The thyroid gland originates from an invagination of which germ layer at which site?

• A. Endoderm invagination at the foramen cecum of the tongue
• B. Neural crest migration to the tongue
• C. Mesodermal invagination from the tongue base
• D. Ectoderm invagination at the stomodeum

Answer: (A)

The thyroid gland begins as an endodermal invagination from the floor of the primitive pharynx, specifically at the foramen cecum on the tongue. This endodermal diverticulum forms the thyroglossal duct and then migrates downward in the midline to reach its final position in front of the trachea. The foramen cecum marks the original site of this thyroid primordium, which is why a lingual thyroid can occur if descent is incomplete. While neural crest cells contribute later to parafollicular C cells (via the ultimobranchial body), they are not the source of the thyroid’s initial tissue; and invaginations from mesoderm or ectoderm at the stomodeum do not give rise to the thyroid.