pharyngeal membrane

**Definition & embryology** - Pharyngeal (branchial) apparatus = arches (mesoderm + neural crest core), clefts/grooves (ectoderm, external), pouches (endoderm, internal), and membranes (where cleft ectoderm directly contacts pouch endoderm). - Each membrane is a 2-layer structure: outer ectoderm (from the cleft) and inner endoderm (from the pouch). In most places, mesenchyme invades and the membrane is obliterated. - Mnemonic: "Ectoderm = Clefts, Endoderm = Pouches, Mesoderm = Arches."

**Derivatives** - 1st membrane: tympanic membrane (eardrum). Ectoderm of 1st cleft forms the outer layer and the external auditory meatus. Endoderm of 1st pouch forms the inner layer and the tubotympanic recess (middle ear cavity, mastoid air cells, eustachian tube). Mesenchyme between them forms the middle fibrous layer. - 2nd–4th clefts: overgrow and are obliterated by the 2nd arch, forming the transient cervical sinus, which normally disappears. Membranes 2–4 are not retained as distinct structures. - Oropharyngeal membrane (buccopharyngeal membrane) is a separate structure. It is an ectoderm–endoderm membrane at the cranial end of the foregut that ruptures at about week 4 to connect the stomodeum and the foregut. Do not confuse it with the pharyngeal membranes. Failure to rupture is rare (oropharyngeal membrane persistence).

**Clinical correlations** - Branchial cleft cyst: remnant of the cervical sinus from incomplete obliteration of the 2nd–4th clefts. It presents as a lateral neck cyst, usually along the anterior border of the sternocleidomastoid. Most are 2nd cleft cysts. - Branchial fistula: a persistent communication between the cleft and pouch, which means the membrane has broken down. The 2nd arch fistula runs from the lateral neck, between the internal and external carotid arteries, to the tonsillar fossa (the 2nd pouch). - 1st arch/cleft anomalies: preauricular pits/sinuses, external ear and EAC malformations (e.g., atresia). These may occur in Treacher Collins syndrome (1st arch neural crest defect, with mandibular hypoplasia, conductive hearing loss and downward-slanting palpebral fissures). - Congenital hearing loss from malformed middle ear structures (ossicles from arches 1 and 2).

**High-yield summary table** - 1st arch → CN V (V2, V3); 2nd → CN VII; 3rd → CN IX; 4th–6th → CN X (superior laryngeal from the 4th arch, recurrent laryngeal from the 6th). - Pouches: 1st = middle ear/eustachian tube; 2nd = palatine tonsil; 3rd dorsal = inferior parathyroids, 3rd ventral = thymus; 4th = superior parathyroids; 4th/5th pouch derivatives also include the ultimobranchial body (parafollicular C cells). - Pouch 3 derivative "3 below 4" for parathyroids: the inferior parathyroids come from the 3rd pouch, and the superior parathyroids from the 4th pouch.