Large scale-invariant fluctuations in normal blood cell counts: A sign of criticality?

All types of blood cells are formed by differentiation from a small self-ma intaining population of pluri-potential stem cells in the bone marrow. Desp ite abundant information on the molecular aspects of division, differentiat ion, commitment and maturation of these cells, comparatively little is know n about the dynamics of the system as a whole, and how it works to maintain this complex "ecology" in the observed normal ranges throughout life. Here we report unexpected large, scale-free, fluctuations detected from the fir st long-term analysis of the day-to-day variability of a healthy animal's b lood cell counts measured over 1000 days. This scale-invariance cannot be a ccounted for by current theoretical models, and resembles some of the scena rios described for self-organized criticality.