A thyroid follicle is a roughly spherical cystlike 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. 21.13). 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 pro duction 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) prepa rations, follicular cells exhibit a slightly basophilic basal cy toplasm with spherical nuclei containing one or more
FIGURE 21.13 • Thyroid gland. This photomicrograph of a human thyroid is from a section stained with H&E. It shows the colloid-containing follicles of the gland. Each follicle consists of a single layer of epithelial cells surrounding a central mass of colloid. The arrows indicate some of the blood capillaries between the follicles. 500.
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 cells (Fig. 21.14), including typical junctional complexes at the apical end of the cell and short microvilli on the apical cell surface. Numerous profiles of rough-surfaced endoplasmic reticulum (rER) are present in the basal region. Small vesicles present in the apical cytoplasm are morphologically similar to vesicles associated with the Golgi apparatus. Abundant endocytotic vesicles, identified as colloidal resorption droplets, and lysosomes are also present in the apical cytoplasm.
 Parafollicular cells (C cells) are located in the periphery of the follicular epithelium and lie within the follicle basal lamina. These cells have no exposure to the follicle lumen. They secrete calcitonin, a hormone that regulates calcium metabolism. In routine H&E preparations, C cells are pale staining and occur as solitary cells or small clusters of cells. Human parafollicular cells are difficult to identify with light microscopy. At the electron microscope level, the parafollicular cells reveal numerous small secretory vesicles, which range in diameter from 60 to 550 nm, and a prominent Golgi apparatus (Fig. 21.15).
FIGURE 21.14 • Electron micrograph of follicular cells in rat thyroid gland. This electron micrograph shows a single layer of epithelium containing low columnar follicular cells. The apical surfaces with visible microvilli (Mv) are in contact with the colloid, whereas basal surfaces of follicular cells rest on the basal lamina (FBL). A narrow extracellular connective tissue space separates the follicular cells from the lumen of the capillary. Note that the fenestrated endothelial cells (En) lining capillary lumen rest on the basal lamina (EBL). Accumulation of lysosomes (L) and colloid resorption droplets (CRD), extensive Golgi apparatus (G), rough endoplasmic reticulum (rER), and presence of enlarged intercellular spaces are indicative of intensive activity of follicular cells. N, nucleus; JC, junctional complex. 14,000. (Courtesy of Dr. Holger Jastrow).