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?