Chapter 52. Branchial Arch Derivatives: The Thyroid Gland - Review of Medical Embryology Book

Introduction: the thyroid gland begins its development by about day 24 (in week 4) from a median entodermal thickening in the floor of the primitive pharynx just caudal to the future site of the tuberculum impar

THE THICKENING forms a downgrowth, the thyroid diverticulum, which grows into the underlying mesoderm, and as the embryo elongates and the tongue grows, the diverticulum descends in front of the neck and pharyngeal gut

The diverticulum is connected to the tongue by a narrow canal, the thyroglossal duct, which opens in the tongue via the foramen cecum, which persists as a vestigial pit on the tongue

THE DIVERTICULUM grows rapidly and forms 2 lobes and by week 7 of embryonic development, it reaches its final position anterior to the trachea, having acquired a small median isthmus and 2 lateral lobes. By then, the thyroglossal duct usually has disappeared

A pyramidal lobe, extending from the isthmus, is seen in about 50% of thyroid glands and is derived from the thyroglossal duct

THE THYROID GLAND begins to function at about the end of month 3, at which time, the first follicles containing colloid can be seen
AT FIRST, THE THYROID PRIMORDIUM is made up of a solid mass of entodermal cells

It later breaks up into a network of epithelial cords or plates by invasion of the surrounding mesenchyme
By week 10, the cords have divided into small cellular groups, and a lumen forms in each cellular cluster. The cells then arrange themselves in a single layer around the lumen
During week 11, colloid is seen in these follicle structures, and even thyroxine can be demonstrated

The normal remains of the thyroglossal duct are the vestigial foramen cecum (of the tongue) and the functional pyramidal lobe of the thyroid gland

Cysts can form anywhere along the course of the developing thyroglossal duct during descent of the developing thyroid gland from the tongue
Remnants of the duct may persist and give rise to cysts in the tongue or in the midline of the neck, usually below the hyoid bone
In about 30% of cases, an opening through the skin is found as a result of perforation following infection of a cyst, and this forms the so-called thyroglossal duct sinus, which usually opens in the midline of the neck in front of the laryngeal cartilages

Very rarely the thyroid fails to descend from the tongue area resulting in a lingual thyroid
Incomplete descent, which is rare, may result in a cervical thyroid that is seen in the neck at or just below the hyoid bone
Accessory thyroid tissue often is fully functional, originates from remnants of the thyroglossal duct, thus can be found anywhere from the level of the tongue to where the thyroid gland comes to rest in the neck