Thyroid gland. Coloured scanning electron micrograph (SEM) of a fracture through the thyroid gland revealing several follicles (orange and green). Between the follicles is connective tissue (red). A follicle consists of a layer of epithelial cells (green) 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 (orange). Thyroid hormones regulate the body's metabolism. Magnification: x35 when printed at 10 centimetres wide.