JOHN BURBIDGE / SCIENCE PHOTO LIBRARY JOHN BURBIDGE / SCIENCE PHOTO LIBRARY
Light micrograph of a section of a human thyroid gland showing the functional units, the thyroid follicles. These units are lined by a single layer of epithelial cells (blue edges round orange areas), which synthesize & secrete the iodine- containing hormones T3 & T4. The thyroid follicle is filled with a glycoprotein substance called thyroglobin or thyroid colloid, which is a storage form of the thyroid hormones (orange). When actively secreting hormones the lining cells are large & cuboidal, & the colloid is diminished. When less active (here) the follicles are distended & the secretory cells flattened. Magnification: x18 at 6x6cm size.
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