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I mean, Hemocytoblast is a stem cell which is constantly being differentiated into daughter cells and leads to formation of all the blood cells (having short and limited life spans), so how come those stem cells remain there and continue to function, even when we grow up and finally grow old (i.e for 70 years approx)? The no. of those stem cells gotta be limited.

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The trick for the stem cells lies in the way the proliferate and differentiate. See the image (from the reference 1):

enter image description here

Stem cells have the problem that they need to differentiate and maintain their population at the same time (subfigure a). This can either be done by asymmetric cell division which leads to one daughter which is still a stem cell and one which has been differentiated (subfigure b). Then it is possible that the cells divide only symmetrical (as shown in subfigure c) so they will either deliver two stem cells or two differentiated cells. And then it is possible (shown in subfigure d) that both possibilities are mixed and that symmetrical and assymetrical division occurs. Either way you get both: Differentiated cells and you still manage to maintain the stem cell pool.

References:

  1. Asymmetric and symmetric stem-cell divisions in development and cancer
  2. Asymmetric cell division of stem and progenitor cells during homeostasis and cancer
  3. Asymmetric cell division
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    $\begingroup$ your first link is broken $\endgroup$ – MattDMo Feb 18 '15 at 17:09
  • $\begingroup$ Can this asymmetric cell division be defined as mitosis? $\endgroup$ – elvarox Feb 18 '15 at 17:21
  • $\begingroup$ Is this the only way, how these stem cells remain active for so many years? I don't think, that even with your answer taken into account, that's possible if we consider 70 years of average life span. $\endgroup$ – elvarox Feb 18 '15 at 17:32
  • $\begingroup$ @MattDMo Thanks for the notification, I fixed this. $\endgroup$ – Chris Feb 18 '15 at 17:35
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    $\begingroup$ @a.aniq yes, it's mitosis. What else would it be? Also, since these cells do remain active for an entire lifetime, your assumptions need to be re-evaluated. $\endgroup$ – MattDMo Feb 18 '15 at 17:37

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