The gill arch is a six-part anatomical system in the early embryonic stage of humans. Various parts of the human body develop from the six relatively independent gill arches during later gestation. If the gill arch is affected by developmental abnormalities, the fetus may experience malformations.
What is the gill arch?
The head gut of all vertebrate embryos proliferates into what is called the gill arch. These are gill-like folds that are relevant only to the fetus and its development. By birth, they form into anatomical structures. In humans, the gill arch develops during the early embryonic period. Between the third and fifth week of embryonic development, the embryonic connective tissue already proliferates and forms a total of six arches. Only four of them are relevant for the later development of the fetus. The fifth gill arch is only rudimentary in all mammals. In internal view, the gill arches appear with gill folds or gullet pockets. In external view, they correspond to gill furrows. The anatomical structure of the gill arch is also known as the bronchial arch or gullet arch. It is also sometimes referred to as the pharyngeal arch or visceral arch.
Anatomy and structure
The individual gill arches in humans are completely metameric, that is, structurally identical. During embryonic development, a cotyledon forms in each gill arch, from which a cartilage, nerve, artery, and muscle later grow. These structures can be assigned to each gill arch individually. That is, together they do not form a coherent system, but exist as a self-contained system to each associated gill arch. The first and second gill arches develop first. This development is followed by the formation of the third and fourth gill arches. The fifth arch is hardly formed. The sixth passes into the fourth later in the embryonic phase. Directly related to the gill arches are the internal pharyngeal pockets, which make up a total of five separate structures.
Function and tasks
Organs develop from the gill arches of the embryo during the later stages of fetal development. These organs are also called branchiogenic organs. The first gill arch forms parts of the face. These mainly include the jaw parts, the palate, and the ossicles malleus and incus. The first gill arch nerve later becomes the fifth cranial nerve. Its muscular anlage becomes the masticatory musculature, with most of its own artery receding. The second gill arch forms into the stapes. The upper hyoid and temporal bones also arise from the second gill arch. The artery of this arch later regresses. Its nerve becomes the seventh cranial nerve and its musculature develops, especially the mimic musculature. The third gill arch later gives rise to the lower hyoid bone. Its muscle becomes the stylopharyngeal muscle, with its artery becoming the internal carotid artery. Its nerve later forms the ninth cranial nerve, called the lingual pharyngeal nerve. The fourth gill arch, in interaction with the sixth gill arch, gives rise in particular to the larynx together with the laryngeal and pharyngeal muscles. Its artery becomes the aortic arch and subclavian artery. Together with portions of the sixth gill arch, the fourth gill arch nerve also develops into the tenth cranial nerve. The merely rudimentary fifth gill arch does not form definite structures. In contrast, anatomical structures do develop from the five gullet pockets or gill slits of the gill arch during the embryonic phase. In particular, the first pharyngeal pocket becomes the eustachian tube and the auditory canal. The second pharyngeal pocket becomes the tonsils of the palate. The third and fourth form the parathyroid gland and the thymus. The fifth pharyngeal pocket becomes C cells, which later populate the thyroid gland.
Diseases
The gill arch may be affected by embryonic developmental disorders. Such a developmental disorder could possibly be due to nicotine or alcohol consumption during pregnancy. Cleft lip and palate is one of the best known phenomena in the context of a developmental disorder of the gill arch. In the gill arch, individual parts of the face develop separately to later grow together.If these individual parts do not fuse or fuse incompletely in the seventh week of pregnancy, a deformed intermaxillary segment may form, for example. The upper jaw bulges from certain parts of the gill arch later fuse with the nasal bulges. They form the left and the right part of the upper lip and also shape the individual sides of the upper jaw. If this development is disturbed or relevant tissue parts reopen during development, a cleft jaw or cleft lip develops, which can be pronounced either unilaterally or bilaterally. Many other abnormalities of the jaw or teeth may be due to developmental abnormalities of the gill arch. Goldenhar syndrome, for example, is a congenital malformation syndrome that can result in asymmetrical corners of the mouth, underdeveloped cheeks and jaw parts, as well as small ears, narrow palpebral fissures, and even missing eyes. Often the children are also affected by a heart defect, kidney damage, or hearing and dental impairments. Medical science now assumes that the cause of the syndrome is a thrombus in the tissues of the first and second gill arches and the first gullet pocket. The thrombus is probably preceded by an interrupted blood supply to these tissues. Little is known about the causes of such a circulatory disorder. The syndrome is not thought to be hereditary.
