Surgical Anatomy of the Thyroid and Parathyroid Glands

EMBRYOLOGY OF THE THYROID GLAND During the fourth week of development, the foramen cecum develops as an endodermal thickening in the floor of the primitive pharynx at the junction between the first and second pharyngeal pouches, immediately dorsal to the aortic sac. The medial thyroid primordium derives as a ventral diverticulum at the foramen cecum. During the fourth to seventh week of gestation, this primitive thyroid tissue penetrates the underlying mesenchymal tissue and descends anterior to the hyoid bone and the laryngeal cartilages to reach its final adult pretracheal position. During its descent, it is first spherical, and then enlarges and becomes bilobed as it grows caudally. The proximal portion of the diverticulum (connecting the gland and the foramen cecum) retracts and forms a solid fibrous stalk early in the fifth week. This thyroglossal duct ultimately atrophies, but any portion of it may persist to become the site of a thyroglossal duct cyst. The distal portion of this duct gives rise to the pyramidal lobe and levator superioris thyroideae in adults. 1 The lateral thyroid primordia (from the fourth and fifth pharyngeal pouches) descend to join the central component during the fifth week of gestation. Calcitonin-secreting parafollicular C cells arise within the ultimobranchial bodies (recognized within the lateral thyroid primordia) from neural crest cells of the fourth pharyngeal pouch. They fuse to the medial thyroid anlage during the fifth week of gestation. These cells are therefore restricted to a zone deep within the middle to upper third of the lateral lobes. The thyroid primordium initially consists of a solid mass of endodermal cells, which later break up into a network of cords with the invasion of the surrounding mesenchyme. 2 The epithelial cords organize into clusters of cells with a central lumen.