7. Cells in Collagen Type I gels

Collagen is a widely used extracellular matrix (ECM) material to study cell physiology in a tissue-mimetic, three dimensional environment. The mechanical properties of collagen are known to influence cell behavior, while, in turn, cells can restructure the matrix and change its mechanical properties by the exertion of traction forces and enzymatic degradation. In this chapter, we study the contribution of cellular traction and network remodeling on the mechanical properties of collagen networks. For this purpose, we employ 3T3 mouse fibroblasts inside collagen networks reconstituted from collagen I extracted from rat tail. Surprisingly, we find no difference in the mechanical properties of collagen networks with and without cells. By using time-lapse microscopy, we can explain this observation in terms of limited cell spreading. Obvious candidate mechanisms to increase cell spreading (changing cell density, serum concentration, collagen concentration and dimensionality) did not improve cell spreading. Using quantitative polymerase chain reaction assays, we verified that the fibroblasts in principle did express collagen binding integrins. Apparently, the cells are unable to form sufficiently mature adhesions to the collagen matrix to allow them to exert significant traction forces. Therefore we recommend to use a different cell line for future experiments, which is more contractile than the 3T3 cell line used here.