Development of the craniofacial structures is a complex process that proceeds in an orderly fashion throughout embryonic and foetal stages of formation. Craniofacial growth occurs due to a relatively rapid and orderly composition of mesodermal and cranial neural crest cells via a complex signaling network. Syndromes of the first and second branchial arches manifest along a spectrum of hypoplasia and aplasia of the structures derived from these arches. The first arches produce mainly the lower jaw – mandible, two bones in the middle ear malleus and incus, muscles for chewing that is muscles of mastication and a dedicated nerve mandibular nerve which is responsible for their supply. The second arch give rise to all of the muscles of facial expression, facial nerve, stapes bone in the middle ear, most of the outer ear, and parts of the bone above the larynx (voice box) along with palatine tonsils and middle ear cavity. The third arch produce the nerves for swallowing and the rest of the bone above the larynx that is glossopharyngeal nerve. The remaining arches give rise to the nerves for the vocal cords and other nerves and cartilage in the neck.
Some differences between abnormalities of the first and second branchial arch derivatives may reflect differences in the embryologic age at the time of the insult with respect to neural crest cell migration. Other changes are related to deregulation of cell- signalling pathways triggered by a combination of genetic and environmental factors. The manifestation and severity of the congenital abnormality depend on the alteration of gene-expression profiles.
