Relationship between apical membrane elasticity and stress fiber organization in fibroblasts analyzed by fluorescence and atomic force microscopy

To investigate the relationship between cellular microelasticity and the structural features of cytoskeletons (CSKs), a microindentation test for apical cell membranes and observation of the spatio-distribution of actin CSKs of fibroblasts were performed by fluorescence and atomic force microscopy (FM/AFM). The indentation depths of apical cell membranes were measured from AFM force-indentation (f-i) curves under equal final loads and mapped two-dimensionally to show the relative distribution of local microelasticity on cell membranes. Intracellular spatial distribution of actin CSKs was visualized fluorescently by high Z-resolution cross-sectional observation of a cell on which indentation mapping analysis had been performed in advance. Structural features of stress fibers (SFs) were observed as three typical patterns of dense SF, sparse SF and sparser SF cell groups, which were quantitated using the degree of orientation in apical SFs (ASFs) that had been defined using two-dimensional Fourier analysis. In indentation depth maps, the upper nuclear region was markedly softer than the pseudopodium region. The mean indentation depth of the upper nuclear region decreased with increased SF density in whole cells and the degree of orientation of ASF, although the pseudopodium region did not exhibit such a trend. The apical membrane of adhered cells was found to tend to stiffen with the increase in both density and degree of orientation of SFs.