Cells adherent on a cyclically stretched substrate with a periodically varying uniaxial strain are known to dynamically reorient nearly perpendicular to the strain direction. We investigate the dynamic reorientation of rat embryonic and human fibroblast cells over a range of stretching frequency from 0.0001 to 20 s(-1) and strain amplitude from 1% to 15%. We report quantitative measurements that show that the mean cell orientation changes exponentially with a frequency-dependent characteristic time from 1 to 5 h. At subconfluent cell densities, this characteristic time for reorientation shows two characteristic regimes as a function of frequency. For frequencies below 1 s(-1), the characteristic time decreases with a power law as the frequency increases. For frequencies above 1 s(-1), it saturates at a constant value. In addition, a minimum threshold frequency is found below that no significant cell reorientation occurs. Our results are consistent for the two different fibroblast types and indicate a saturation of molecular mechanisms of mechanotransduction or response machinery for subconfluent cells within the frequency regime under investigation. For confluent cell layers, we observe similar behaviors of reorientation under cyclic stretch but no saturation in the characteristic time with frequency, suggesting that cell-cell contacts can play an important role in the response machinery of cells under mechanical strain.

Cells have the ability to measure and respond to extracellular signals like chemical molecules and topographical surface features by changing their orientation. Here, we examined the orientation of cultured human melanocytes exposed to grooved topographies. To predict the cells' orientation response, we describe the cell behavior with an automatic controller model. The predicted dependence of the cell response to height and spatial frequency of the grooves is obtained by considering the symmetry of the system (cell + substrate). One basic result is that the automatic controller responds to the square of the product of groove height and spatial frequency or to the aspect ratio for symmetric grooves. This theoretical prediction was verified by the experiments, in which melanocytes were exposed to microfabricated poly(dimethylsiloxane) substrates having parallel rectangular grooves of heights (h) between 25 and 200 nm and spatial frequencies (L) between 100 and 500 mm(-1). In addition, the model of the cellular automatic controller is extended to include the case of different guiding signals acting simultaneously.

Little is known about how functional regulation failure in genetically altered cells is influenced by topographical confinement of cells, a situation often present in tissues in vivo. We used cultured melanocytes derived from human skin samples as a model system for such investigations. Normal melanocytes have a very well defined shape consisting of a cell body and two dendrites arranged 180 degrees relative to each other. In contrast, neurofibromin 1-melanocytes (NF1-melanocytes) have up to a 50% reduction of neurofibromin 1, which results in an altered morphology that can be easily measured. NF1-melanocytes deviate from the defined structure of normal melanocytes by forming more than two dendrites per cell. We show that morphology consequences of genetically altered melanocytes can be canceled if cells interact with substrates microstructured by stripes that apply mechanophysical signals in the form of physical topography. The strength of the mechanophysical signal was varied systematically by increasing the height of the microstructures. Melanocytes respond to surface topographical features that are larger than 50 nm and have lateral confinements smaller 4 microns. The response of normal and NF1-melanocytes to different topographies was analyzed quantitatively by determining density distributions for the number of dendrites per cell, the angles between dendrites, and the orientation imprinted in the substrate. The synthesis of melanin, a pigment produced by melanocytes, differs in the case of genetically altered NF1- and normal melanocytes. In both cases, the interaction with microstripes enhanced melanin production significantly. This enhanced melanin production is speculated to be caused by the mechanical stabilization of the dendrites by substrate guidance.

Cellular ageing can lead to altered cell mechanical properties and is known to affect many fundamental physiological cell functions. To reveal age-dependent changes in cell mechanical properties and in active mechanoresponses, the stiffness of human fibroblasts from differently aged donors was determined, as well as the cell's reaction to periodic mechanical deformation of the culture substrate, and the two parameters were correlated. A comparison of the average Young's moduli revealed that cells from young donors (<25 years) are considerably stiffer than cells from older donors (>30 years). The reduced stiffness of cells from the older donor group corresponds to the measured decrease of actin in these cells. Remarkably, cells from the older donor group show a significantly faster reorganization response to periodic uniaxial tensile strain than cells from the young donor group. The impact of a reduced amount of actin on cell stiffness and cell reorganization kinetics is further confirmed by experiments where the amount of cellular actin in cells from the young donor group was decreased by transient siRNA knockdown of the actin gene. These cells show a reduced stiffness and enhanced reorganization speed, and in this way mimic the properties and behavior of cells from the older donor group. These results demonstrate that mechanical properties of human fibroblasts depend on the donor's age, which in turn may affect the cells' active responses to mechanical stimulations.