The discovery of the Hayflick limit represented a radical change in the way science looked at cellular reproduction. Before the doctor's discovery, cells were thought to be capable of immortality. Although the phenomenon of the Hayflick limit has been studied only in vitro, it eventually came to generally be accepted in the scientific community as fact. For decades, it looked like the limit was insurmountable, and it still appears that way. In 1978, however, the discovery of a segment of non-replicating DNA in cells called telomeres shed light on the possibility of cellular immortality.
Telomeres are repetitive strings of DNA found at the ends of chromosome pairs within diploid cells. These strings are usually compared to the plastic ends of shoelaces (called aglets) that keep the laces from fraying. Telomeres provide the same protection to chromosomes, but the telomere on the end of each chromosome pair is shortened with each cellular division. Eventually, the telomere is depleted, and apoptosis begins.
The discovery of telomeres supported the Hayflick limit; after all, it was the physical mechanism by which cells entered senescence. Just under a decade later, however, another breakthrough in cellular aging was uncovered. Telomerase is a protein that's found in all cells, but in normal cells, it's turned off -- it doesn't do anything. In abnormal cells like tumors and germ cells, however, telomerase is quite active: It contains an RNA template capable of producing new telomeres on the ends of chromosomes in aging cells.
Telomerase has the aging research community excited for two reasons. First, since it's naturally active in tumors and can be detected in urine samples, testing for the presence of telomerase can lead to more effective testing of cancer patients. Second, researchers have figured out how to extract telomerase and synthesize it. Potentially, if active telomerase is added to normal adult cells, they'll continue to replicate long beyond their Hayflick limit. In one study that supports this notion, researchers reported that cells to which they'd introduced telomerase had replicated 20 more times than their normal life span would indicate -- and were still dividing [source: Cherfas].
Science has yet to definitively prove that telomerase can produce cellular immortality. There seems to be myriad factors involved in programmed cellular death beyond the destruction of telomeres. As long as humans fear death, though, there will always be research into overcoming these natural obstacles to our immortality, cellular or otherwise.