Cell rheology: Mush rather than machine

One key function of the eukaryotic cell is deformability, which is well characterized phenomenologically yet remains poorly understood fundamentally. The cell's nucleus, which is stiffer than the cytoskeleton, also shares some of these same features. Importantly, it is well recognized that these elastic cellular structures are porous and dispersed in water, which is of course the principal cellular constituent. Still, although the implications of water flow on cellular mechanics are known to be important, the underlying mechanism has never been clear. Clearly, had the cell's cytoskeleton been much softer than that of the mat, it would have been mechanically impossible for the cell to penetrate into it. On the other hand, a cytoskeleton that is substantially stiffer would have made motility within the mat metabolically wasteful. Efficient motility, therefore, should favor the adaptation of the cell's mechanical properties to match those of the energy-rich mush.