Straps between tissues (left) can be created by small movements that draw in collagen fibers.

To enlarge a drawing, an artist might use a pantograph, a pivoting frame that mechanically amplifies small movements into larger ones. On page 1083, Sawhney and Howard propose that cells use a similar mechanical arrangement to amplify small-scale traction forces into large-scale reorganization of the surrounding matrix. Their model may describe a general mechanism underlying morphogenesis.

When explants of fibroblasts are placed in collagen gels, they exert mechanical forces that lead to the formation of ligament-like straps between explants. Though this system is considered a good model for tissue morphogenesis, it remained unclear how micrometer-scale cellular traction forces generate millimeter-scale structural changes in the matrix.

Using a novel computer algorithm to monitor the patterning of the matrix, the authors found that small cellular movements bring about nearly simultaneous reorganization in the collagen gel, suggesting that the collagen forms a mesh of interconnected fibers. The results can be explained by a geometric model in which a small movement along one axis of the collagen mesh (the axis running between explants) generates a large movement perpendicular to the axis, which draws collagen into the strap. Because biological meshes are widespread, this could be a general mechanism by which small, slow movements bring about the large, rapid changes required for tissue remodeling, development, and cell movement. ▪