third pharyngeal arch

**Overview:** Pharyngeal (branchial) arches are mesenchymal (mesoderm and neural crest) swellings that form in weeks 4-5. Each arch has a cartilage rod, a muscular component, a cranial nerve, and an aortic arch artery. Arches are separated externally by clefts (ectoderm) and internally by pouches (endoderm).

**Third arch derivatives:** - **Nerve:** CN IX (glossopharyngeal). - **Muscle:** Stylopharyngeus only. It elevates the pharynx and larynx during swallowing and speech. It is the only muscle supplied by CN IX. - **Cartilage (Reichert's is arch 2):** The third arch cartilage forms the greater horn and the lower part of the body of the hyoid bone. - **Artery (aortic arch 3):** The common carotid arteries and the proximal part of the internal carotid arteries. The distal internal carotid comes from the dorsal aorta. The external carotid sprouts from the aortic sac. - **Third pharyngeal pouch:** Dorsal wings form the **inferior parathyroid glands**, and ventral wings form the **thymus**. These migrate caudally, and the inferior parathyroids end up inferior to the superior ones, which come from pouch 4. Mnemonic: "3rd pouch = 3 syllables... thymus and inferior parathyroids" (the inferior glands come from the third pouch, "3 below 4").

**Clinical correlations:** - **DiGeorge (22q11.2 deletion) syndrome:** Abnormal development of pouches 3 and 4 gives thymic aplasia (T-cell deficiency with recurrent viral and fungal infections) and hypocalcemia from parathyroid hypoplasia (tetany, seizures). Related findings are cardiac outflow defects (truncus arteriosus, tetralogy of Fallot) and abnormal facies (CATCH-22). Patients may show deficits tied to pouches 3 and 4 and to neural crest. - **Ectopic parathyroid or thymic tissue:** The inferior parathyroids have a long migratory path, so they are more variable in location (for example, in the anterior mediastinum or within the thymus). This matters in parathyroid surgery. - **CN IX lesion:** Loss of the gag reflex afferent limb (the efferent limb is CN X), loss of taste from the posterior one-third of the tongue, and impaired stylopharyngeus function. Glossopharyngeal neuralgia gives lancinating throat pain triggered by swallowing. - **Carotid sinus reflex:** CN IX carries afferent signals from the carotid sinus baroreceptors and carotid body chemoreceptors, which ties to the arch 3 artery.

**Arch summary for boards:** - Arch 1: CN V2/V3, muscles of mastication, Meckel's cartilage (malleus, incus), maxillary artery. Treacher Collins is a first-arch defect. - Arch 2: CN VII, muscles of facial expression, stapes, styloid process, lesser horn of hyoid (Reichert's cartilage), stapedial artery. - Arch 3: CN IX, stylopharyngeus, greater horn of hyoid, common carotid and proximal internal carotid. - Arch 4: CN X (superior laryngeal branch), most pharyngeal constrictors, cricothyroid, levator veli palatini, thyroid and epiglottic cartilages, left aortic arch and right subclavian (proximal). - Arches 6: CN X (recurrent laryngeal), intrinsic laryngeal muscles (except cricothyroid), pulmonary arteries and ductus arteriosus.

**Key point:** Arch 3 is typically tested by the pairing "stylopharyngeus = CN IX," along with "3rd pouch = thymus and inferior parathyroids" (DiGeorge).