A 40-year-old woman undergoing neck surgery for hyperparathyroidism has all four glands identified in their usual positions, yet serum calcium remains elevated. Imaging reveals ectopic parathyroid tissue in the superior mediastinum near the thymus. Embryologically, this ectopic gland is best explained by:
- A Failure of migration of the superior parathyroid from the fourth pouch
- B Descent of the inferior parathyroid along with the thymus from the third pouch ✓
- C Persistence of the ultimopharyngeal body from the fifth pouch
- D Abnormal migration of neural crest cells into the third arch
Correct answer: B. Descent of the inferior parathyroid along with the thymus from the third pouch
Explanation
The inferior parathyroids develop from the third pharyngeal pouch together with the thymus. As the thymus descends into the superior mediastinum, the inferior parathyroid may be dragged with it, producing mediastinal ectopic parathyroid tissue. This paradoxical relationship, where the inferior gland arises above the superior one, is a classic examination point. The superior parathyroids from the fourth pouch remain close to the thyroid and rarely migrate into the chest.
Reference: Langman's Medical Embryology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.