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. 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. 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.

Each pharyngeal arch has a cartilaginous bar, a muscle component which differentiates from the cartilagenous tissue, an artery, and a cranial nerve.

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 arches

First pharyngeal arch

The first pharyngeal arch is also called the "mandibular arch". It is located between the stomodeum and the first pharyngeal groove. This arch gives rise to such things as the maxilla, mandible, the incus and malleus of the middle ear, and the muscles of mastication.

Second pharyngeal arch

• Also called the "hyoid arch"
• Skeletal contributions
  • Stapes, styloid process, hyoid (Lesser Horn and Part of Body)
• Muscular contributions
  • Muscles of facial expression, buccinator, platysma, stapedius, stylohyoid, digastric posterior belly
• Nerve
  • Facial nerve (cranial nerve VII)

Third pharyngeal arch

• Skeletal contributions
  • Hyoid (Greater Horn and Part of Body)
• Muscular contributions
  • Stylopharyngeus
• Nerve
  • Glossopharyngeal nerve (cranial nerve IX)

Fourth and sixth pharyngeal arches

• Skeletal contributions
  • thyroid cartilage, cricoid cartilage, epiglottic cartilage, arytenoid cartilages
• Muscular contributions
  • intrinsic muscles of larynx, pharynx; levator palati
• Nerve
  • Vagus nerve (cranial nerve X)

Branchial pouches

First pouch

• This is the only pouch in which the endoderm and ectoderm remain close together, as the tympanic membrane. There is minimal mesoderm in the tympanic membrane.
• The endoderm becomes the auditory tube, middle ear and mastoid antrum.
• The ectoderm becomes the external acoustic meatus

Second pouch

• Contributes to the middle ear, tonsils, supplied by the glossopharyngeal nerve.

Third pouch

• Derivatives include the inferior parathyroid glands and thymus

Fourth pouch

• Derivatives include the superior parathyroid gland and parafollicular C-Cells of the thyroid gland.

Fifth pouch

• Rudimentary structure, becomes part of the fourth pouch.

References

• 2006 - 2007 Dental Decks
• McMinn, R., 1994. Last's anatomy: Regional and applied (9th ed).
• Larsen Embryology.

External links

• The role of the endoderm in the development and evolution of the pharyngeal arches

Anatomy | Animal anatomy

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