Pharyngeal arch

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.

Structure

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 branchial 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. Some fish species have a second set of jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation.

First pharyngeal arch

The first pharyngeal arch also mandibular arch (corresponding to the first branchial arch or gill arch of fish), is the first of six pharyngeal arches that develops during the fourth week of prenatal development. It is located between the stomodeum and the first pharyngeal groove.

Processes

This arch divides into a maxillary process and a mandibular process, giving rise to structures including the bones of the lower two-thirds of the face and the jaw. The maxillary process becomes the maxilla (or upper jaw), and palate while the mandibular process becomes the mandible or lower jaw. This arch also gives rise to the muscles of mastication.

Meckel's cartilage

Meckel's cartilage forms in the mesoderm of the mandibular process and eventually regresses to form the incus and malleus of the middle ear, the anterior ligament of the malleus and the sphenomandibular ligament. The mandible or lower jaw forms by perichondral ossification using Meckel's cartilage as a 'template', but the mandible does not arise from direct ossification of Meckel's cartilage.