Fig. 5.
Fate control. a A stem cell (type 1) can either regenerate, or differentiate into a terminal post-mitotic cell (type 2), or have a third alternative fate that leads to a new branch. Negative feedback of the population of terminal cells acts on the probability of regeneration and differentiation, which necessarily leads to a positive feedback on the alternative fate as the three probabilities must sum to 1. b The effect of sinusoidal variation in d (the disturbance) on |S|, the ‘sensitivity function’, as a function of disturbance frequency. |S| is related to the size of the corresponding fluctuation of the population of terminal cells. n reflects the strength of feedback. For this fate control model p r (x 2) is the same as in the renewal control case, while p d (x 2) is taken to be p r (x 2)/2. As in renewal control, stronger feedback results in better disturbance rejection (better robustness) at lower frequencies, at the price of poor disturbances rejection at mid-frequencies. Unlike renewal control, however, the system has significantly more capacity for disturbance rejection (more overall robustness), as indicated by the much larger area below the gray line