Aging cells (second of 3 images). Light micrograph of a culture of cells which have been genetically- engineered to age rapidly. The cells are seen here in a late phase of their life, and have stopped dividing and producing new cells. The reasons behind aging are complex, and include genetic, physiological and environmental factors. One of the best known causes of aging is the Hayflick effect, in which certain types of cells are genetically programmed to divide only a limited number of times before they die. Magnification: x400 at 35mm size. (See photo's G200/066 and G200/068 for aging sequence to death).