Dynamics of cell mass and size control in multicellular systems and the human body

Cellular processes, in particular homeostasis and growth, require an intricate and complex exchange of matter between a cell and its surroundings. Yet experimental difficulties have prevented a detailed description of the dynamics of a cells mass and volume along different cellular processes, limiting our understanding of cell physiology in health and disease1-4. It has been recently observed that single mammalian cells fluctuate their mass in a timescale of seconds5. This result challenges central and longstanding cell growth models, according to which cells increase their mass either linearly or exponentially throughout the cell cycle4,6. However, it remains unclear to what extent cell mass fluctuations may be sustained in multicellular organisms. Here I provide a mathematical model for cell mass fluctuations and explore how such fluctuations can be successfully sustained in multicellular organisms. I postulate that cells do not synchronise their mass fluctuations, but they are executed in a random fashion with their phases uniformly distributed. I derive an inequality to estimate the resulting mass shift between fluid compartments in an organism due to cell mass fluctuations. Together with a new estimate for the number of human cells in the body, I demonstrate that my hypothesis leads to shifts of mass between the intracellular and extracellular fluid compartments in the human body that are approximately or smaller than 0.25 mg and, therefore, perfectly viable. The proposed model connects cell physiology with information theory and entropy.