Which pharyngeal pouch forms the ultimobranchial body (parafollicular cells)?

• A. Third pharyngeal pouch
• B. Fourth pharyngeal pouch
• C. Second pharyngeal pouch
• D. First pharyngeal pouch

Answer: (B)

The idea here is that parafollicular cells (the C cells) of the thyroid arise from neural crest cells that migrate into the ultimobranchial body, which itself comes from the ventral part of the fourth pharyngeal pouch. This pouch-specific origin explains why these calcitonin-secreting cells are associated with the thyroid, even though their early lineage is not endodermal thyroid tissue. The fourth pharyngeal pouch also gives rise to structures like the superior parathyroid glands, but its distinctive contribution to the ultimobranchial body is what leads to the parafollicular cells. By contrast, the third pouch forms the inferior parathyroids and thymus, the second pouch contributes to the epithelium of the palatine tonsil, and the first pouch forms parts of the middle ear and auditory tube. Therefore, the pouch that forms the ultimobranchial body and its parafollicular cells is the fourth pharyngeal pouch.