Embryology, Parathyroid

Excerpt

The parathyroid glands are a group of endocrine glands located on the posterior thyroid. These glands are responsible for the secretion of parathyroid hormone (PTH), and play a role in calcium homeostasis. Typically, there are four glands divided into pairs - the superior parathyroid glands and the inferior parathyroid glands. The superior parathyroids generally are located superior to the inferior thyroid artery and posterolaterally to the recurrent laryngeal nerve, bilaterally. The inferior parathyroids are most commonly near the inferior pole of the thyroid and anteromedially to the recurrent laryngeal nerve, bilaterally. Branches of the inferior thyroid artery deliver arterial blood supply. Venous drainage is via the thyroid veins (superior, middle, inferior). The number and location of the parathyroid glands can be variable, with patients presenting with anywhere from three to eight parathyroid glands. Ectopic glands generally appear along the path of embryologic descent, within the carotid sheath, inside the thyroid gland, or within the superior mediastinum.

Embryologically, the parathyroid glands derive from the endoderm of the third and fourth pharyngeal pouches. The third pharyngeal pouch gives rise to the inferior parathyroid glands, while the superior parathyroids arise from the fourth pharyngeal pouch. Due to their relatively long course of descent, the final location of the inferior glands is more variable.