Pharyngeal arches

Six branchial or pharyngeal arches are formed originally. Each arch is lined by ectoderm on the outside and endoderm on the inside. Somatic mesoderm and neural crest cells form the core of the arch. The neural crest develops into cartilage, bone, and connective tissue in the arch. The ectoderm between adjacent arches forms the pharyngeal clefts. Endoderm between adjacent arches forms pharyngeal pouches. Pharyngeal arch 5 involutes without forming any major structures, so the arches are numbered 1,2,3,4 and 6. Each arch has an associated arterial or aortic arch and cranial nerve.

Derivatives of pharyngeal arches and associated structures

Number	Artery	Nerve	Muscle	Neural crest derivatives
1 (Mandibular)	Maxillary	Trigeminal (V)	Splits into maxillary and mandibular prominences. Mandibular gives rise to muscles of mastication, mylohyoid, anterior belly of digastric, tensor veli palatini and tensor tympani	Maxilla, zygomatic bone, squamous part of the temporal bone, palatine bone, vomer, mandible, Meckel’s cartilage, sphenomandibular ligament, incus and malleus
2 (Hyoid)	Stapedial	Facial (VII)	Muscles of facial expression, posterior belly of digastric, stapedius, stylohyoid	Lesser horn of hyoid, upper half of body of hyoid bone, stapes, styloid process, stylohyoid ligament
3	Common carotid and internal carotid artery	Glossopharyngeal (IX)	Stylopharyngeus	Greater horn and lower half of body of hyoid bone
4	Arch of aorta, right subclavian artery	Superior laryngeal branch of the vagus nerve (X)	Cricothyroid and cricopharyngeus; muscles of soft palate except tensor veli palatini; muscles of pharynx except stylopharyngeus	Thyroid, cricoid, arytenoid, corniculate and cuneiform cartilages
6	Pulmonary arteries and ductus arteriosus	Recurrent laryngeal branch of Vagus nerve (X)	Skeletal muscles of the esophagus and intrinsic muscles of the larynx except for the cricothyroid	Thyroid, cricoid, arytenoid, corniculate and cuneiform cartilages

Pharyngeal Pouches

Number	Derivatives
1	Auditory tube and cavity of the middle ear
2	Crypts of palatine tonsil
3	Inferior parathyroid glands, thymus
4	Superior parathyroid glands, C cells, or parafollicular cells of the thyroid*

Pharyngeal Clefts

Number	Derivatives
1	External auditory meatus**
2,3 and 4	Obliterated

*previously thought to arise from neural crest but now proved to be endodermal in origin.

** The first pharyngeal membrane between the ectoderm and endoderm of the first arch forms the tympanic membrane.

Congenital cervical cysts and fistulas

Preauricular cysts or fistulas arise from pharyngeal cleft 1 while remnants of clefts 2-4 present as cervical cysts or fistulas along the anterior border of the sternocleidomastoid muscle.

The thyroid gland develops from an endodermal thyroid diverticulum and forms the follicles. It is connected to the tongue at the foramen caecum located at the junction of the anterior ⅔ and posterior ⅓. The thyroglossal duct extends from the foramen caecum to the thyroid gland. It is normally obliterated.

Ectopic thyroid tissue

It is typically located in the midline, anywhere from the base of the tongue to lower neck along the route taken by the thyroid during its migration from the tongue. Most common location is the lingual thyroid. High cervical, superior mediastinal and paracardiac thyroids may be seen occasionally. It may cause hypothyroidism.

Thyroglossal cyst and fistula

Both thyroglossal cyst and fistula are present in the midline. They form when the thyroglossal duct fails to obliterate fully. Infection is the most common complication. Definitive treatment is by Sistrunk procedure where the whole tract along with a part of the hyoid bone and foramen caecum are removed.

Face and tongue

The anterior ⅔ of the tongue is formed by the fusion of the median and lateral tongue buds originating from the floor of the first pharyngeal arch. Occipital myoblasts invade the developing tongue and give rise to intrinsic muscles of the tongue. Posterior ⅓ of the tongue develops from swellings arising from the floor of the third and fourth pharyngeal arches.

The face is formed from the two mandibular prominences, two maxillary prominences, and the frontonasal prominence. They are derived from the neural crest mesenchyme. The nasal pits divide the frontonasal process into two lateral nasal and two medial nasal prominences. The mandibular prominence forms the mandible, lower lip, and jaw. The maxillary prominence on either side fuses with the lateral nasal process. A groove is formed between these two processes, forming the nasolacrimal duct and lacrimal sac. The two medial nasal processes fuse with each other to form the midline of the nose and philtrum of the upper lip. The lateral nasal process forms the alae of the nose. The upper part of the frontonasal process forms the forehead and bridge of the nose. Maxillary prominences form the cheek and lateral part of the upper lip. The maxillary prominences also fuse with the medial nasal prominences.

The primary palate is formed by the fusion of the two medial nasal processes. The secondary palate is formed by the fusion of the palatine shelves, which grow from the maxillary processes.

Unilateral cleft lip

Failure of the maxillary prominence on the affected side to join with the merged medial nasal prominences, resulting in a persistent labial groove

Bilateral cleft lip/ harelip

Failure of the maxillary prominences to merge with the medial nasal prominences

Median cleft lip

Very rare; caused by a mesodermal deficiency. Partial or complete failure of the medial nasal prominences to merge and form the intermaxillary segment.

Cleft lip is seen in Mohr’s syndrome (transmitted as an autosomal recessive trait).

Cleft Palate

It may be either unilateral or bilateral. It is often associated with cleft lip.

Clefts of the anterior or primary palate

Occur anterior to the incisive foramen and are caused by a failure of the lateral palatine processes to meet and fuse with the primary palate

Clefts of the anterior and posterior palate

Involve both the primary and secondary palate and are caused by failure of the lateral palatine processes to meet and fuse with each other, the primary palate, and the nasal septum

Clefts of the posterior or secondary palate

These clefts are posterior to the incisive foramen and are caused by a failure of the lateral palatine processes to meet and fuse with each other and the nasal septum.