Schematic of developing fetus with first, second,and third arches labeled.
Floor of pharynx of human embryo about twenty-six days old.
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In the development of vertebrates, the pharyngeal arches (which develop into the branchial arches or gill arches in fish) are primordia for a multitude of structures. In the human embryo (where the vasculature of the pharyngeal arches is also known as the aortic arches), they develop during the fourth week as a series of mesodermal outpouchings on both sides of the developing pharynx. In fish, the branchial arches support the gills.
During human and all vertebrate development, a series of pharyngeal arch pairs form in the developing embryo. These project forward from the back of the embryo toward the front of the face and neck. Each arch develops its own artery, nerve that controls a distinct muscle group, and skeletal tissue. The arches are numbed from 1 to 6, with 1 being the arch closest to the head of the embryo, and arch 5 existing only transiently. :318-323
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 of these is surrounded by mesenchyme. Arches do not develop simultaneously but instead possess a "staggered" development.
Pharyngeal pouches (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.
There are six pharyngeal arches, but in humans the fifth arch exists only 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.
The recurrent laryngeal nerve is produced from the nerve of arch 6, and the laryngeal cartilages from arches 4 and 6. The superior laryngeal branch of the vagus nerve arises from arch 4. Its arteries, which project between the nerves of the fourth and sixth arches, become the left-side arch of the aorta and the right subclavian artery. On the right side, the artery of Arch 6 is obliterated while, on the left side, the artery persists as the ductus arteriosus; circulatory changes immediately following birth cause the vessel to close down, leaving a remnant, the ligamentum arteriosum. During growth, these arteries descend into their ultimate positions in the chest, creating the elongated recurrent paths.:318–323
In other animals
The pouches line up with the clefts, and these thin segments become gills in fish.
Use in staging
The development of the pharyngeal arches provides 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.
This article uses anatomical terminology; for an overview, see anatomical terminology.
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