Normal tissue cells are generally not viable when suspended in a fluid and are therefore said to be anchorage dependent. Such cells must adhere to a solid, but a solid can be as rigid as glass or softer than a baby's skin. The behavior of some cells on soft materials is characteristic of important phenotypes; for example, cell growth on soft agar gels is used to identify cancer cells. However, an understanding of how tissue cells-including fibroblasts, myocytes, neurons, and other cell types-sense matrix stiffness is just emerging with quantitative studies of cells adhering to gels (or to other cells) with which elasticity can be tuned to approximate that of tissues. Key roles in molecular pathways are played by adhesion complexes and the actinmyosin cytoskeleton, whose contractile forces are transmitted through transcellular structures. The feedback of local matrix stiffness on cell state likely has important implications for development, differentiation, disease, and regeneration.

/pdf/tissue-cells-feel-and-respond-to-the-stiffness-of-their-6dx0rumap6.pdf

Tissue Cells Feel and Respond to the Stiffness of Their Substrate

Cell Migration: A Physically Integrated Molecular Process

During the past 20 years, it has become generally accepted that the modulation of fibroblastic cells towards the myofibroblastic phenotype, with acquisition of specialized contractile features, is essential for connective-tissue remodelling during normal and pathological wound healing. Yet the myofibroblast still remains one of the most enigmatic of cells, not least owing to its transient appearance in association with connective-tissue injury and to the difficulties in establishing its role in the production of tissue contracture. It is clear that our understanding of the myofibroblast its origins, functions and molecular regulation will have a profound influence on the future effectiveness not only of tissue engineering but also of regenerative medicine generally.

/pdf/myofibroblasts-and-mechano-regulation-of-connective-tissue-nzvxuu0xbh.pdf

Myofibroblasts and mechano-regulation of connective tissue remodelling

http://chemotaxis.semmelweis.hu/CHTXhpg/Durotaxis/LoCM_BiophysJ_2000.pdf

Cell Movement Is Guided by the Rigidity of the Substrate

Responses of cells to mechanical properties of the adhesion substrate were examined by culturing normal rat kidney epithelial and 3T3 fibroblastic cells on a collagen-coated polyacrylamide substrate that allows the flexibility to be varied while maintaining a constant chemical environment. Compared with cells on rigid substrates, those on flexible substrates showed reduced spreading and increased rates of motility or lamellipodial activity. Microinjection of fluorescent vinculin indicated that focal adhesions on flexible substrates were irregularly shaped and highly dynamic whereas those on firm substrates had a normal morphology and were much more stable. Cells on flexible substrates also contained a reduced amount of phosphotyrosine at adhesion sites. Treatment of these cells with phenylarsine oxide, a tyrosine phosphatase inhibitor, induced the formation of normal, stable focal adhesions similar to those on firm substrates. Conversely, treatment of cells on firm substrates with myosin inhibitors 2,3-butanedione monoxime or KT5926 caused the reduction of both vinculin and phosphotyrosine at adhesion sites. These results demonstrate the ability of cells to survey the mechanical properties of their surrounding environment and suggest the possible involvement of both protein tyrosine phosphorylation and myosin-generated cortical forces in this process. Such response to physical parameters likely represents an important mechanism of cellular interaction with the surrounding environment within a complex organism.

/pdf/cell-locomotion-and-focal-adhesions-are-regulated-by-4qm8y4o672.pdf

Cell locomotion and focal adhesions are regulated by substrate flexibility

When tissue cells are cultured on very thin sheets of cross-linked silicone fluid, the traction forces the cells exert are made visible as elastic distortion and wrinkling of this substratum. Around explants this pattern of wrinkling closely resembles the "center effects" long observed in plasma clots and traditionally attributed to dehydration shrinkage.

http://science.sciencemag.org/content/sci/208/4440/177.full.pdf

Silicone rubber substrata: a new wrinkle in the study of cell locomotion

To make visible the traction forces exerted by individual cells, we have previously developed a method of culturing them on thin distortable sheets of silicone rubber1. We have now used this method to compare the forces exerted by various differentiated cell types and have examined the effects of cellular traction on re-precipitated collagen matrices. We find that the strength of cellular traction differs greatly between cell types and this traction is paradoxically weakest in the most mobile and invasive cells (leukocytes and nerve growth cones). Untransformed fibroblasts exert forces very much larger than those actually needed for locomotion. This strong traction distorts collagen gels dramatically, creating patterns similar to tendons and organ capsules. We propose that this morphogenetic rearrangement of extracellular matrices is the primary function of fibroblast traction and explains its excessive strength.

Fibroblast traction as a mechanism for collagen morphogenesis

Connective tissue morphogenesis by fibroblast traction: I. Tissue culture observations

Peritoneal macrophages were plated out on a series of artificial haptotactic substrata consisting of grid patterns of vacuum-evaporated palladium metal alternating with hydrophobic untreated polystyrene and with hydrophilic sulphonated polystyrene. By their active locomotion the macrophages accumulated preferentially onto the less hydrophilic of either pair of these alternative substrata. This order of substratum preference is precisely the opposite to that shown by fibroblastic cells. Macrophages were also found to accumulate preferentially onto roughened culture surfaces as opposed to smooth ones, which is again opposite to the behaviour of fibroblasts. These opposite substratum preferences shown by macrophages may reflect physical as well as functional differences in their surfaces, and could serve as assay criteria for macrophages and cell types putatively equivalent to macrophages.

/pdf/anomalous-preferences-of-cultured-macrophages-for-4o66eofife.pdf

Albert K. Harris

Papers

Silicone rubber substrata: a new wrinkle in the study of cell locomotion

Fibroblast traction as a mechanism for collagen morphogenesis

Connective tissue morphogenesis by fibroblast traction: I. Tissue culture observations

Anomalous preferences of cultured macrophages for hydrophobic and roughened substrata.

Is Cell sorting caused by differences in the work of intercellular adhesion? A critique of the Steinberg hypothesis.