Cell Polarity-Driven Instability Generates Self-Organized, Fractal Patterning of Cell Layers

As a model system to study physical
interactions in multicellular
systems, we used layers of Escherichia coli cells, which exhibit little or no intrinsic coordination
of growth. This system effectively isolates the effects of cell shape,
growth, and division on spatial self-organization. Tracking the development
of fluorescence-labeled cellular domains, we observed the emergence
of striking fractal patterns with jagged, self-similar shapes. We
then used a large-scale, cellular biophysical model to show that local
instabilities due to polar cell-shape, repeatedly propagated by uniaxial
growth and division, are responsible for generating this fractal geometry.
Confirming this result, a mutant of E. coli with
spherical shape forms smooth, nonfractal cellular domains. These results
demonstrate that even populations of relatively simple bacterial cells
can possess emergent properties due to purely physical interactions.
Therefore, accurate physico-genetic models of cell growth will be
essential for the design and understanding of genetically programmed
multicellular systems.