Thyroid Embryology Part 1

Thyroid Gland Embryology

The modern head and neck surgeon / endocrine surgeon should have a complete grasp of the embryogenesis of the thyroid gland as well as knowledge of the possible congenital anomalies arising out of this gland, as they may influence the completeness of the surgery as well as the complications arising from operations of the thyroid gland.

The thyroid gland is the first of the body’s endocrine glands to develop, appearing as an outpouching of the primitive foregut around the third week of gestation (roughly the 24th day).The thyroid gland forms as a proliferation (thickening) of the endodermal epithelial cells found on the median surface of the developing pharyngeal floor. The site of this development lies between two key structures, the tuberculum impar and the copula, and is recognized as the foramen cecum.

The foramen cecum located at the base of the tongue is the site of origin of the thyroid gland (medial thyroid anlage) at the junction between the first and second branchial (pharyngeal) pouches, immediately dorsal to the aortic sac.

The thyroid gland has a double embryologic origin:

• The primitive pharynx
• The neuroectodermal / neural crest.

The thyroid gland initially arises caudal to the tuberculum impar, which is also known as the median tongue bud. This embryonic swelling develops from the first branchial arch and occurs on the midline of the floor of the developing pharynx, eventually helping form the tongue as the two lateral lingual swellings overgrow it.

The foramen cecum begins rostral to the copula, also known as the hypobranchial (hypopharyngeal) eminence. This median embryologic swelling consists of mesoderm that arises from the second branchial pouch (although the third and fourth branchial pouches are also involved).

The thyroid gland, therefore, originates between the first and second branchial pouches.