Telomeres are the protective caps at the end of chromosomes. They shorten with cell division, and so are part of the clock which decides when a cell reaches the Hayflick limit and ceases dividing. There is much more to it than this, however: telomere length across all the cells in a piece of living tissue is dynamic, as there are processes that lengthen telomeres as well - such as the activity of telomerase.
In general average telomere length erodes with age, reflecting the progressive breakdown of the body's ability to maintain itself - but this proceeds quite differently in different tissues and different species. It can even be reversed in the short term if the health of an individual improves, though in the long term the overall progression is still downhill.
Shorter average telomere length has been correlated with measures of health in statistical studies, but data allowing prediction of longevity for an individual has proven elusive to date. Here, however, a more sophisticated measure of telomere dynamics is show to be predictive of life span in individual mice:
Aberrantly short telomeres result in decreased longevity in both humans and mice with defective telomere maintenance. Normal populations of humans and mice present high interindividual variation in telomere length, but it is unknown whether this is associated with their lifespan potential. To address this issue, we performed a longitudinal telomere length study along the lifespan of wild-type and transgenic telomerase reverse transcriptase mice.
We found that mouse telomeres shorten ∼100 times faster than human telomeres. Importantly, the rate of increase in the percentage of short telomeres, rather than the rate of telomere shortening per month, was a significant predictor of lifespan in both mouse cohorts, and those individuals who showed a higher rate of increase in the percentage of short telomeres were also the ones with a shorter lifespan. These findings demonstrate that short telomeres have a direct impact on longevity in mammals, and they highlight the importance of performing longitudinal telomere studies to predict longevity.