Thyroid gland. Comparison between a scanning election micrograph (SEM left) and transmission electron micrograph (TEM right). of part of the thyroid. As seen in the TEM image the follicles are lined by a single layer of cuboidal epithelial cells (blue), which synthesise and secrete the iodine-containing hormones T3 (tri-iodothyronine) and T4 (thyroxine). The thyroid follicle is filled with a glycoprotein substance called thyroglobulin or thyroid colloid (green), which is a storage form of the thyroid hormones. When actively secreting hormones the lining cells are large and cuboidal, when less active the follicles are distended and the secretory cells flattened.In the SEM image the surface outline of numerour follicle cells (blue) are visible with green colloid seen in a few fractured follicles above and below. Magnification: SEM x1000, TEM x 3500 when printed 10cm high. For a series of comparisons between SEMs and TEMs see images C047/7006 to C047/7034.