Branchial apparatus

The branchial apparatus consists of a series of six mesodermal arches formed during early gestation, separated externally by ectodermally lined branchial clefts and internally by endodermally lined pharyngeal pouches. The second arch increases in thickness and proliferates caudally to meet the enlarging ridge of the fifth arch; this causes the second, third and fourth branchial clefts to become enclosed in a cavity, called the cervical sinus of His. Normally, this sinus of His will soon obliterate.

Specific adult structures are derivated from each branchial arch and the related cleft and pouch. The first branchial cleft persists as the epithelium of the external auditory canal; the other branchial clefts are obliterated as the sinus of His disappears. From each mesodermal branchial arch, specific osseous, cartilagenous and vascular structures develop. The first branchial arch contributes to facial development; the other arches also contribute to facial development, and are involved in the formation of the pharynx and larynx. The first branchial pouch results in the eustachian tube. The second branchial pouch is involved in the formation of the palatine tonsil. The third branchial pouch forms the thymus and inferior parathyroid glands, while the fourth branchial pouch forms the superior parathyroid glands.

2. Pathology

Defects of the branchial apparatus manifest as cysts, sinuses, fistulas and ectopic glands. Anomalies of the branchial complex are vestigial remnants, resulting from incomplete obliteration of the branchial apparatus or buried epithelial cell rests.

First branchial anomalies

First branchial cleft anomalies may originate anywhere along the nasopharynx, middle ear cavity or external auditory canal; these sinuses, fistulas or cysts may extend downwards below the angle of the mandible, where they can present as an inflammatory mass lesion (Fig.1). Branchial pouch anomalies most commonly give rise to a short eustachian tube, or rarely to a nasopharyngeal cyst (see eustachian tube dysfunction (VI:2), Fig. 1).

Second branchial anomalies

These are the most frequent branchial anomalies. They may present as a fistula between the tonsillar fossa and an external skin opening, typically located along the anterior border of the sternocleidomastoid muscle. Such a fistula passes between the internal and external carotid artery. A second branchial cyst must lie somewhere along this path; the most common location is anterior to the sternocleidomastoid muscle, displacing the carotid vessels medially and the submandibular gland anteriorly (Fig.2).

Third branchial anomalies

A complete third branchial fistula has a course similar to the second branchial fistula, but posterior to the common and internal carotid artery, and opens into the pyriform sinus. Most third branchial anomalies originate in the posterior neck compartment.

Fourth branchial anomalies

Very rare lesions which arise from the pyriform sinus and extend downwards in the neck anterior to the carotid artery.

Thymic anomalies

Anomalies can result from incomplete descent of the thymus into the chest, sequestration of foci of thymic tissue along the normal pathway of descent, or thymic cysts resulting from failure of involution of the thymopharyngeal duct.

Parathyroid anomalies

Several theories have been formulated about the formation of parathyroid cysts; they may be embryological remnants of third and fourth branchial pouches. They are seen most commonly inferior and posterior to the thyroid gland.

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