Branchial arch

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Branchial arch

Schematic of developing fetus with first, second and third arches labeled.

Floor of pharynx of human embryo about twenty-six days old.
Gray's	subject #13 65
Carnegie stage	10
MeSH	Branchial+Arches
Dorlands/Elsevier	a_57/12149648

In the development of vertebrate animals, the branchial arches (or pharyngeal arches) develop during the fourth and fifth week in utero as a series of mesodermal outpouchings on the left and right sides of the developing pharynx. They are sometimes called gills, but that name is a misnomer (originating in the now discredited recapitulation theory).

1 Development
2 Relations
3 Specific arches
4 Use in staging
5 See also
6 References
7 External links

[edit] Development

These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium. By differential growth the neck elongates and new arches form, so the pharynx has six arches ultimately.

Each pharyngeal arch has a cartilaginous bar, a muscle component which differentiates from the cartilagenous tissue, an artery, and a cranial nerve. Each of these is surrounded by Mesenchyme. Arches do not develop simultaneously, but instead possess a "staggered" development.

[edit] Relations

Pharyngeal or branchial pouches form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches.

The pouches line up with the clefts, and these thin segments become gills in fish.

In mammals the endoderm and ectoderm not only remain intact, but continue to be separated by a mesoderm layer.

[edit] Specific arches

There are six pharyngeal arches, but in humans the fifth arch only exists transiently during embryologic growth and development. Since no human structures result from the fifth arch, the arches in humans are I, II, III, IV, and VI.

More is known about the fate of the first arch than the remaining four. The first three contribute to structures above the larynx, while the last three contribute to the larynx and trachea.

Pharyngeal arch	Muscular contributions	Skeletal contributions	Nerve and Artery
1st (also called "mandibular arch")	muscles of mastication (temporalis, masseter, medial pterygoid, lateral pterygoid), anterior belly of digastric, mylohyoid, tensor tympani, tensor veli palatini	maxilla, mandible (only as a model for mandible not actual formation of mandible), the incus and malleus of the middle ear, also Meckel's cartilage	Trigeminal nerve (V2 and V3) Maxillary artery
2nd (also called the "hyoid arch")	Muscles of facial expression, buccinator, platysma, stapedius, stylohyoid, digastric posterior belly	Stapes, styloid process, hyoid (lesser horn and Part of Body), Reichert's cartilage	Facial nerve (VII)
3rd	Stylopharyngeus	Hyoid (greater horn and Part of Body)	Glossopharyngeal nerve (IX), Common carotid/Internal carotid
4th and 6th	intrinsic muscles of larynx, pharynx; levator palati	thyroid cartilage, cricoid cartilage, epiglottic cartilage, arytenoid cartilages	Vagus nerve (X) Pulmonary artery

[edit] Use in staging

The development of the pharyngeal arches provide a useful morphological landmark with which to establish the precise stage of embryonic development. Their formation and development corresponds to Carnegie stages 10 to 16 in mammals, and Hamburger-Hamilton stages 14 to 28 in the chicken.