Thyroglossal Duct Cysts and Ectopic Thyroid Tissue

Embryology

The median anlage of the thyroid originates from the endodermal segment in the floor of the primitive pharynx at the foramen cecum located in the midline at the junction of the anterior two-thirds of the tongue (first branchial arch derivative) and posterior one-third (third branchial arch derivative) (see Chapter 2 , Applied Embryology of the Thyroid and Parathyroid Glands). Between 5 and 7 weeks of gestation, this central element of what will become the thyroid gland migrates caudally from the foramen cecum to its normal position below the thyroid cartilage. The path of descent is closely associated with the hyoid bone and is usually anterior to it but can also be seen posteriorly or within the bone. The lateral thyroid anlage is derived from the ultimobranchial body, a descending diverticulum of the fourth to fifth pharyngeal pouch. The ultimobranchial body ultimately derives from the neural crest and brings C cells to the thyroid gland. Between 7 and 10 weeks of gestation, the thyroglossal epithelial tract obliterates. Failure of the thyroglossal duct tract to obliterate can result in formation of thyroglossal duct cysts (TGDCs).