Branchial (Pharyngeal) Arches

Page 1 of 3COURSES > HUMAN ANATOMY II, DDS09, AUT06 > CONCISE ORAL HISTOLOGY > BRANCHIAL ARCHESBranchial ArchesCourse DocumentsBranchial (Pharyngeal) ArchesThe contribution of the mandibular and maxillary processes to the formation of the face and oral cavity isreadily apparent. Nonetheless, these processes arise from only one of several paired structures havingsimilar anatomical features:1. The branchial (pharyngeal) arches2. The pharyngeal pouches3. The branchial clefts (grooves)4. The branchial membranesThe appearance of the branchial arches proceeds in a craniocaudal sequence from the 1st to the 6th,but only four pairs are actually discernible on the external surface.Each pair of arches has the same basic form:• A central cartilaginous rod serves as its skeleton• A muscular component (branchiomere)• A transient vascular component (aortic arch)• One or more cranial nerves.Branchial arches, and their associated pouches and clefts, contribute significantly to the development ofthe head and neck, and most congenital malformations of the head and neck (including cleft palate)arise during the transformation of these embryonic structures into their adult derivatives.The 1st and 2nd Branchial ArchesDuring the 4th week of development the mesoderm of the ventral foregut becomes segmented, andforms a series of five bilateral processes. The initial mesodermal core of the first two arches is quicklyaugmented by the migration of neural crest tissue (ectomesenchyme). Mesodermally-derivedmesenchyme gives rise to muscle and vascular tissues, while the ectomesenchyme supplies theskeletal and connective tissues of 1st and 2nd arch.The contributions of the 1st (mandibular) arch to the formation of the face are:1. The mandible and the maxilla.2. Its cartilage is named Meckel's cartilage, and although it does not contribute substantially to theformation of the mandible, portions of Meckel's cartilage persist as:◦ The malleus and the incus of the middle ear◦ The spine of the sphenoid bone◦ The anterior ligament of the malleus◦ The sphenomandibular ligament.3. The musculature associated with the 1st arch includes:◦ The muscles of mastication (temporalis, masseter, medial and lateral pterygoids◦ The mylohyoid◦ The anterior digastric◦ The tensor tympani and the tensor veli palatini.All of these muscles are innervated by the maxillary and mandibular divisions of cranial nerve V. Theopthalmic division of the trigeminal nerve does not innervate any branchial arch structure.

Page 2 of 3The 2nd (hyoid) arch contains Reichert's cartilage, which forms:1. Much of the hyoid bone2. Most of the stapes3. The styloid process of the temporal bone4. The stylohyoid ligamentThe muscles of the second arch form all of the muscles of facial expression, as well as the stylohyoid,stapedius and posterior digastric. All are innervated by cranial nerve VII.Nerve fibers of specific cranial nerves enter the mesoderm and initiate muscle development. Musclesthat develop from these mesodermal cores then adapt to the adult branchial arch derivatives andmigrate from their sites of origin. The original nerve connections are maintained during their migration,and this accounts for the tortuous, even somewhat devious routes of many cranial nerves in the adult.Branchial Grooves (Clefts) and Pharyngeal PouchesThe branchial arches are separated:• Externally by ectodermally-lined branchial grooves or clefts• Internally by endodermally-lined pharyngeal pouches.Where the groves and pouches approximate one another they form branchial membranes. Of all thegrooves, pouches and membranes, only those associated with the 1st and 2nd arches actually persist inthe adult.The 2nd, 3rd and 4th branchial grooves are obliterated by the overgrowth of the 2nd branchial arch(hyoid operculum), thereby providing a smooth contour to the neck. Failure to completely eliminate theresulting cervical sinus may lead to the formation of a cervical (branchial) fistula and/or cyst.The pharyngeal pouches also give rise to a variety of important adult structures.1. The 1st pouch contributes to the developing ear.2. The 2nd pouch can be divided into ventral and dorsal portions.◦ The ventral part is obliterated by the developing tongue◦ The dorsal portion persists and the endodermal lining and the underlying mesenchymedifferentiate into the palatine tonsils.3. The derivatives of the third pouch include:◦ The thymus◦ The inferior parathyroid4. The fourth pouch contributes:◦ The superior parathyroid◦ The ultimobranchial body (precursor to the parafollicular or C cells of the thyroid gland)Development of the EarThe three parts of the human ear (external, middle and inner) arise from quite separate and diverseembryonic structures.• The external auditory meatus forms from the 1st branchial groove.• The future external ear (auricle) becomes recognizable as a series of six swellings or hillockssurrounding the dorsal part of the groove. Differential growth of the face and neck leads to the"migration" of the ear to its normal adult position.• The middle ear develops from the tubotympanic recess, which is derived from the 1st pharyngealpouch.• The endodermal lining of the pharyngeal pouch and the ectoderm of the 1st branchial grooveform the tympanic membrane (eardrum).

Page 3 of 3The embryonic origins of the malleus, incus and stapes were discussed earlier.Development of the TongueThe tongue develops in the ventral wall of the oropharynx from contributions of several branchial archesand by the invasion of muscle tissue from the occipital somites.• During the fourth week, paired lateral proliferations of mesenchyme (lateral lingual swellings)appear on the internal aspects of the 1st branchial arches.• Between and caudal to them is a median eminence called the tuberculum impar (literally meansunpaired swelling). The caudal border is marked by a blind pit, the foramen caecum, whichmarks the point of origin of the thyroid, which migrates to its adult position in the neck to becomethe thyroid gland.• As the tongue continues to develop, the lateral lingual swellings and the tuberculum impar mergeto provide the ectodermally derived mucosa of the anterior 2/3 of the tongue.• The ventral-medial parts of the 3rd and 4th arches elevate to form a united midventralprominence known as the copula (some textbooks also include a second structure called thehypobranchial emminence), which becomes the posterior 1/3 of the tongue. A v-shapeddepression (the sulcus terminalis) represents an approximate anatomical dividing line between1st arch and 3rd/4th arch derived mucosa in the adult.• The intrinsic muscles of the tongue arise in the floor of the pharynx opposite to the origin of thehypoglossal nerve. This muscle mass pushes forward as the hypoglossal cord from its site oforigin into the tongue.The complex development of the tongue from the branchial arches and from occipital somites isreflected in its innervation.• The intrinsic muscles of the tongue are innervated by cranial nerve XII.• The mucosal contributions of the 1st arch are seen in the sensory supply of the anterior 2/3 ofthe tongue by the lingual nerve (mandibular division of the C.N. V).• The posterior 1/3 of the tongue is innervated by the glossopharyngeal nerve• The vagus nerve make a small contribution to the epiglottis.With respect to gustatory sensation:• The chorda tympani (2nd arch) invades 1st arch territory and innervates taste buds in theanterior 2/3 of the tongue• Innervation of tastebuds in the posterior 1/3 of the tongue and the pharynx is by a mixedcontribution of cranial nerves IX and X.