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Polarised light micrograph of crystals of cholecalciferol, commonly known as vitamin D3. Vitamin D is present in dairy produce & fish oil. It is also produced by the body after exposure of the skin to sunlight. Deficiency of vitamin D produces rickets in children by interfering with calcium uptake during the formation of bone. In elderly persons the deficiency produces osteomalacia, or a softening of the bones. Maagnification X63 (at 35mm size). (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID PARKER.)