Pharyngeal Arches in Vertebrates
In vertebrates, the pharyngeal arches are derived from all three germ layers. Neural crest cells enter these arches where they contribute to craniofacial features such as bone and cartilage. However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest-independent mechanisms of pharyngeal arch development. The first, most anterior pharyngeal arch gives rise to the oral jaw. The second arch becomes the hyoid and jaw support . In fish, the other posterior arches contribute to the brachial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus. The genetic and developmental basis of pharyngeal arch development is well characterized. It has been shown that Hox genes and other developmental genes such as dlx are important for patterning the anterior/posterior and dorsal/ventral axes of the branchial arches. Interestingly, some fish species have jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation.
Read more about this topic: Pharyngeal Slit
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“No annual training or muster of soldiery, no celebration with its scarfs and banners, could import into the town a hundredth part of the annual splendor of our October. We have only to set the trees, or let them stand, and Nature will find the colored drapery,—flags of all her nations, some of whose private signals hardly the botanist can read,—while we walk under the triumphal arches of the elms.”
—Henry David Thoreau (1817–1862)