How many cells are needed?

Dawn DawnC at bigfoot.com
Tue Sep 16 08:29:33 EST 1997

I believe it's generally accepted that, even by age 100, our cells still
have enough telomeres to divide. Furthermore, as you know, cells can
reach a state of quiescence where they don't divide but still carry out
their normal functions. 

So, while Telomere shortening is involved with aging, I don't think it's
the MAIN reason why we live 75-100 years.

Suresh Rattan wrote:
> While teaching a course on biogerontology I have got stuck with this
> question: how many cells are needed to live a life of 100 years?
> The background for this question is that for the last 35 years the dominant
> paradigm in cellular ageing research is that normal diploid cells have a
> limited division potential, known as the Hayflick limit, which for human
> beings is about 50 to 70 doublings, depending upon the age of the donor,
> cell type etc etc etc. Therefore, even if one considers the first normal
> diploid cell, the zygote, having a Hayflick limit of, say 70, then it can
> potentially give rise to 10 to the power 21 (I cannot type superscript on
> this e-mail format!!); that is 1 followed by 21 zeroes number of cells. Is
> this number sufficient to take us through life?
> 1:  The answer papears to be YES if you calculate this  (highly highly
> simplified/simplistic) way: The protein weight of a single cell is about
> 250 pigogram; so a 100 Kg person will have about 10 raised to the power 14
> cells; even if 100% of the cells have to be replaced everyday for 100
> years, one needs a total of 10 to the power 19 cells maximally; thus
> thepresent estimates of the Hayflick limit provide enough cells to live.
> 2:  The answer appears to be NO if you calculate in another way (the way
> Harry Rubin has done in his recent critical review of in vitro versus in
> vivo ageing, published in the Mechansism of Ageing and Development, vol.
> 98; pp. 1-35; 1997): take an example of epidermal layer of the skin; basal
> cells have to divide once every 10 days or so for the regular
> differentiation and turnover of keratinocytes; so in a year they have to
> divide about 36 times, and in 100 years, about 3600 times; the same may be
> the case for other dividing cell populations (RBC turnover time about 3
> times a year, so about 300 times in 100 years, and so on. Considering this
> way, the present estimates of the Hayflick limit are too low to be
> meaningful in real terms.
> So, what is correct? Even if we consider that normal diploid cells have a
> limit, how much it is and is it relevant to ageing? Or there is no limit
> like that in the body, and all that stuff which people have been doing for
> the last 35 years with respect to finding regulatrors of proliferative
> capacity is crap?

Time is the best teacher. Unfortunately, it kills all of its students.

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