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The wikipedia article on cellular senescence states:

Cellular senescence is the phenomenon by which normal diploid cells cease to divide. In culture, fibroblasts can reach a maximum of 50 cell divisions before becoming senescent. This phenomenon is known as "replicative senescence", or the Hayflick limit.

My question is, if many cells in diploid organisms (e.g. humans) have an inbuilt limit of the number of cell divisions, how is it that a human can develop from a single celled zygote to a organism comprised of many 10's of trillions of cells - if many cells can only divide 50 or 60 times?

Wouldn't the cells "use up" their available telemeres during development? or is it a simple mathematical relationship, that whilst yes, existing cells are "burning up" their telemeres as they divide during development, but because the number of cells are increasing exponentially they develop all the tissue mass the organism needs before hitting the hayflick limit?
e.g. if cell numbers are doubling during each division phase, it only takes 30 doublings to achieve a billion cells, after 40 doublings you have a trillion cells.
i.e. after 40 or 45 doublings it has developed all the tissue mass of a mature organism, and still has some telemeres left over for additional tissue generation during the organisms lifetime?

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  • $\begingroup$ Read about telomerase - the Wikipedia article you mention talks about it specifically as a mechanism for getting around the limit - but yes, you can also get an awful lot of cells with 50 or 60 divisions; 10^18. $\endgroup$ – Bryan Krause Dec 29 '16 at 3:14
  • $\begingroup$ One cell divides into two. Each of those divides into two. Each of those divides into two... Etc. If this happened 50 times, you would get 2^50=1,125,899,906,842,620 cells. That's plenty and then some. $\endgroup$ – David Bahry Dec 31 '16 at 6:51
  • $\begingroup$ Yes this article expresses it as a formula: $ Nt = N_0 2^{tf} $ where Nt is number of cells at time t, N0 number of cells initially, t is time (days), f is frequency of cell cyces per unit time $\endgroup$ – the_velour_fog Dec 31 '16 at 7:22
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You pretty much have answered your own question. It is a math problem. Cells are burning up their telomeres during fetal development onward but the cells are multiplying exponentially in number.

But biology is a bit messy, embryonic stem cells do have telomerase activity and their telomerase are maintain. So the clock doesn't start counting down immediately. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2360127/

Temporal wise hES cells are around 4–5 days post fertilization, at which time they consist of 50–150 cells. https://en.wikipedia.org/wiki/Embryonic_stem_cell.

So you don't quite start with an n of 1 cell. But of an n of 50-150 (in humans)... before you start to see telomere shortening. With that head start those 50 doubling can make a lot more cells. By my guestimate with excel ~1.69E17 cells ~320 tons. More than enough you to last a life time.)

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  • $\begingroup$ Thanks, the links you provided were useful. I also found an article on the maths behind cell division It would be interesting to come across an article which specifically addressed the "race" between tissue mass created through cell division and the hayflick limit. But I guess we need to figure that out ourselves, and then as you mention there is always the fact that biology is a bit messy :) $\endgroup$ – the_velour_fog Dec 29 '16 at 4:01

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