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.

To make visible the traction forces exerted by individuals cells, we have previously developed a method of culturing them on thin distortable sheets of silicone rubber. 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.

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Many embryonic cells generate substantial contractile forces as they spread and crawl.These forces mechanically deform each cell's local environment, and the resulting distortions can alter subsequent cell movements by convection and the mechanisms of contact guidance and haptotaxis. Here we develop a model for the cumulative effects of these cell-generated forces and show how they can lead to the formation of regular large-scale patterns in cell populations. This model leads to several predictions concerning the effects of cellular and matrix properties on the resulting patterns. We apply the model to two widely studied morphogenetic processes: (a) patterns of skin-organ primordia, especially feather germ formation, and (b) the condensation of cartilagenous skeletal rudiments in the developing vertebrate limb.

Active locomotion by individual marine and freshwater sponges across glass, plastic and rubber substrata has been studied in relation to the behavior of the sponges' component cells. Sequential tracing of sponge outlines on aquarium walls shows that sponges can crawl up to 160 microns/hr (4 mm/day). Time-lapse cinemicrography and scanning electron microscopy reveal that moving sponges possess distinctive leading edges composed of motile cells. Sponge locomotion was found to be mechanically similar to the spreading of cell sheets in tissue culture both with respect to exertion of traction (which causes the wrinkling of rubber substrata) and with respect to the patterns of adhesive contacts formed with the substratum (as observed by interference reflection microscopy). Other similarities include the orientation of sponge locomotion along grooves and the preferential extension onto more adhesive substrata. Neither the patterns of wrinkling produced in rubber substrata nor the distributions of adhesive contacts seen by interference reflection microscopy show evidence of periodic, propagating waves of surface contractions, such as would be expected if the sponges' mechanism of locomotion were by peristalsis or locomotory waves. Our observations suggest that the displacement of sponges is achieved by the cumulative crawling locomotion of the cells that compose the sponge's lower surface. This mode of organismal locomotion suggests new explanations for the plasticity of sponge morphology, seems not to have been reported from other metazoans, and has significant ecological implications.