Pharyngeal arches

Embryological background

The pharyngeal arches appear in the fourth week of embryological development when neural crest cells migrate into the future head and neck regions, specifically the mesenchymal region.

The shape of future pharyngeal arches is regulated by the expression of hox genes. The arches are formed bilaterally as adjacent c-shaped swellings or rolls. Each arch is separated from the next on the embryo’s external surface by pharyngeal grooves (clefts), and on the internal surface by depressions called pharyngeal pouches. Pharyngeal pouches are developed from the endoderm and they open towards the pharyngeal clefts. Initially, there are six arches in amniotes. Yet, before development is complete, the fifth arch in humans regresses or is very little expressed according to some sources. Therefore, the numbering of first, second, third, fourth, sixth pharyngeal arches remains.

The first pair of pharyngeal arches to appear are the primordium of the jaws. These are seen as two elevations lateral to the future pharynx. These elevations are further divided into maxillary (dorsal and upper positioned) and mandibular (ventral and lower) prominences. After the first pair of pharyngeal arches, the second one successively appears as well as the others. The remaining arches appear on each side of the future head and neck region right before the first pair.
By the end of the fourth week, four visible pairs of pharyngeal arches are revealed while the fifth and sixth arches are invisible on the surface of the embryo.

Anatomy and supply

The pharyngeal apparatus contains cells from all three germ cell layers (endoderm, mesoderm and ectoderm) and from the neural crest. The core of each pharyngeal arch consists of embryonic connective tissue, known as the mesenchyme, which is derived in the third week from mesoderm and neural crest tissue. The mesenchyme is covered externally with ectoderm and internally with endoderm.
The ectoderm forms the skin and has a role in sensory neuronal differentiation, while the endoderm forms the inner surface of the pharynx, taste buds and glands including the thyroid, parathyroid and thymus. At the same time, the nearby mesoderm moves into each pharyngeal arch, forming a foundation for future muscle development (paraxial mesoderm), while the neural crest cells contribute to skeletal and connective tissue.

Each pharyngeal arch includes cartilaginous, arterial, muscular and nervous components. These will subsequently give rise to structures of the developing fetus where the cartilaginous part will form the skeleton of each arch and the muscular part will differentiate into muscles of the head and neck. Each pharyngeal arch will develop its own motor and sensory innervation supplied by the cranial nerves.

Expand your knowledge with the following video about cranial nerves:

Now each arch will be listed in order, including details of the musculature derived from the mesoderm, the skeletal structures that develop from the neural crest, the cartilage present, the connective tissue structures and the nerve assigned to that particular arch. Note that the first two arches are named (as the mandibular and hyoid, respectively) while the rest of the arches remain unnamed.

First pharyngeal arch

The first pharyngeal arch (mandibular) gives rise to the structures of the jaw, two of the middle ear bones and the muscles of mastication. The maxillary artery (terminal part) is equally derived from the first arch. The arch separates into maxillary (dorsal) and mandibular (ventral) prominences.

The maxillary prominence forms the maxilla, zygomatic bone, part of the vomer and short limb of the incus.

The mandibular prominence gives rise to the temporal bone, the malleus and the mandible. From the paraxial mesoderm of the first pharyngeal arch the muscles of mastication, mylohyoid, anterior belly of digastric, tensor veli palatini and the tensor tympani muscles are formed. 

The cartilage of the first pharyngeal arch (Meckel’s cartilage) is related to the development of the ear (dorsal end). The proximal part of this cartilage evolves into the malleus and incus while the rest forms the sphenomandibular ligament and the anterior ligament of the malleus.

The trigeminal nerve (CN V; maxillary and mandibular branches) supplies derivatives of the first pharyngeal arch. Branches from this nerve give sensory supply to the head and neck, including the facial skin, teeth, mucous membranes of the nasal cavities, palate, mouth and tongue. It also provides motor innervation to the muscles of mastication and the other muscles derived from this first arch.

Second pharyngeal arch

The second pharyngeal arch (hyoid) forms the muscles of facial expression, parts of the face and ear.

The second pharyngeal arch cartilage (Reichert’s cartilage) ossifies at its ventral and dorsal aspects to give rise to the stapes, long limb of incus, styloid process of temporal bone, superior part and the lesser cornu of the hyoid bone, respectively. The stylohyoid ligament and connective tissue of the tonsil make up the connective tissue structures for this arch.

The second pharyngeal arch forms the following muscles: the stapedius, stylohyoid, posterior belly of digastric muscle, auricular muscle and the muscles of facial expression. From its vascular component, this arch forms the embryonic stapedial artery and the caroticotympanic artery.

The entire arch is innervated by the facial nerve (CN VII), providing the anterior tongue with taste sensation.

Third pharyngeal arch

The third pharyngeal arch forms part of the hyoid bone, the common carotid artery and part of the internal carotid artery.

The third pharyngeal arch cartilage ossifies into the inferior part and the greater cornu of the hyoid bone. ​The third arch produces the stylopharyngeus muscle, from its mesoderm. The vascular components will give rise to the common carotid artery and the proximal portion of the internal carotid artery.

The glossopharyngeal nerve (CN IX) innervates structures derived from the third pharyngeal arch, providing taste and sensation to the posterior tongue.

Fourth pharyngeal arch

The fourth pharyngeal arch forms the laryngeal cartilages, muscles of the soft palate and pharynx, part of the subclavian artery and the arch of the aorta.

There are no skeletal structures in the fourth pharyngeal arch. The fourth and sixth pharyngeal arches cartilages merge to give rise to the laryngeal cartilages. The exception is the epiglottis, which develops from the third and fourth pharyngeal arches. More specifically, it arises from the hypopharyngeal eminence of the embryonic pharynx’s mesenchyme. The mesoderm of the fourth pharyngeal arch forms the cricothyroid muscle, levator veli palatini and the constrictor muscles of the pharynx.

These structures of the fourth pharyngeal arch are innervated by the superior laryngeal branch of the vagus nerve (CN X).

Fifth pharyngeal arch

​The fifth pharyngeal arch is transient and rudimentary in amniotes. It forms with the others in the very early stages and then regresses almost immediately. When visible, it looks like a small appendage between the fourth and sixth arches. Authors have conflicting opinions about the existence and regression of this arch. Similarly, there is a debate about abandoning the comparative anatomy numbering and renaming the sixth into the fifth pharyngeal arch.

Sixth pharyngeal arch

The sixth pharyngeal arch forms the laryngeal cartilages, the intrinsic muscles of the larynx, the pulmonary arteries and the ductus arteriosus.

There are no bones and ligaments in this pharyngeal arch. Notice that the fourth and sixth pharyngeal arches combine to contribute to the development of the laryngeal cartilages with the exception of the epiglottis. The musculature component of the sixth pharyngeal arch forms the intrinsic muscles of the larynx.

Like the fourth arch, this arch is innervated by the vagus nerve (CN X), specifically its recurrent laryngeal branch.

Would you like to expand your knowledge of embryology? Take a step further and complete this journey of exploring the pharyngeal apparatus with the following article: