Re: Can the life span of an individual be determined by counting the Telomeres?

Doru, sorry it took so long to answer your question, but it intrigued me 
enough to do some additional research.  Since other people may be 
interested in reading this response to your question, let me first relate 
some background information on cellular aging.  When cells are cultured in 
petri dishes (in vitro) they grow and divide.  When they reach confluence 
(i.e. carpet the bottom of the dish), they are split into separate dishes 
where they begin to divide again.  Each split is known as a passage.  In 
the 1960s Leonard Hayflick observed that fibroblasts grown in such an 
experiment had a defined number of passages.  That is, they could be 
passaged about 60 times but after that the cells just wouldn't divide any 
longer; the cells had reached a state of replicative senescence.  This 
became known as the Hayflick limit.  The Hayflick limit is cell type 
specific, which is to say that different types of cells (firbroblast vs. 
liver cell for example) have different Hayflick limits.  Another 
observation that Hayflick made was that if he took fibroblasts derived from 
human embryos and fibroblasts from human adults, the cells from the embryos 
divided many more times than those from the adult.  This suggested that 
replicative senescence might be related to the aging process.  Additional 
evidence for this is found by examining the Hayflick limit of fibroblasts 
from different species.  In general, fibroblasts from species with longer 
maximum lifespans have larger Hayflick limits.  Fibroblasts from rats have 
an average population doubling (PD - another term for the Hayflick limit) 
of between 10-20, humans of between 35-70, and Galapagos tortoises of 
between 100-120.  
The question remained as to why there are a defined number of divisions for 
these cells.  This is where telomeres come in.  Telomeres are the 
structures that cap the ends of chromosomes and consist of short terminal 
DNA repeats.  It was first suggested in the 1970s that because of the way 
DNA replicates, during normal replication one strand would be replicated to 
the end whereas the other strand will have a short gap at the 5' end.  In 
principle, each time the DNA was replicated, the chromosome would shorten 
progressively from each end, i.e. the length of the telomeres would 
shorten.  This is, indeed the case in many cells types.  However, it was 
also discovered that an enzyme called telomerase acts in certain cells to 
keep the length of the telomeres constant.  The telomerase enzyme was 
initially found in stem cells, germ cells, and some cancerous cells.  The 
finding of telomerase activity in cancerous cells correlated with the 
theory that replicative senescence was an evolutionary adaptive anti-cancer 
mechanism.
Both the observations by Hayflick and the observations of telomere 
shortening resulted in the proposition that telomere length and telomerase 
activity could be important determinants in an organisms aging - as per 
your question.  During the past several years, this has been a fairly 
active area of research in the aging community.  The findings can be summed 
up as follows.  While in vitro senescence of somatic cells is well 
documented, many cells in the body (in vivo) do not proliferate enough 
during the lifetime of the animals to reach their Hayflick limit.  Cells 
that do proliferate regularly have been shown to have telomerase activity 
or are replaced continuously from stem cells.  In addition, there does not 
appear to be a correlation between telomere length and maximal lifespan of 
different species.  Regardless, it is also important to point out that the 
variation in cell division over time for each tissue type and the variation 
in the shortening of the telomeres during each division would make it 
nearly impossible to predict the lifespan of an individual organism with 
any accuracy.  Having said that, there is some merit in the notion that 
diseased tissues might have differences in telomere length compared to 
healthy tissues, which could provide a potential diagnostic tool in the 
future.