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Vitamin B12. Polarised light micrograph of crystals of Vitamin B12 (cyanocobalamin). This is one of the water-soluble B-complex vitamins, and is necessary for the synthesis of nucleic acids and the maintainance of myelin in the nervous system. Vitamin B12 deficiency affects almost all body tissues. The most serious effects of this are pernicious anaemia and a degeneration of the nervous system. Vitamin B12 is contained only in foods of animal origin, especially liver, fish and eggs. It is also produced by the growth of certain micro-organisms. Recommended adult daily intake is 3-4 microgrammes. Magnification: x180 when printed 10cm wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/CLAUDE NURIDSANY & MARIE PERENNOU)