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Insulin. Polarised light micrograph (PLM) of crystals of the hormone insulin. The crystals appear hexagonal in shape. The insulin molecule is made up of two chains of amino acids (A & B chains), which are held together by two sulphur bonds. Insulin is a hormone produced in the pancreas by cells of the islets of Langerhans. Its function is to regulate blood sugar levels. Insufficient production of insulin leads to an accumulation of glucose in the blood, causing the condition diabetes mellitus. Interference contrast. Magnification: x117 at 6x7cm size. Magnification: x200. (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA/SCIENCE PHOTO LIBRARY)