Thyroid gland. Coloured scanning electron micrograph (SEM) of a fracture through the thyroid gland revealing several follicles. Between the follicles is connective tissue. A follicle consists of a layer of epithelial cells around a central storage chamber. The cells produce the thyroid hormones tri-iodothyronin (T3) and thyroxine (T4) and secrete them into the central chamber where they are stored as the glycoprotein thyroid colloid (purple). Thyroid hormones regulate the body's metabolism. Magnification: x80 when printed at 10 centimetres wide