Comparison of cellular strain with applied substrate strain in vitro

Strain magnitudes within tenocytes undergoing substrate tensile strain are not well defined. It was hypothesized that strain magnitudes at the cellular level would reflect those of the applied substrate (equibiaxial or uniaxial) strain. A vacuum-operated device was used to apply equibiaxial or uniaxial tension to a flexible substrate upon which tenocytes were cultured in monolayer. Images of tenocytes labeled with Fura-2, to detect free intracellular calcium ions, and MitoFluor Green, to detect mitochondria, were taken prior to strain and for 20 min during application of static strain. A custom-written, texture correlation program computed strain magnitudes in the cell based on the change in pixel pattern displacements between images of non-strained and strained cells. On average, cellular strain was approximately 37+/-8% and 63+/-11% of the applied equibiaxial and uniaxial substrate strain, respectively. The largest cell strains were detected in cells oriented parallel to the direction of applied uniaxial tensile strain. However, strain magnitudes within a cell were heterogeneous. The variance in strain magnitude within and among tenocytes is dependent on cell orientation, cell stiffness, cytoskeleton organization, subcellular organelles, or placement and type of cell-substrate contacts. Results of the present study indicate that cultured tenocytes experience a moderate fraction of the applied substrate strain.