High sensitivity of human hearing is believed to be achieved by cochlear amplification. The basis of this amplification is thought to be the motility of mammalian outer hair cells (OHCs), i.e., OHCs elongate and contract in response to acoustical stimulation. Thus, the generated force accompanying the motility amplifies the vibration of the basilar membrane. This motility is concerned with both the cytoskeleton beneath the OHC plasma membrane and the protein motors distributed over the plasma membrane, because it is presumed that the cytoskeleton converts the area change in the plasma membrane induced by the conformational change of the protein motors into OHC length change in the longitudinal direction. However, these factors have not yet been clarified. In this study, therefore, the ultrastructure of the cytoskeleton of guinea pig OHCs and their mechanical properties were investigated by using an atomic force microscope (AFM). The cortical cytoskeleton, which is formed by discrete oriented domains, was imaged, and circumferential filaments and cross-links were observed within the domain. Examination of the morphological change of the cytoskeleton of the OHC induced by diamide treatment revealed a reduction of the cross-links. Results of the examination indicate that the cortical cytoskeleton is comprised of circumferential actin filaments and spectrin cross-links. Mechanical properties in the apical region of the OHC were a maximum of three times greater than those in the basal and middle regions of the cell. Moreover, Young's modulus in the middle region of a long OHC obtained from the apical turn of the cochlea and that of a short OHC obtained from the basal or the second turn of the cochlea were 2.0 ± 0.81 kPa and 3.7 ± 0.96 kPa, respectively. In addition, Young's modulus was found to decrease with an increase in the cell length.
|Title of host publication
|Biomechanics at Micro- and Nanoscale Levels
|Subtitle of host publication
|World Scientific Publishing Co.
|Number of pages
|Published - 2005 Jan 1