PHARYNGEAL ARCHES: FORMATION AND COMPONENTS - ANATOMY AND PHYSIOLOGY - 2024

The pharyngeal or branchial arches are mesodermal bars that displace the mesodermal tissue that surrounds the pharyngeal intestine of the human embryo. The gill arches appear between the fourth and fifth week of development.

The branchial arches are the most important structures for the formation of the head and neck during intrauterine embryonic development. These arches appear in the pharyngeal wall, being six cylindrical thickenings of which the fifth is a transitory structure in humans.

Image source: slideshare.net

They separate the primitive stomach (common oral and nasal cavity of the embryo) from the developing heart. On the outside they are separated by small slits called "gill grooves" and on the inside by small depressions that form the so-called "pharyngeal bags".

As embryonic development continues, each branchial arch forms its own cartilaginous and muscular components, as well as its own artery and nerve. Some cartilaginous portions of these arches eventually disappear, but others persist for the rest of life as cartilage or bone structures.

The muscles developed in these arches migrate to adjacent regions, but their origin can always be traced, since they retain the same innervation as the original branchial arches.

The pharyngeal arches give rise to Meckel's cartilage, the ear ossicles, the styloid process, the hyoid bone, and the laryngeal cartilages of the adult.

Formation of the pharyngeal arches

Diagram of the pharyngeal arches (Source: Loki austanfell via Wikimedia Commons)

The term "branchial arch" was traditionally used to describe the development system of the embryonic arches of fish and amphibians, which is why some authors currently prefer to refer to these structures as "pharyngeal arches".

The formation of the branchial arches begins around the fourth week of intrauterine embryonic development. These consist of six pairs of arches that decrease caudally in size.

Each arch is made up of four essential types of tissue, namely: cartilage, muscle, nerve, and artery. These serve as building blocks for the face, neck, and oropharynx. However, the fronto-facial prominence does not derive from the branchial arches.

Diagram of a 4 week human embryo (Source: Laird.sheldahl via Wikimedia Commons)

The nerves of the branchial arches have motor neurons that connect with the skeletal muscles derived from each arch and are also called branchial motor neurons.

They also include sensory neurons, originating from the ectoderm, and visceral sensory neurons from the ectoderm, which cover the primitive intestine.

The branchial arches are prominences or protrusions of mesenchyme that appear along the upper part of the primitive intestine. Its external part is covered with ectoderm, while the endoderm covers its internal surface.

Components

The components that originate from each branchial arch are described below. The fifth pharyngeal or branchial arch is not described, since it is a transitory structure that completely disappears in humans. The origin of the tongue is described separately, since it originates from various branchial arches.

First branchial arch

This branchial arch is called the mandibular arch. It consists of two parts, a small dorsal part called the maxillary process that extends forward under the region that corresponds to the eye; and a much larger ventral portion called the mandibular process or Meckel's cartilage.

As development continues, the maxillary process and Meckel's cartilage disappear as such, except for two small portions at the distal ends that give rise to the incus and hammer respectively (ossicles of the ear).

The lower jaw is formed by intramembranous ossification of the mesodermal tissue that surrounds Meckel's cartilage. A part of this cartilage becomes fibrous and becomes the sphenomaxillary ligament.

The nerve of the first branchial arch is the trigeminal nerve that is distributed in the skin of the lower jaw and in the anterior two thirds of the lingual mucosa. This nerve gives a branch called the "inferior maxillary nerve", which supplies the muscles of the mandibular arch. These would be the chewing muscles, the anterior belly of the digastric muscle, and the hammer muscle.

Second branchial arch

The second branchial arch, or hyoid arch, forms a cartilage called Reichert's cartilage that gives rise to the following structures: the styloid process of the temporal bone, the stapes, the stylohyoid ligament and, in its ventral part, the lesser horn and the superior hyoid bone.

The nerve of the second branchial arch is the facial nerve. The muscles that develop from this arch and that are innervated by the facial are the stylohyoid muscle, the posterior belly of the digastric muscle, the stapes muscle, and the muscles of facial expression.

Third branchial arch

The cartilage of this pharyngeal or branchial arch forms the lower portion of the body of the hyoid bone and the greater horn of this bone.

The soft structures derived from this arch are a large part of the tongue and the stylopharyngeal muscle. The nervous component of this arch is the glossopharyngeal nerve. This nerve provides part of the sensory innervation to the tongue and innervates the stylopharyngeal muscle.

Fourth and sixth gill arches

The cartilaginous components of these branchial arches fuse and give rise to the thyroid, cricoid, and arytenoid cartilages of the larynx.

The muscles of the fourth arch are the cricothyroid and the constrictor muscles of the pharynx. These muscles are innervated by the superior laryngeal nerve, a branch of the vagus nerve.

The muscles derived from the sixth branchial arch are the intrinsic muscles of the larynx and are innervated by the recurrent laryngeal nerve, also a branch of the vagus nerve.

Language

In a four-week embryo, the tongue appears as a structure made up of three protrusions (two lateral and one medial). These three bulges come from the proliferation of the mesoderm in the ventral portion of the mandibular arch (first branchial arch).

The mesoderm of the second, third and part of the fourth branchial arch form the hypobranchial eminence. The posterior portion of the fourth branchial arch forms the epiglottis. As the mucosa that covers the tongue comes from the first branchial arch, it is innervated by the inferior maxillary nerve, a branch of the trigeminal nerve.

The posterior portion of the tongue comes from the second and third branchial arch and part of the fourth. In the adult, the sensory innervation of the posterior part of the tongue is given by the glossopharyngeal nerve belonging to the third branchial arch, which indicates that this portion grows much more than that from the second branchial arch.

The epiglottis is innervated by the superior laryngeal nerve, suggesting that it originates from the fourth branchial arch, since that nerve comes from that arch.

References

1. Elumalai, G., Chodisetty, S., Usen, BO, & Patel, RD (2016). "Patent Ductus Caroticus" embryological basis and its clinical significance. Elixir Physio. & Anatomy, 98, 42439-42442.
2. Langman, J. (1977). Medical embryology. Williams and Wilkins, Baltimore, MD.
3. Larsen, WJ (1998). Essentials of human embryology (pp. 123-172). New York: Churchill livingstone.
4. Moore, KL, Herbst, M., & Thompson, M. (1988). Essentials of human embryology. BC Decker.
5. Peck, JE (1994). Development of Hearing. Part II: Embryology. Journal-American Academy of Audiology, 5, 359-359.