# Why doesn't cellular, replicative senescence (or the hayflick limit) constrain the normal development of an organism?

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?

• 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. Dec 29, 2016 at 3:14
• 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. Dec 31, 2016 at 6:51
• 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 Dec 31, 2016 at 7:22