second pharyngeal arch

**Mnemonic:** "Second arch = Seven" (CN VII). Arch 1 = chew (CN V), Arch 2 = smile (CN VII), Arch 3 = swallow/stylopharyngeus (CN IX), Arches 4-6 = swallow/speak (CN X).

**Derivatives** - Cartilage (Reichert cartilage): stapes, styloid process of temporal bone, lesser horn of hyoid, upper part of body of hyoid, and the stylohyoid ligament. (Mnemonic: "Stapes, Styloid, Stylohyoid ligament, Stylohyoid muscle, Seven") - Muscles: muscles of facial expression (orbicularis oculi/oris, buccinator, platysma, frontalis, etc.), stapedius, stylohyoid, posterior belly of digastric, auricular muscles. - Nerve: facial nerve (CN VII) – motor to the above muscles. Chorda tympani carries taste from the anterior 2/3 of the tongue (special visceral afferent) and parasympathetics to submandibular/sublingual glands. - Artery: hyoid and stapedial arteries (the stapedial artery normally regresses in humans).

**Embryology** - Arches consist of mesoderm/mesenchyme core, neural crest-derived cartilage, ectodermal covering (clefts), and endodermal lining (pouches). - The 2nd arch grows caudally over the 3rd and 4th arches and fuses with the epipericardial ridge in the lower neck, burying the 2nd–4th clefts in the temporary cervical sinus, which normally obliterates. - Pouch correlate: 2nd pouch forms the epithelial lining of the palatine tonsil crypts (the mesoderm contributes tonsillar lymphoid tissue).

**Clinical correlations** - Branchial (cervical) cleft cysts: most commonly arise from failure of obliteration of the 2nd cleft/cervical sinus. They present as a lateral neck mass anterior to the sternocleidomastoid, often along the anterior border; the tract can run between the internal and external carotid arteries toward the tonsillar fossa (a fistula opens externally at the SCM border and internally at the tonsillar area). - Facial nerve palsy: Bell palsy produces ipsilateral upper AND lower facial paralysis (LMN lesion), hyperacusis (stapedius paralysis), loss of taste in anterior 2/3 of tongue, and decreased lacrimation/salivation. Upper motor neuron lesion spares the forehead. - Reichert cartilage anomalies: stapes fixation/ossicular malformation causing conductive hearing loss; Klippel-Feil is not a 2nd arch disorder (cervical vertebral fusion). Congenital facial palsy can occur with Möbius syndrome (CN VI and VII). - Teratogens: retinoic acid and thalidomide can disrupt the 1st/2nd arch derivatives (ear and facial malformations, hearing loss). Hemifacial microsomia/Goldenhar syndrome (oculo-auriculo-vertebral spectrum) involves 1st and 2nd arch defects. - Treacher Collins syndrome is mainly a 1st arch disorder (TCOF1, neural crest migration failure), though 2nd arch structures (ear ossicles, external ear) can be involved.

**High-yield comparison** - 1st arch: Meckel cartilage → malleus, incus, sphenomandibular ligament; muscles of mastication, mylohyoid, ant. belly of digastric, tensor tympani, tensor veli palatini; CN V2/V3. - 2nd arch: as above, CN VII. - 3rd arch: greater horn and lower body of hyoid; stylopharyngeus; CN IX; common carotid and proximal ICA. - 4th–6th arches: laryngeal cartilages and pharyngeal/laryngeal muscles; CN X (superior laryngeal from 4th, recurrent laryngeal from 6th).