Thyroid gland

The thyroid gland is located in the anterior neck region adjacent to the larynx and trachea

The thyroid gland is a bilobate endocrine gland located in the anterior neck region and consists of two large lateral lobes connected by an isthmus, a thin band of thyroid tissue. The two lobes, each ~5 cm in length, 2.5 cm in width, and 20 to 30 g in weight, lie on either side of the larynx and upper trachea. The isthmus crosses anterior to the upper part of the trachea. A pyramidal lobe often extends upward from the isthmus. A thin connective tissue capsule surrounds the gland. It sends trabeculae into the parenchyma that partially outline irregular lobes and lobules. Secretory follicles constitute the functional units of the gland.

The thyroid gland develops from the endodermal lining of the floor of the primitive pharynx

The thyroid gland begins to develop during the fourth week of gestation from a primordium originating as an endodermal thickening of the floor of the primitive pharynx. The primordium grows caudally and forms a duct-like invagination known as the thyroglossal duct. The thyroglos-sal duct descends through the tissue of the neck to its final destination in front of the trachea, where it divides into two lobes. During this downward migration, the thyroglossal duct undergoes atrophy, leaving an embryologic remnant, the pyramidal lobe of the thyroid, which is present in about 40% of the population. About the ninth week of gestation, endodermal cells differentiate into plates of follicular cells that become arranged into follicles. By week 14, well-developed follicles lined by the follicular cells contain colloid in their lumen. During week 7, epithelial cells lining the invagination of the fourth branchial pouches (sometimes called the fifth branchial pouches) known as the ultimobranchial bodies start their migration toward the developing thyroid gland and become incorporated into the lateral lobes. After fusing with the thyroid, ultimobranchial body cells disperse among the follicles, giving rise to parafollicular cells that become incorporated into the follicular epithelium.

The thyroid follicle is the structural unit of the thyroid gland

A thyroid follicle is a roughly spherical cyst-like compartment with a wall formed by a simple cuboidal or low columnar epithelium, the follicular epithelium. Hundreds of thousands of follicles that vary in diameter from about 0.2 to 1.0 mm constitute nearly the entire mass of the human thyroid gland. The follicles contain a gel-like mass called colloid (Fig. 20.12). The apical surfaces of the follicular cells are in contact with the colloid, and the basal surfaces rest on a typical basal lamina.

Follicular epithelium contains two types of cells: follicular and parafollicular cells

The parenchyma of the thyroid gland is composed of epithelium containing two types of cells:

• Follicular cells (principal cells) are responsible for production of the thyroid hormones T4 and T3. These cells vary in shape and size according to the functional state of the gland. In routine hematoxylin and eosin (H&E) preparations follicular cells exhibit a slightly basophilic basal cytoplasm with spherical nuclei containing one or more prominent nucleoli. The Golgi apparatus has a supranuclear position. Lipid droplets and PAS-positive droplets can be identified with appropriate staining. At the ultrastructural level the follicle cells reveal organelles commonly associated with both secretory and absorptive k k

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