Relationship between the local stiffness of the outer hair cell along the cell axis and its ultrastructure observed by atomic force microscopy

As electromotility may arise from a conformational change of the molecules' 'protein motors', which might be distributed along the outer hair cell (OHC) lateral wall, the force generated by the OHC electromotility would be related not only to the conformational change of the protein motors but also to the mechanical properties of the lateral wall. Therefore, a detailed understanding of the mechanical properties of the OHC lateral wall is important. In our previous reports, to understand the difference in the stiffness along the cell axis, the local deformation of the OHC in response to hypotonic stimulation was analyzed by measuring the displacement of microspheres attached randomly to the cell lateral wall, and the distribution of Young's modulus along the cell axis was obtained using the contact mode of an atomic force microscope (AFM). These investigations revealed that the stiffness of the cell in the apical region was greater than that in other regions where the stiffness is constant. In this study, the ultrastructure of the OHC lateral wall was investigated with the oscillation imaging mode of the AFM (Tapping Mode™), and the relationship between the stiffness along the cell axis and the ultrastructure that was observed by the AFM imaging was analyzed. From the analysis, it was concluded that the circumferential filaments observed in the tapping mode AFM are actins which are part of the cortical lattice, and that the difference between the intervals of the circumferential filaments in the apical region and those in other regions is one factor that causes the high stiffness in the apical region.