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To find out, Hayflick harvested cells from fetal tissue and transferred them to a Petri dish. Freed from the responsibility of doing anything to keep a larger organism alive, the cells did the only other thing they knew how to do: divide. Shortly after they were placed in culture, they doubled their number. Then they doubled the doubling. The cycle repeated itself about 100 times, until all at once it stopped. From then on, the cells did something a lot like aging. They consumed less food; their membranes deteriorated; and the culture as a whole languished. Hayflick repeated the experiment, but this time used cells from a 70-year-old, and found that the cellular aging began a lot earlier, after 20 or 30 doublings. Clearly, it seemed, the cells from the older human were older themselves.
"What we were seeing," says Hayflick, now a professor at the University of California, San Francisco, "was the concept of cellular aging: growing old in the microcosm of a Petri dish."
For gerontologists, this was monumental stuff. If human tissue behaved in the body the same way it did in the dish, they felt, it meant that somewhere in the nanoviscera of each cell there was an actuarial hourglass that gave it only so much time to live and no more. If the clock could be found--and, more important, reset--both the cells and the larger corpus that gave rise to them might be made immortal. Of course, hypothesizing the existence of such a cellular timekeeper was one thing; finding it and manipulating it were something else again. In the years since, senescence scientists have taken two approaches to achieving this goal.
The first idea researchers have explored is broadly thought of as the cellular-damage model of aging. For any complex system--whether it's made of inorganic metal or protoplasmic goo--the mere act of doing the work it was designed to do carries a price. No sooner does the hardware begin operating than its parts begin wearing out and its journey to the junkyard begins. Cells are not spared this fate, and one of the functions that takes the most out of them is the job of processing food.
Like all organisms, cells produce waste as they metabolize energy. One of the most troublesome by-products of this process is a species of oxygen molecule known as a free radical--essentially an ordinary molecule with an extra electron. This addition creates an electrical imbalance that the molecule seeks to rectify by careening about, trying to bond with other molecules or structures, including DNA. A lifetime of this can lead to a lot of damaged cells, which may lead to a range of disorders, including cancer and the more generalized symptoms of aging like wrinkles and arthritis.