Aging cells (third of 3 images). Light micrograph of a culture of cells which have been genetically- engineered to age rapidly. The cells seen here are dead and dying, and have not produced new cells to replace them. 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 G200/067 for aging sequence).