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Both animal and plant tissue exhibit a nonlinear rheological phenomenon knownas compression stiffening, or an increase in moduli with increasing uniaxialcompressive strain. Does such a phenomenon exist in single cells, which are thebuilding blocks of tissues? One expects an individual cell to compressionsoften since the semiflexible biopolymer-based cytoskeletal network maintainsthe mechanical integrity of the cell and in vitro semiflexible biopolymernetworks typically compression soften. To the contrary, we find that mouseembryonic fibroblasts (mEFs) compression stiffen under uniaxial compression viaatomic force microscopy (AFM) studies. To understand this finding, we uncoverseveral potential mechanisms for compression stiffening. First, we study asingle semiflexible polymer loop modeling the actomyosin cortex enclosing aviscous medium modeled as an incompressible fluid. Second, we study atwo-dimensional semiflexible polymer/fiber network interspersed witharea-conserving loops, which are a proxy for vesicles and fluid-basedorganelles. Third, we study two-dimensional fiber networks withangular-constraining crosslinks, i.e. semiflexible loops on the mesh scale. Inthe latter two cases, the loops act as geometric constraints on the fibernetwork to help stiffen it via increased angular interactions. We find that thesingle semiflexible polymer loop model agrees well with our AFM experimentsuntil approximately 35% compressive strain. We also find for the fiber networkwith area-conserving loops model that the stress-strain curves are sensitive tothe packing fraction and size distribution of the area-conserving loops,thereby creating a mechanical fingerprint across different cell types. Finally,we make comparisons between this model and experiments on fibrin networksinterlaced with beads as well as discuss the tissue-scale implications ofcellular compression stiffening.

Loops versus lines and the compression stiffening of cells