The pharyngeal arches appear around the 4th to 5th week of development and develop into structures of the head and neck. They are bars of mesoderm tissue forming around the pharynx and demarcated internally by the pharyngeal pouches (grooves on the endoderm) and externally by the corresponding pharyngeal clefts (grooves on the ectoderm). There are six arches, but only five are considered as the 5th arch is only transiently present.

Derivatives of the pharyngeal arches

The pharyngeal arches mostly contain paraxial mesoderm, which become muscles of the head and neck. Each arch is associated with a specific cranial nerve, such that muscle innervation is based on their arch of origin. The 1st arch is special in that it divides into the maxillary and mandibular prominences (of facial development) so innervation is therefore split such that the prominences are supplied by CN Vb and Vc respectively. Cranial neural crest cells from the neural tube invade the arches to form cartilages (from which arise bones and ligaments), although mesoderm also contributes to the (laryngeal) cartilages of the 4th and 6th arches.

Structures associated with and formed by each arch is summarised as follows:

Arch	Cranial nerve	Muscles	Cartilages & derivatives
I	Vb, Vc	muscles of mastication, oral tongue, tensor tympani, tensor palati, ant. belly of digastric, mylohyoid	Meckel’s cartilage: alisphenoid, malleus, incus, remnants in jaw
II	VII	muscles of facial expression, stapedius, pos. belly of digastric	Richert’s cartilage: stapes, styloid process, stylohyoid ligament, upper body and lesser cornu of hyoid
III	IX	stylopharyngeus, pharyngeal tongue	lower body and greater cornu of hyoid
IV	X (superior laryngeal)	criothyroid, soft palate muscles (minus tensor palati)	thyroid and epiglottic cartilages
VI	X (recurrent laryngeal)	intrinsic laryngeal muscles (minus cricothyroid)	remaining laryngeal skeleton

CLINICAL CORNER

The a rare autosomal dominant Treacher Collins syndrome (mandibulofacial dysostosis) involves underdeveloped facial bones, palate abnormalities and irregular outer/middle ear causing bilateral conductive hearing loss. It is caused by TCOF1 gene mutations (implicated in ribosome formation, chromosome segregation and regulation of oxidative stress), resulting in apoptosis in the neural tube which depletes the number of neural crest cells that can travel to and proliferate in the 1st and 2nd arches. Antioxidants in maternal diet may be protective.

DiGeorge syndrome (VCFS) involves minor craniofacial defects, agenesis of thymus and parathyroid glands, cardiovascular problems (e.g. TOF, PTA) plus severe middle and inner ear defects. It is caused by deletion of chromosome part 22q11 which contains the Tbx1 gene (encoding a transcription factor). Expression of Tbx1 in mesoderm, directs pharyngeal pouch formation, and in ectoderm and endoderm regulates expression of chemokine Cxcl12 to guide neural crest cells expressing receptor Cxcr4 to the arches.