first pharyngeal arch

**Embryology:** Pharyngeal arches form during weeks 4-5. Each arch has a core of mesenchyme (neural crest plus mesoderm) and is covered externally by ectoderm and internally by endoderm. Each arch contains an artery, a cartilage rod, a nerve, and muscle. Mnemonic: ACNM (Aortic arch, Cartilage, Nerve, Muscle).

**Nerve:** CN V (trigeminal). The maxillary (V2) and mandibular (V3) divisions supply the arch. The ophthalmic division (V1) does not.

**Artery:** The first aortic arch artery becomes the maxillary artery (it also contributes to the external carotid).

**Cartilage (Meckel cartilage):** - Malleus and incus (the stapes comes from arch 2) - Sphenomandibular ligament - Anterior ligament of the malleus - The mandible forms by intramembranous ossification around Meckel cartilage. It is not formed from the cartilage itself.

**Skeletal derivatives of the maxillary prominence (intramembranous ossification):** Maxilla, zygomatic bone, palatine bone, vomer, and part of the temporal bone (squamous part).

**Muscles (all CN V3):** - Muscles of mastication: masseter, temporalis, medial pterygoid, lateral pterygoid - Mylohyoid - Anterior belly of the digastric - Tensor tympani - Tensor veli palatini

**Other:** The anterior two-thirds of the tongue gets its general sensation from CN V3 (lingual nerve). Taste comes from the chorda tympani (CN VII, arch 2). The tongue's mucosa develops from the first arch (lingual swellings and tuberculum impar).

**Clinical correlations:** - **Treacher Collins syndrome (mandibulofacial dysostosis):** Autosomal dominant. Usually due to a TCOF1 mutation, which causes failure of neural crest migration into the first arch. Findings: mandibular and malar hypoplasia, downslanting palpebral fissures, coloboma of the lower eyelid, conductive hearing loss (ossicle and external ear anomalies), and a normal intellect. - **Pierre Robin sequence:** Micrognathia causes posterior displacement of the tongue (glossoptosis). This causes a cleft palate and airway obstruction. It is associated with SOX9 abnormalities. - **Hemifacial microsomia / Goldenhar syndrome:** Unilateral first and second arch hypoplasia. - **Cleft lip:** Failure of the maxillary prominence to fuse with the medial nasal prominence (intermaxillary segment). It is usually unilateral. - **Cleft palate:** Failure of the lateral palatine processes (from the maxillary prominences) to fuse with the nasal septum and each other.

**High-yield comparison:** - Arch 1: CN V2/V3 (muscles of mastication) - Arch 2: CN VII (muscles of facial expression, stapes, styloid process, lesser horn and upper body of the hyoid) - Arch 3: CN IX (stylopharyngeus, greater horn and lower body of the hyoid) - Arch 4 and 6: CN X (superior laryngeal and recurrent laryngeal branches)

**Test tip:** If a muscle involved in chewing is affected, or a patient has mandibular hypoplasia with ear anomalies, think first arch and CN V.