Pharyngeal (Branchial) Arches - Overview
The pharyngeal arches (branchial arches), contribute significantly to the formation of structures in the head and neck.
They are distinctly visible in the head region of the embryo by the 5th week of development. They are indicated by Roman numerals, I to IV on the image to the right.
A Vth arch makes a transitory appearance and an additional arch, arch VI, is thought to develop in humans.
For all practical purposes, arches I to III are considered individually and arches IV, V and VI are lumped together.
A horizontal cut through the head region of a 5 week old embryo reveals the structure of the pharyngeal arches.
Each consists of:
- Pharyngeal Cleft (Groove) - Ectoderm lined surface groove
- Pharyngeal Pouch - Endoderm lined outgrowth of the developing pharynx
- Pharyngeal Arch - Mesenchymal core consisting of neural crest & mesoderm
mesoderm to form muscles and CT
- Nerve - supplies the specific muscles derived from each arch
- Artery - forms the major vascular structures of the head, neck & thorax.
Pharyngeal Arch I
The first pharyngeal (branchial) arch (Arch I) is also known as the mandibular arch. It divides into maxillary and mandibular processes and contributes 3 bones to the facial skeleton, mandible and two bones of the middle ear cavity. Its mesenchyme also provides eight pairs of muscles.
Cartilage
The cartilage of the first arch, like that of the second and third arches, is derived from neural crest.
It is called Meckel’s cartilage and largely regresses, except for the proximal portion in the mandibular process. which gives rise to the malleus and incus of the middle ear.
In the maxillary process, neural crest, forms mesenchymal cells that undergo intramembranous ossification to produce:
- maxillary bone
- zygomatic bone
- squamous part of temporal bone
Mesenchyme
The mesodermal mesenchyme of the first arch gives rise to eight pairs of muscles:
Four pairs of muscles of mastication.
- temporalis
- masseter
- medial and lateral pterygoids
- anterior belly of digastric
- mylohyoid
- tensor velli palatini
- tensor tympani.
All of these muscles are innervated by the trigeminal nerve (CN V), the nerve of pharyngeal arch I.
Pharyngeal Arch II
The second pharyngeal arch is known as the hyoid arch. It gives rise to parts of the hyoid bone, styloid process and one bone of the middle ear. Several pairs of muscles that attach to these bones as well as muscles of facial expression are derived from the mesoderm.
Cartilage
The cartilage of the second arch, which is derived from neural crest, is known as Reichert's cartilage. It gives rise to the following structures:
- stapes of the middle ear cavity
- styloid process
- lesser horn of hyoid bone
- upper part of the body of the hyoid bone
Mesenchyme
Several pairs of muscles are derived from the mesenchyme of the second branchial arch. The more important ones of these include:
- stapedius
- stylohyoid
- posterior belly of digastric
- mimetic muscles, including
- orbicularis oris
- orbicularis oculi
- buccinator
Pharyngeal Arch III
The third pharyngeal arch is not named, though its cartilage forms a major portion of the hyoid bone. Its mesoderm gives rise to a single muscle.
Cartilage
The cartilage of the third branchial arch gives rise to the greater horn and inferior part of the body of the hyoid bone.
Mesenchyme
Mesodermal mesenchyme of the third arch produces a single muscle, stylopharyngeus.
It is the sole muscle innervated by glossopharyngeal nerve (CN IX), the nerve of the third arch.
Pharyngeal Arches IV thru VI
Cartilage
The cartilages of arches IV through VI remain as cartilage forming the skeleton of the larynx, and consisting of both paired and unpaired cartilages.
These cartilages are derived from lateral plate mesoderm.
Mesenchyme
The muscles formed by the mesodermal mesenchyme of the IV to Vi arches include constrictors of the pharynx as well as intrinsic and extrinsic muscles of the palate and larynx.
All muscles formed by these arches are innervated by branches of the vagus nerve (CN X).
Aortic Arches
In the head region of the embryo, there are paired dorsal and ventral aortas that are connected to each other by aortic arches that pass through the pharyngeal arches.
These vessels will form the aorta and major arteries of the upper body, head and neck.
Some of the six numbered aortic arches will persist and others disappear.
The first aortic arch forms part of the external carotid artery and the maxillary artery, a terminal branch of the external carotid (not shown).
The second aortic arch forms the hyoid and stapedial arteries.
The third aortic arch forms the common carotid and proximal part of the internal carotid.
The fourth aortic arch has different fates on the right and left, becoming the subclavian artery on the right and aorta on the left.
The fifth aortic arch never actually develops.
The sixth aortic arch becomes the right pulmonary artery on the right and the ductus arteriosus on the left.
Pharyngeal Pouch Endoderm
The pharyngeal pouches are endodermal lined, lateral out growths of the foregut region. Each pharyngeal pouch differentiates into separate components of the head and neck.
Pouch I Endoderm
The endoderm of the first pharyngeal pouch forms the lining of the pharyngotympanic tube and middle ear cavity all colored in yellow.
The three ear ossicles, malleus, incus and stapes are formed from cartilage of pharyngeal arches I and II.
Pouch II Endoderm
The endoderm of the second pharyngeal pouch forms the lining of the tonsillar fossa for the palatine tonsils.
Many years ago, a student suggested I take a picture of her 'impressive' palatine tonsils. So I did.
Click on the image to the left if interested.
Pouch III and IV Endoderm
The endoderm of the third pharyngeal pouch forms a dorsal bud and a ventral bud.
The ventral buds of the third pouches, fuse in the midline and migrate into the superior mediastinum to form the thymus.
The dorsal bud of each pouch forms an inferior parathyroid gland located behind the inferior pole of each lobe of the thyroid gland.
The fourth pharyngeal pouch forms a dorsal and ventral bud as does the third pouch. The dorsal buds of the fourth pharyngeal pouches form the superior parathyroid glands located behind the superior poles of the thyroid glands.
The ventral buds of the fourth pharyngeal pouches interact with the endoderm of the fifth pharyngeal pouches and neural crest cells to form what is called the ultimobranchial body. This contributed to the development of the parafollicular (C-cells) of the thyroid gland.
Pharyngeal Groove Ectoderm
The pharyngeal (branchial) grooves are ectoderm lined spaces between adjacent pharyngeal arches. The first groove is the only one of any consequence. The others usually become absorbed into the tissues of the lateral neck.
The ectoderm of the first pharyngeal groove forms the lining of the external auditory meatus and lateral surface of the tympanic membrane highlighted in blue in the diagram.
The tympanic membrane is formed from ectoderm of the branchial groove, endoderm of the branchial pouch and intervening mesoderm.
The cochlea and vestibular apparatus, also derived from ectoderm are also colored blue. Their development will be discussed separately.
Tongue
The tongue begins development around the 4th week, with the appearance of primordial in the first through fourth pharyngeal arches.
Arch I
- Lateral lingual swellings - paired
- Tuberculum impair - unpaired
- Hypobranchial eminence
These structures provide the mucosa of the tongue. The muscles are formed from mesoderm derived from occipital myotomes.
The mucosa of the anterior 2/3 of the tongue, being derived from the first branchial arch is innervated by the lingual branch of V, the nerve of the first arch.
The mucosa of the posterior 1/3 of the tongue, being derived from the third branchial arch is innervated by the lingual branch of glossopharyngeal nerve and the root of the tongue from the fourth arch is supplied by the vagus nerve.
The muscles of the tongue generally have "glossus" as part of the name. All except one, are derived from mesoderm of the occipital myotomes and are innervated by the hypoglossal nerve, (CN XII).
he exception is palatoglossus, which is derived from mesoderm of pharyngeal arch IV and therefore is innervated by the vagus nerve, (CN X).
In fact, palatoglossus, in spite of having "glossus" as part of its name, is more of a palate muscle than a tongue muscle.
Thyroid Gland
Development of the thyroid gland begins as a ventral endodermal diverticulum in the floor of the developing oral cavity. The origin of the thyroid is marked by the presence of the foramen cecum on the surface of the tongue.
Initially, the thyroid gland is connected to the oral cavity by way of a thyroglossal duct, which eventually disappears.
The thyroid gland migrates inferiorly through the anterior neck to become located over the anterior surface of the upper tracheal rings. As a result, ectopic thyroid tissue may be located anywhere along the migratory path. A common location for these masses of thyroid tissue is in the tongue.
Thyroid follicular cells, that produce active thyroid hormone, thyroxine, are developed from endodermal cells.
Neural crest cells that interact with the ultimobranchial body form a second type of endocrine cell within the thyroid gland. These are the parafollicular cells that secrete calcitonin, a calcium regulating hormone.
Clinical Correlates
Ectopic Locations
Because of the dynamic nature of development and the migration of cells from different developmental cell lines like the neural crest or pharyngeal pouch endoderm, there is plenty of opportunity for ectopic migrations to occur resulting in the appearance of normal or abnormal glandular tissue in locations other than where they usually reside. Understanding the migration of the pharyngeal endoderm can help frame a differential diagnosis of masses in the neck region. Below are a few examples:
Ectopic Thymus
The patient is a 3 year old who was seen by his pediatrician for a lateral neck mass. The T2 weighted MRI image shows a large mass of thymic tissue that failed to migrate into the superior mediastinum, its usual location.
Ectopic Parathyroids
The adult patient presented with elevated parathyroid hormone levels and radiologic evidence of bone depletion. The CT shows the presence of a parathyroid adenoma due to ectopic parathyroid tissue that had migrated into the superior mediastinum. The adenoma is located posterior to the trachea and to the right of the esophagus.
It is likely that in its migration in the neck, this inferior parathyroid went too far or produces some additional parathyroid tissue that went along with the thymus into the mediastinum.
Ectopic Thyroid
The thyroid may be arrested anywhere along its path of migration to the anterior neck. The images on the right are from three different patients each with a diagnosis of ectopic thyroid.
Click on each thumbnail image for a larger, labeled view.
Case 1 is a 13 year old who presented with no thyroid tissue in the anterior neck on physical exam. CT imaging with contrast(radio labeled iodine) showed a mass of thyroid tissue within the posterior tongue. Lingual thyroid tissue accounts for about 90% of ectopic occurrences.
Case 2 is a 35 year old with diffuse thyroid tissue in the usual location, but also with a pyramidal lobe and ectopic thyroid tissue in the region of the hyoid.
Case 3 is a young adult with absence of thyroid tissue in the usual location as confirmed by a thyroid scan. Ectopic thyroid tissue is localized in the floor of the oral cavity, adjacent to the hyoid bone.
Case 1
Case 2
Case 3
Cystic Formations
Persistence of ducts or spaces that would usually disappear can become fluid filled resulting in the appearance of a 'lump' in the neck. Two common conditions are cervical sinus cysts, which tend to be lateral in the neck and thyroglossal duct cysts, which usually occupy the midline and which move upward upon protrusion of the tongue.
Cervical Sinus Cysts
Only the first pharyngeal cleft persists as it forms the external ear canal (external auditory meatus).
Arch 2 normally expands caudally and overgrows the 2nd, 3rd and 4 pharyngeal clefts, which disappear.
Occasionally the cleft persists as a sinus, which may fill with fluid and become visible on the lateral neck. They usually overlie the sternocleidomastoid muscle. If the sinus erodes to the surface, a fistula has developed, which may be evidenced by the appears of liquid exudate on the neck surface.
These CT images are from the same patient with a left sided cervical sinus cyst.
Thyroglossal Duct Cyst
The thyroglossal duct normally undergoes atresia, its original attachment to the floor of the oral cavity is marked by the foramen cecum.
Occasionally a section of the duct may persist in the anterior midline and may become cystic, presenting as a mid line mass that is non tender and which rises in the midline of the neck when the patient is instructed to stick out their tongue.
Click on the lump in the patient's neck to see a mid sagittal CT of a different patient with the same condition.
Ankyloglossia
Ankyloglossia or 'tongue tie' is due to failure of tissue to degenerate in the floor of the oral cavity. Normally only the frenulum of the tongue anchors it to the floor.