The Branchial Region: From Embryo to Adult Structures
The branchial region consists of transient structures that appear early in the embryonic development of vertebrates, including humans. Located in the head and neck of the developing embryo, these structures are crucial for forming many adult features of the face, neck, and throat. This developmental area shapes various tissues, including bones, muscles, nerves, and glands.
Building Blocks of the Branchial Region
During the fourth and fifth weeks of human embryonic development, the branchial region develops distinct components: branchial arches, branchial grooves, and pharyngeal pouches. The branchial arches, also known as pharyngeal arches, are mesenchymal prominences bulging from the lateral walls of the developing pharynx. Six pairs of these arches form, though the fifth arch is often rudimentary or merges with the fourth and sixth.
Each branchial arch has a core of mesenchyme, derived from paraxial and lateral plate mesoderm, with significant neural crest cell contribution. Neural crest cells migrate into the arches, forming skeletal and connective tissues. The arches are covered externally by ectoderm and lined internally by endoderm.
Between adjacent arches, external indentations called branchial grooves (or pharyngeal clefts) form from the ectoderm. Internal to the arches, endodermal outpocketings called pharyngeal pouches develop, extending laterally from the pharyngeal lumen towards the branchial grooves. Each arch is thus flanked by an external groove and an internal pouch. The arches are numbered in a cranial-to-caudal sequence, starting with the first arch closest to the developing head.
From Embryo to Adult: What the Branchial Region Becomes
The first branchial arch, often called the mandibular arch, gives rise to structures of the jaw and ear. Its skeletal derivatives include the malleus and incus bones of the middle ear, parts of the temporal bone, and the maxilla and mandible. Muscles derived from this arch include those involved in chewing, such as the temporalis, masseter, and pterygoid muscles, as well as the mylohyoid and the anterior belly of the digastric muscle. The trigeminal nerve (cranial nerve V) supplies these muscles and provides sensation to the face.
The second branchial arch, known as the hyoid arch, contributes to structures around the ear and neck. Skeletal elements formed from this arch include the stapes bone of the middle ear, the styloid process of the temporal bone, and the lesser horn and upper part of the body of the hyoid bone. Muscles originating from the second arch are those of facial expression, including the platysma, orbicularis oculi, and buccinator. The facial nerve (cranial nerve VII) innervates these muscles.
The third branchial arch forms the greater horn and the lower part of the body of the hyoid bone. The stylopharyngeus muscle is also derived from this arch. This arch is associated with the glossopharyngeal nerve (cranial nerve IX), which supplies the stylopharyngeus muscle and provides taste and general sensation to the posterior tongue. The common carotid artery and the proximal part of the internal carotid artery develop from the arterial arch of this segment.
The fourth branchial arch contributes to the formation of the thyroid and cricoid cartilages of the larynx. Muscles derived from this arch include the cricothyroid muscle and parts of the pharyngeal constrictors. The superior laryngeal branch of the vagus nerve (cranial nerve X) innervates these muscles. The right subclavian artery and a portion of the aortic arch originate from the fourth arterial arch.
The sixth branchial arch is responsible for forming the remaining laryngeal cartilages, such as the arytenoid, corniculate, and cuneiform cartilages. Most of the intrinsic muscles of the larynx develop from this arch. The recurrent laryngeal branch of the vagus nerve (cranial nerve X) supplies these laryngeal muscles. The pulmonary arteries and the ductus arteriosus arise from the sixth arterial arch.
The pharyngeal pouches also differentiate into various adult organs. The first pharyngeal pouch expands to form the tympanic cavity of the middle ear and the auditory (Eustachian) tube. The second pharyngeal pouch gives rise to the palatine tonsils. The third pharyngeal pouch differentiates into the inferior parathyroid glands and the thymus gland.
The fourth pharyngeal pouch develops into the superior parathyroid glands. It also forms the ultimobranchial body, which fuses with the thyroid gland and contributes parafollicular C cells.
The branchial grooves also have a developmental fate. The first branchial groove forms the external auditory meatus. The remaining second, third, and fourth branchial grooves typically disappear as the second arch grows caudally and covers them, forming the cervical sinus.
When Development Goes Awry: Common Branchial Anomalies
Abnormal development or incomplete obliteration of branchial structures can lead to various congenital anomalies, known as branchial anomalies. These conditions typically manifest as cysts, sinuses, or fistulas in the head and neck.
A branchial cleft cyst is the most common anomaly, resulting from incomplete closure of a branchial groove, most frequently the second. These cysts are typically found along the side of the neck, often near the angle of the jaw or in the upper neck, appearing as a soft, non-tender lump. They can become noticeable when infected, leading to pain, redness, and swelling.
A branchial sinus is a narrow tract opening either externally onto the skin or internally into the pharynx. An external branchial sinus typically presents as a small opening or dimple on the side of the neck, often discharging mucus or pus, especially if infected. These sinuses are most commonly associated with the second branchial groove, but can occur from any groove. Internal sinuses are less common and may present with recurrent throat infections.
A branchial fistula is a complete tract connecting the skin surface to the pharynx. These fistulas can allow for fluid or air passage, sometimes leading to recurrent infections or drainage from the neck. Like cysts and sinuses, fistulas are most frequently derived from second branchial arch anomalies. Their occurrence is often sporadic, arising from a failure of the branchial apparatus to properly fuse and resorb during embryonic development.