Figure 2.

SAOS-A2 cells were seeded on (a) D-periodic, or (b) non-periodic collagen matrices and allowed to spread for 45 min. Subsequently, cells were glutaraldehyde/paraformaldehyde-fixed and AFM deflection images representing the error signal were recorded while scanning the sample in contact mode. White arrows indicate the orientation of the collagen fibrils within the matrices. (a) On D-periodic collagen, cells polarize strongly and deform the D-periodic matrix perpendicular to the fibril direction, as indicated by the exposed surface. Collagen fibrils are bundled at the front and back of the cell without rupturing. The inset (3 μm × 3 μm) shows an AFM contact mode topograph of ≈ 3 nm thick collagen matrices assembled on freshly-cleaved mica in the presence of potassium ions. (b) Cell adhesion causes frequent rupture of non-periodic collagen fibrils, as demonstrated by the frayed appearance of the fibril ends and the widespread exposure of the mica surface in the cell periphery. The inset (3 μm × 3 μm) shows an AFM contact mode topograph of ≈ 3 nm thick collagen matrices assembled in the absence of potassium ions. (c) Model illustrating how differences in matrix rigidity between D-periodic and non-periodic collagen matrices affect cell polarization. Upon seeding, cells explore the mechanical properties of the surrounding D-periodic or non-periodic matrix by forming protrusions in all directions. Subsequently, cells form adhesion complexes and begin to exert pulling forces on the matrix. The high tensile strength of D-periodic collagen fibrils permits the establishment of strong cellular traction along the fibril direction. In contrast, the high pliability of the fibrils prevents traction when cells pull perpendicular to the fibril orientation. As a result of the directional traction the cells elongate. The low tensile strength of non-periodic collagen fibrils avoids traction build-up in the fibril direction, preventing cells from polarizing (Reproduced with modifications and proper permission obtained from Elsevier as published in Friedrichs et al., 2007)