Collagen 1 solution

hSMECs or hS/PCs were suspended at a density of 3 × 10⁵ cells/ml in a collagen I solution (Corning Inc.) (3 mg/mL) at 4°C, and aliquots of 200 µl of cell suspension were added to BSA-coated wells of a 24-well plate (Cell Treat, Pepperell, MA). Samples were incubated at 37°C for 30 min before the addition of 500 µl FBS-free RPMI cell culture media. Cells were allowed to spread in the collagen gel overnight before the initiation of contraction with the addition of FBS. After 16 h of incubation at 37°C, the cell culture media was changed with the same media supplied with 10% FBS, and images of the gels were taken at 0 min. Gels were then photographed at indicated time points, and the average gel diameter for each experimental group (n=8) was calculated. Following the completion of the experiments, collagen gels immediately fixed using 4% paraformaldehyde for 30 min and followed by three PBS washes. The collagen gels were subsequently treated with the permeabilization solution (PBS containing 0.1% TritonX-100) for 45 min, stained with AlexaFluor 568-conjugated phalloidin and DAPI (Thermo Fisher) for 1 h and 30 min, respectively. After multiple PBS washes. samples were imaged with ZEISS LSM 880 (ZEISS, Pleasanton, CA) confocal microscope under fluorescence and differential reflectance modes to visualize collagen fibril, cytoskeleton and nuclei.