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Fans of the television series Star Trek: The Next Generation may remember an episode entitled "Up the Long the Ladder," in which the star ship Enterprise receives a distress signal from a planet called Mariposa. On this colony all the inhabitants are clones of the survivors of the crashed ship that brought its first settlers, and recent clones had failed to develop to maturity due to a phenomenon called "replicative fading." The clones surreptitiously collected DNA from Commander Riker and other members of the crew, but the officers of the Enterprise finally insisted that the colony replenish its population the old fashioned way, by sex.
It turns out that this episode of Star Trek reflected science that actually had been worked out about 30 years earlier by a researcher named Leonard Hayflick.
Dr. Hayflick's Chicken Cells
In the early 1960's, Dr. Leonard Hayflick was a young professor of anatomy at the University of Pennsylvania. (He is as of the writing of this article still alive, and an emeritus professor at the University of California at San Francisco.) In the early 1960's, it was an accepted scientific principle that organisms, for example, people and animals and plants, die, but cells live forever.
A Nobel Laureate named Alexis Carrell had proved the theory of cellular immortality, or at least so most scientists thought.
Carrell continued to feed chicken cells to his chicken cells and had live chicken cells in the flask for 20 years.
Hayflick, however, noticed that his own experimental culture of chicken cells in fact was not immortal. The cells could divide to continue themselves, but only 40 to 60 times, when they became "senescent," or old. Old chicken cells could live on for a while, but they could not undergo the process of mitosis to produce new chicken cells. Hayflick found that cells actually aren't immortal, that they reach something called a replicative limit, and they die.
The principle also applied to human cells, Hayflick and collaborators determined in the late 1970's. And this finding had profound implications for the science of life extension.
The Hayflick Limit
From time to time human cells renew themselves by a process called mitosis. The coils of DNA in the center of the cell straighten out so they can copy themselves. Then they make copies of themselves so one cell can become two. The older copy of the cell then can die and be disposed of by the immune system, while the new cell can take its place in the tissue. As long as this process happens on a regular basis, cells and tissues and organs are rejuvenated.
Somewhere between 40 and 60 replications, however, when a cell is between 80 and 120 years old, it simply cannot replicate itself any more. It does not immediately shut down, but it may become quiescent, just "there," rather than performing its vital role in tissue. When this happens, tissues and organs and people malfunction.