THYROID GLAND

THYROID GLAND

The parenchyma of the thyroid gland (glandula thyroidea) has an unmistakable microscopic structure, as it consists of follicles filled with colloid. The spherical enclosed follicles reach a diameter of 50-200 μm, so that they are clearly visible in the microscope at low magnification. The colloid appears as an amorphous substance which is PAS reactive. The surface of the thyroid gland is covered by a capsule of irregular dense connective tissue from which the septa emanate and divide the parenchyma into lobules.

The follicle wall is composed of a single layer of follicular cells that sit on the basement membrane. The height of the follicular cells reflects their functional state; cuboidal cells are most commonly found, but with increased function the cells elongate into columnar cells, while a hypofunctional gland is characterized by flattened follicular cells. With the change in the height of the glandular epithelium, the amount of colloid changes (hyperfunction is characterized by small follicles that contain less colloid, whereas hypofunction is reflected by a high amount of colloid and larger follicles). The cytoplasm of follicular cells has a richly developed rough endoplasmic reticulum and therefore appears basophilic. The Golgi apparatus, mitochondria, lysosomes and numerous vesicles can also be found in the cytoplasm. The basolateral plasmalemma of follicular cells contains a membrane protein, called pendrin, which functions as an ion carrier and allows the uptake of iodine anions (iodides). On the apical surface, these secretory cells produce the glycoprotein thyroglobulin, which is stored inside of the follicle. The follicular cells also resorb the thyroglobulin through the apical surface to convert it into the active thyroid hormones, triiodothyronine and tetraiodothyronine (thyroxine), which they transport into the blood on the basal surface into the adjacent network of fenestrated blood capillaries that surround each follicle externally.

The synthesis of thyroglobulin, thyroid hormones and their function is described in detail in the electronic course for the lecture Endocrine System II.

The second population of epithelial cells found in the thyroid gland are the parafollicular cells (C cells). These cells are not part of the follicles but are most often found in clusters between them; when included in the follicles, they never reach its lumen. These cells are larger than follicular cells and their cytoplasm is lighter due to the lower content of rough endoplasmic reticulum. Electronmicroscopically, the cisterns of the Golgi apparatus and numerous secretory vesicles can be detected here. These cells also show endocrine activity, as they produce and release the polypeptide hormone calcitonin, which is important for the regulation of calcium and phosphorus metabolism.

The function is explained in more detail in the e-course Endocrine System II.
Both cell populations of the glandular epithelium have different origins. Follicular cells are of endodermal origin while parafollicular cells are derived from the neural crest.