Skin sags. Hair grays. Organs don't work quite like they used to. A gradual wearing out and running down of the body's tissues seems an inherent part of growing older. Rejuvenation of skin, muscles, and other body parts naturally declines with the passing years. Scientifically speaking, however, this observation is much less self-evident. Some cells in a person's body can resist the tide of aging. Consider the reproductive cells a person carries that will become the cells of newborn children who have 80-plus years of life to look forward to. Generation after generation, these reproductive cells form an unbroken line stretching for millennia.