6 mesodermal thickenings on each side of primitive pharynx:

1. Outer ectodermal covering (Externally – 5 grooves – Pharyngeal clefts)
2. Mesodermal core
3. Inner endodermal lining (Internally – 4 grooves – Pharyngeal pouches)

Basic structure of pharyngeal arch:

1. Mesoderm – Cartilagenous bar (forms cartilage, ligaments and bones)
2. Mesoderm – Striated muscle (special viceral muscle of head and neck)
3. Aortic arch artery
4. Own nerve (motor + posttrematic sensory)
5. Next arch’s nerve (pretrematic sensory)

NB: 5th arch disappears

Derivatives of the arches:

Ectoderm, endoderm, mesoderm derivatives and shape of each pharyngeal arch:

Anomalies:

1. Branchial cyst – along anterior border of sternocleidomastoid muscle due to failure of cervical sinus to obliterate
2. Branchial sinus – Branchial cyst opens into skin by narrow canal
3. Branchial fistula – Branchial cyst opens into lumen of pharynx
4. Duplication of external auditory meatus – 1st branchial anomaly

Note: If left untreated, may become repeatedly infected and inflamed. Recurrent inflammation makes surgical resection more difficult. Excellent prognosis if lesion is completely resected. 

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