Embryology of Parathyroid Glands

The parathyroid glands develop from the third and fourth pharyngeal pouches.2 The upper parathyroid glands originate from the dorsal tips of pharyngeal pouch IV.3 The ventral portion of pharyngeal pouch IV consists of the ultimobranchial body, which is incorporated into the lateral part of the developing thyroid and eventually supplies the parafollicular or C cells. The common embryologic origin of the lateral part of the thyroid and the upper parathyroids accounts for the occasional intrathyroidal location of upper parathyroids, although this is a rare observation.4

The lower parathyroid glands arise from the dorsal part of pharyngeal pouch III. The thymus, which originates from the ventral part of pharyngeal pouch III, and the lower parathyroid gland descend as a complex in a plane ventrally to pharyngeal pouch IV. Therefore, the lower parathyroids are usually found in a more anterior position than the upper parathyroids. At the caudal descent, the lower parathyroid usually dissociates from the thymus and is ultimately located near the lower pole of the thyroid. The caudal migration of the complex of the lower parathyroid and thymus can vary widely. In the case of absence of migration, the lower parathyroid gland is found cranially to the upper pole of the thyroid, mimicking a superior parathyroid. Thymic tissue surrounding the undescended parathyroid can clarify the true origin of the ectopic parathyroid gland. The absence of thymic tissue caudally to the thyroid is another indicator of an undescended lower parathyroid. When the lower parathyroid remains attached to the thymus during caudal migration, it becomes positioned in the anterosuperior mediastinum.

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