Progress toward quantitative design principles of multicellular systems

One of the important challenges in biology is to quantitatively explain how multicellular systems' behaviors arise from the genetic circuits inside each cell and the interactions among these cells. Communication among cells is one of the primary means for cells to interact with each other. One cell can influence how the other cell behaves by "talking" to that cell, for example, through the secretion of a signaling molecule that the other cell can respond to. Each cell can typically communicate with multiple cells located at various locations. Thus we can represent multicellular systems as complex communication grids. Discovering the common principles that govern such multicellular communication grids is crucial for tying together a wide range of multicellular systems such as tissues, embryos, and populations of microbes, under a common quantitative framework. But it has been difficult to find such principles. One difficulty is that we do not yet have generally applicable strategies for judiciously reducing the number of parameters in a multicellular system with a large number of intracellular and intercellular components. Another difficulty is that we usually do not know what quantitative metrics can characterize multicellular behaviors of interest. For example, it is unclear in many multicellular systems what quantities one should use to measure or model the degree to which cells in a tissue coordinate their behaviors to regulate the expression level of a gene that is common to them. Here we outline some possible methods for overcoming these difficulties and review recent studies that suggest that these methods may be potent.