The Hayflick limit 
In the 1970s the American cellular biologist Leonard Hayflick discovered that most types of human cells have a natural limit to the number of times they can divide, or reproduce. 
Some types of cells, such as those that produce red and white blood corpuscles, can divide millions of times. Others, such as most nerve cells, do not reproduce at all. If a cell's Hayflick limit is 50, for example, it will divide 50 times and then become senescent. It withers and dies.  When enough of our cells die, we die. 

Immortal cells 
Some cells have no Hayflick limit. Barring trauma from outside, they are immortal. They can be killed, but they do not age.  The "lowly" bacteria are immortal. They can be killed -- by starvation, radiation, lack of water, or being eaten by another organism. But they do not age. Bacteria keep on dividing forever, until some outside agency kills them. 
Cancer cells are similarly immortal. They keep on dividing and dividing, endlessly, unless they are killed or their host dies. "HeLa" cells, taken from the tumor of Henrietta Lacks in 1951, are still reproducing as vigorously as they did nearly 50 years ago. 
Human germline cells -- ova and sperm cells -- also show no Hayflick limit. 
Why can some cells keep on going and going like the pink bunny in the TV commercials, while others curl up and die after a certain number of divisions? 

Telomeres 
Some biogerentologists (scientists who study aging) believe the answer lies in our telomeres. 
Inside the nucleus of virtually every cell in your body are the long strands or filaments called chromosomes. Human cells have forty-six chromosomes, except for the sex cells, which have half that number. The chromosomes contain DNA. DNA makes up the cell's genes. 
At the tip of each spindle-shaped chromosome is a sort of cap, called a telomere. Telomeres somewhat resemble the aglets on the ends of shoelaces. The telomeres keep the ends of the chromosomes from sticking together, and from sticking onto other chromosomes. Bacterial DNA does not have telomere caps, and tends to loop itself into a ragged circle, like a snake swallowing its tail. Telomeres keep the individual strands of DNA in our cells from looping or connecting to one another. They also play an intriguing role in cellular aging. Some researchers believe that telomeres are a sort of cellular clock that sets the rate at which the cells age and eventually die. Each time a cell divides, its telomeres shorten. When the telomeres become short enough, cell division stops and the cell soon dies. But cancer cells regrow their telomeres after every division.  Michael Fossel, professor of clinical medicine at Michigan State University, says quite clearly, "Telomeres [are] the clocks of aging." He and other researchers believe that telomere shortening is responsible for cellular aging and, eventually, cellular death.  Most biologists do not accept so simple an explanation. And yet... 

Telomerase 
In January 1998 researchers announced that they had extended the lifespan of human cells "indefinitely" in a laboratory experiment in which telomerase was added to the cells. Telomerase is the enzyme that essentially builds new telomeres. 
Cancer cells produce plentiful telomerase. Normal human cells do not -- even though they have the telomerase gene in their DNA. In normal human cells, that gene is suppressed, deactivated.  The researchers, from Geron Corporation and the University of Texas Southwest Medical Center, inserted an activated telomerase gene into the cells. The cells reproduced well past their Hayflick limits, giving powerful evidence that telomeres have a decisive influence on cellular senescence and may indeed be "the clock of aging." 
Writing in the prestigious journal Science, biologist Titia de Lange, of Rockefeller University's Laboratory for Cell Biology and Genetics, commented, "The doubt [about telomeric influence on aging] has now come to an end with a report...describing direct evidence for a causal relation between telomere shortening and cellular senescence." 

Telomerase and Cancer 
Many researchers are interested in finding how to prevent cancer cells from producing telomerase. If a telomerase "off" switch could be found, it will become possible to stop tumors before they grow large enough to be trouble. 
The fact that normal cells possess the telomerase gene but do not employ it may be a warning signal. Activate that gene and the cell may start runaway cancerous growth. 
The goal, then, is to control telomerase production well enough to remove the cell's Hayflick limit and allow the cell's owner -- maybe you! -- to live forever. Without causing cancer. 

MORF4  
Meanwhile, researchers at Baylor College of Medicine in Houston have cloned a gene that makes some types of human cells live more than twice as long as they would normally. 
It may also cure cancer. 
When added to cancerous cells, the MORF4 gene (Mortality Factor from human chromosome number 4) stops the cells from reproducing. The tumor stops growing and becomes senescent. A mutated form of MORF4, when added to normal human cells, allows them to keep on reproducing far beyond the normal limits of their lifespans. 
Another road to immortality? Don't bet against it. 

Human growth hormone  
One protein that seems to have a measurable effect in extending lifespan is human growth hormone (hGH), also known as somatotropin. 
Human growth hormone is secreted by the pituitary, a tiny gland located at the base of the brain, not far from the pineal. The pituitary is known as the master of the endocrine system, because it regulates the secretion of hor-mones by the body's other ductless glands, such as the thyroid, adrenals, gonads, etc., which, in turn, secrete the hormones that regulate the body's metab-olism. 
Human growth hormone has many vital functions, rang-ing from building bones and muscles to strengthening the immune system and helping to heal wounds. Dwarfism is a consequence of lack of hGH, and abnormally small children are treated with it to help them to grow closer to normal size. 
While growth hormone is secreted in large amounts by adolescents during sleep, in adulthood its production diminishes, often as much as ten to 15 percent every ten years in men. Women maintain higher levels until they reach menopause, then their hGH production rapidly declines. Resear-chers have not missed this obvious hint that slowdown in hGH may be related to aging. 
Low levels of hGH are associated with loss of muscle leanness and accumulation of fat. Worse, as the individu-al's weight increases, hGH levels drop more, setting up a negative feedback loop. Injection of hGH improves muscle strength and leanness, apparently helping the body to build protein instead of fat. Dr. Daniel Rudman of the Medical College of Wisconsin tested hGH's anti-aging possibilities with a group of male volunteers in their 60s and 70s. Half the volunteers received no hGH injections; they were the control group. The others received hGH injections three times a week for six months, so that their hGH levels were returned to the amounts they had as young men. 
While the men in the control group showed the normal deterioration of muscle, bone and organs expected for men of their age, those who received the hGH injections not only stopped aging -- in some ways their aging was reversed. 
They put on new muscle mass. Their skin increased in thickness by almost ten percent. Internal organs such as the spleen and liver also gained mass. Some of the deteri-orative effects of aging had been stopped and even turned around. 
 To make certain these effects were due to hGH, the researchers stopped the injections. The "youthful" group began to age normally once again. 
It is far too early to be certain, but if there is a single elixir of youth, human growth hormone might be it. Still, hGH needs to be tested over long time spans, and its possible side effects must be tracked down. It is known, for example, that overly large doses of hGH can cause or aggravate hypertension, lead to diabetes, enlarge the heart and effect the joints. 
Where we are heading 
Between hGH, telomerase, MORF4, and other possibili-ties such as organ regeneration (not to mention nanomachines or cryonics), the avenues toward immortality are many and the roadblocks are coming down. 
Commenting on the future possibili-ties of various therapies for aging, molecular biologist John Medina, of the University of Washington School of Medicine, puts it this way: 
What this means is nothing less than a bombshell. There are active research-ers in the field today who think that we may soon have protocols that could double or even treble normal human life spans. 
Doubling or tripling your life span means you will live to 150 or 225, at least. And by that time, biomedical research will have uncovered another century or two of life-extending techniques for you. 
While scientists are quick to point out that there is a long way between extending the lifetimes of cells in a Petri dish and extending human lifespan, such work is a major step toward the day when aging is banished and death itself becomes an option rather than an inevitability. 
The repercussions  
Science is constantly creating new opportunities for us, but new opportunities always gener-ate new problems for society. These social problems inevi-tably fall into the hands of our politicians. 
What can politicians do about biomedical knowledge that will extend human lifespans? 
Their first instinct, prompted by religious zealots, will be to ban it. It's new, it's something that's never been done before, therefore it must be wrong -- perhaps evil. 
After all, what was the immediate reaction to the news that a sheep had been cloned? "Ban it!" cried the presidents of the U.S. and France, as well as the Pope and many others. "The scientists are trying to play God!" 
The Clinton administration wants to pass a law out-lawing human cloning. 
But while many people are justifiably wary of cloning humans, the offer of virtual immortality will be too powerful to sweep under the rug. Right or wrong, good or bad, no matter what the eventual consequences to society, most people do not want to die. Most people will leap at the chance to extend their lives indef-initely, and to be youthful and vigorous for centuries. Or millennia. 
The politicians' second instinct will be to control the research. Write laws and set up bureaucracies that keep the scientists on a tight leash. Control the purse-strings for research so that only compliant scientists can receive government funding. 
Again, the pressures to push such research forward as rapidly as possible and make its results available to everyone will be overwhelming. We are talking of life and death here, and the basic animal drive to stay alive is far too powerful to deny or even delay. 
No matter how expensive it may be, no matter how it warps our society, people will want to extend their lives as far as they can. 
What happens to Social Security, Medicare, the insurance industry and our whole society when people live to be 200? Remember, by the time they get to 200, scientists will undoubtedly have found out how to extend lifespan even farther, and to maintain youthful vigor indefinitely. 
Instead of nibbling at the edges of institutions that were created when the average life expectancy was 60, politicians should understand that many of today's voters are going to live for centuries. Or longer. 
And these will be vigorous, active men and women, not shuffling old folks. They won't need retirement homes, they'll needs jobs or other ways of generating income. 
The insurance industry will concentrate on annuities that can provide very long-term dividends. Retirement will be a thing of the past, but education and retraining for new careers will become more important than ever. Lifelong educa-tion and the entertainment industry will merge. 
The long perspective 
One great advantage of a world in which human life-spans are measured in centuries is that it will finally allow us (or force us) to tackle the truly long-term problems that we face. 
Today, most people hardly ever think about the long-term future. Why worry about global warming if it's not going to have any real impact until after I'm dead and gone? Who cares about budget deficits? Or population growth? 
The long-term problems of our environment, of our race relations, of government deficits, of the economic disparities between rich and poor -- people who live for many centuries will not be able to ignore them or pass them on to future generations. 
Immortality will bring not only wisdom, but responsi-bility. It will also bring us starflight; taking centuries to reach the stars will no longer be a barrier. 
The human race will end its adolescence and attain true adulthood, at last. 
 Live long and prosper. 
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This article is taken from Ben Bova's forthcoming book, Immortality, is available from Avon Books.  To order your copy online Click HERE.