity of the tympanic membrane and auditory tube, also contributing to the tympanic cavity.
• The second pouch mostly disappears with only a small tonsillar sinus remnant..
• From the third pouch, the inferior parathyroid glands develop.
• The fourth pouch produces the superior parathyroid glands and ulomobranchial body regions, contributing to the thymus.

Pharyngeal Grooves (Clefts)

• Four grooves (clefts) are present on each side of the embryo's head and neck region during the fourth and fifth weeks.
• The external acoustic meatus is the only groove that becomes a postnatal structure, persisting during development.
• All other grooves disappear into a cervical sinus.

Pharyngeal Membranes

• Pharyngeal membranes form where pouches and grooves come into contact during development, and they enclose pharyngeal pouches.
• These membranes are double layered, consisting of endoderm from the pouches and ectoderm from the grooves, which gradually regress.
• Only the pharyngeal membrane of the first groove persists, forming the tympanic membrane.

Cervical (Branchial) Sinuses and Fistulas

• External sinuses arise from incomplete closure of the second pharyngeal groove and cervical sinus; often along the anterior border of the sternocleidomastoid muscle.
• Internal sinuses open into the tonsillar sinus or near the palatopharyngeal arch; they're less frequent but result from persisting remnants of the second pharyngeal pouch.
• Complete closure of the pharyngeal grooves typically occurs before birth.

Cervical (Branchial) Cysts

• Remnants of the pharyngeal cavity or second groove can form cysts—small, spherical, or elongated—located just inferior to the angle of the jaw and the anterior border of the sternocleidomastoid muscle, or in the preauricular region.
• They become apparent during late childhood or early adulthood and are characterized by a slowly enlarging, painless swelling.

First Pharyngeal Arch Syndrome

• Abnormal development of the first pharyngeal arch (jaw) affects the eyes, ears and soft tissues of the face and mouth.
• Two major types are Treacher Collins syndrome and Pierre Robin sequence.

DiGeorge Syndrome

• Also known as 22q11.2 deletion syndrome, characterized by the absence of a thymus, underdeveloped parathyroid glands, and abnormalities in the cardiac outflow tracts.
• Symptoms include congenital heart defects, immune deficiencies (thymus is essential for T-cell development), and varying degrees of hypoparathyroidism.
• The disease may manifest with ectopic (in an unusual place) glandular tissue in some cases.

Ectopic Parathyroid Glands

• Parathyroid glands may be positioned near or within the thyroid gland or thymus.
• Superior glands are typically in a stable position, while the inferior glands are less so.
• Supernumerary parathyroid glands may develop due to divisional abnormalities from the primordia.