Contraction of hydrated collagen gels by fibroblasts: evidence for two mechanisms by which collagen fibrils are stabilized.

Studies were conducted to learn more about the mechanism by which fibroblasts contract hydrated collagen gels, a process that may be important in the supramolecular organization of the extracellular matrix. Removal of cells from contracted gels by two different methods, treatment with detergent or treatment with trypsin/EDTA solution, had no visible effect on the bundles of collagen fibrils that had been organized in frameworks around and in between the cells. There was, however, a portion of the collagen gels that expanded after the cells were removed. We conclude that during concentration of collagen gels by fibroblasts, rearranged collagen fibrils were stabilized in place by two different mechanisms. At first, the fibrils were mechanically held in place by the cells. Subsequently, the fibrils were stabilized by non-covalent chemical interactions that are independent of cells. A model system for studying collagen gel reorganization in the absence of cells was developed based on centrifugation of the gels. The overall features of collagen gel reorganization by centrifugation were similar to the features of collagen gel contraction by cells. The collagen fibrils of gels reorganized by centrifugation were at first stabilized mechanically and the gels expanded after centrifugation was stopped. With additional time, the collagen fibrils were stabilized by non-covalent chemical interactions, and then the gels no longer expanded after centrifugation was stopped.