Fig. 3.

Renewal control. a A stem cell (type 1) can either regenerate or differentiate into a terminal post-mitotic cell (type 2). Negative feedback acts to affect the probability of regeneration. b The effect of sinusoidal variation in d (the disturbance) on |S|, the so-called ‘sensitivity function’, as a function of disturbance frequency. |S| is in turn related to the size of the corresponding fluctuation of the population of terminal cells (type 2). n reflects the strength of feedback, with stronger feedback resulting in better disturbance rejection (better robustness) at lower frequencies, at the price of amplifying the effect of disturbances at mid-frequencies (fragility)