Development of an in vitro model of contraction by fibroblasts

Abstract

Dermal scars in adult humans are mechanically and functionally inferior to normal skin and can be physically disfiguring. The contraction of the wound by fibroblasts has been linked to the formation of scar. The mechanical and chemical signals, which control the contraction, are being investigated through the use of models of fibroblast contraction to understand the conditions which promote tissue regeneration. A cell force monitor (CFM) was designed and constructed to measure quantitatively the contraction of a highly-porous, collagen-GAG matrix by fibroblasts. Using this device, contractile force, displacement, and kinetics were compared for different values of cell density and total stiffness resisting fibroblast contraction. In addition, observation of live cells contracting individual matrix struts established the cellular mechanisms responsible for the matrix contraction measured in the CFM. Observation of live cells revealed that macroscopic contraction of the collagen-GAG matrix was the result of forces generated during cell elongation.

(cont.) Contractile force normalized by the number of attached cells (1 nN per cell) was independent of cell density (400 - 2,000 cells/mm3) and total stiffness resisting contraction (0.7 - 10.7 N/m). Total contractile force was dependent on the cell density. These results indicated that the contractile force developed during fibroblast elongation was determined at the level of individual cells (not cooperatively) and was limited by force per cell (not displacement per cell). The kinetics of macroscopic matrix contraction were also independent of cell density and system stiffness; contractile force reached an asymptotic value in 15 h. Observation of live cells found the macroscopic time dependence likely resulted from the stochastic nature of cell elongation initiation and the time required for the fibroblasts to elongate completely (-2-4 h). Therefore, the time dependence of macroscopic matrix contraction did not reflect the time dependence of force generation by individual fibroblasts, but rather an average for the entire population.