Polarised light micrograph of crystals of vitamin B12 (cyanocobalamin), a substance necessary for the synthesis of nucleic acids and the maintenance of myelin in the nervous system. A deficiency of vitamin B12 affects nearly all of the body tissues, particularly those containing rapidly dividing cells. The most serious effects of a deficiency are pernicious anaemia and degeneration of the nervous system. Vitamin B12 is manufactured only by certain microorganisms and is contained only in foods of animal origin. Good sources are liver, fish and eggs. Magnification: x61 at 35mm size.