The field of cellular gerontology, studying aging at the cellular level, was pioneered by Leonard Hayflick. Almost 50 years ago, he discovered that cultured normal human cells have a limited capacity to divide, after which they become senescent. The most convincing evidence that normal human cells have a finite lifespan came from several elegant experiments that he performed while working with Paul Moorhead (Hayflick, 1965; Hayflick and Moorhead, 1961). While culturing primary human fibroblasts, they observed and described three phases of life for a cell population: Phase I—the primary culture, Phase II—months of copious cell growth and division, and Phase III—the period when cell replication diminished and ultimately stopped (Hayflick andMoorhead, 1961). To support the idea that normal human cells have a limited number of divisions, Hayflick and Moorhead cultured three separate populations of human fibroblasts simultaneously: one derived from a male at population doubling (PD) 14, one derived from a female at PD 10, and one that contained an equal number of male (PD 14) and female (PD 10) fibroblasts. When the older unmixed male-derived cell population stopped dividing, they investigated the mixed population and discovered that only female cells were present (Hayflick andMoorhead, 1961). Besides providing additional evidence that cells have a limited lifespan, this experiment demonstrated that the older cells
