third pharyngeal pouch

**Embryology** - Pharyngeal pouches are endoderm-lined outpouchings between the pharyngeal arches. Clefts are ectoderm and arches are mesoderm/neural crest. - The third pouch has two parts: - Dorsal wings: inferior parathyroid glands (chief cells make PTH, oxyphil cells). - Ventral wings: thymus (thymic epithelial reticular cells, Hassall corpuscles). The thymus migrates caudally and medially to the anterior mediastinum. - The inferior parathyroids come from the third pouch and the superior parathyroids from the fourth pouch. The inferior glands migrate with the thymus past the superior glands, so they end up lower. This is a common board trap. - Thymic lymphocytes come from bone marrow-derived hematopoietic progenitors that seed the thymus. The thymic epithelium is endodermal (3rd pouch) with some ectodermal (3rd cleft) contribution.

**Pouch review (mnemonic: "Ear, Tonsils, Bottom-to-top: 1-2-3-4")** - 1st pouch: middle ear cavity, eustachian tube, mastoid air cells (tympanic membrane forms from the 1st membrane). - 2nd pouch: epithelial lining of the palatine tonsil. - 3rd pouch: inferior parathyroids (dorsal) and thymus (ventral). - 4th pouch: superior parathyroids. Also the ultimobranchial body, which gives parafollicular C cells of the thyroid (the neural crest origin is traditional teaching, but newer data support endodermal contribution).

**Clinical significance: DiGeorge syndrome (22q11.2 deletion; "CATCH-22")** - Defective development of the 3rd and 4th pouches, with a neural crest migration defect (TBX1 gene implicated). - Features: Cardiac defects (truncus arteriosus, tetralogy of Fallot, interrupted aortic arch), Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia. - Thymic aplasia causes absent or low T cells with recurrent viral and fungal infections (e.g., Candida, PJP). There is no thymic shadow on CXR. Flow cytometry shows low CD3+ T cells. B cells are usually present, and antibody levels are often normal or variable. - Absent or hypoplastic parathyroids cause low PTH and hypocalcemia, which can present as neonatal tetany or seizures, with a prolonged QT interval, and hyperphosphatemia. - Management: calcium and vitamin D for hypocalcemia, and cardiac surgery as needed. Thymic transplant or HSCT may be used for complete DiGeorge. Give irradiated, CMV-negative blood products and avoid live vaccines in T-cell deficiency.

**Other associations** - Ectopic or undescended inferior parathyroid tissue (in the thymus or anterior mediastinum) is a classic cause of persistent hyperparathyroidism after neck exploration. Remember the inferior glands have a more variable location than the superior glands. - Thymic cysts or ectopic thymic tissue along the path of descent (angle of the mandible to the mediastinum) are derived from the 3rd pouch. - Thymoma is not an embryologic malformation, but it arises from thymic epithelium (associated with myasthenia gravis, pure red cell aplasia, hypogammaglobulinemia/Good syndrome).

**Differential for T-cell and pouch disorders** - DiGeorge: low T cells and low Ca2+. - SCID: low T and B cells with a thymic shadow that is absent or small, but without hypocalcemia or the characteristic cardiac/facial anomalies. - Chronic mucocutaneous candidiasis: T-cell dysfunction against Candida without parathyroid failure. - Not to be confused with a 22q11 variant with isolated 4th pouch involvement or with the 3rd and 4th pouch fistulas (a 4th pouch sinus tract is associated with acute suppurative thyroiditis in the left lobe).