The cochlea of the inner ear transduces sound energy into electrical signals that are essential for audition. This transduction is processed in nano-scale vibration of the cochlear sensory epithelia. In mammals, the epithelia contain various cells and structures: inner hair cells, outer hair cells (OHCs), Deiters’ cells, basilar membrane, reticular lamina, etc. The sound elicits vibration in all these constituents. Among them, only OHCs cell body actively and periodically changes in length in association with the vibration. The unique mechanical activity of OHCs modifies the sound elicited vibration in the epithelia with a feedback mechanism. Although the modification is considered to critically contribute to the high sensitivity and sharp tuning in hearing through sensory IHCs, the real motion of OHCs remains uncertain. Vibrometrical studies of cochlear mechanics has revealed important vibration of the cell bodies involving the epithelia. However, difference in vibration pattern of the apical and basal ends of the cell has remain uncertain due to low spatial resolution of the system and low reflectivity of the cells. We performed a spectral domain OCT (SD-OCT) vibrometry by using the modified commercial SD-OCT system. Because the broad spectral bandwidth and strong power of the light source improve a performance of OCT systems in both of imaging and vibrometry, we introduced a supercontinuum light source into the commercial system. Our system achieved cellular-level tomographic imaging and subnano-scale vibration measurement in the transparent epithelia with the recording time of 100 ms in in vivo animal.
The sensory epithelium in the cochlea of the inner ear transduces sound energy into electrical signals, which are essential for a neurotransmitter release. Nevertheless, the in vivo behavior and correlation of the dynamics among each layer remains unclear. To assess the wide-range motion within a living organism, we develop multifrequency-swept optical coherence microscopic vibrometry (MS-OCMV). This method employed the wide-filed heterodyne interferometric vibration measurement technique, which can capture the full-field vibration amplitude and phase distributions with optical coherence microscopy (OCM). In this system, we newly introduced the broadband multifrequency supercontinuum (SC) as a light source. Manipulating the frequency interval of the multifrequency SC can determine the depth of the region of interest where the two-dimensional biological vibration distributed as well as to conduct 3D tomographic volumetric imaging. In this manuscript, we demonstrate the accuracy of two-dimensional surface vibration measurement and the underlying performance of 3D tomographic imaging. We report the measurement results of vibrational motions induced by sound stimuli on the sensory epithelium of a living guinea pig using MS-OCMV.
Multifrequency sensing technique adopting the wide field heterodyne detection technique is demonstrated for interior
surface vibration measurements in thick biological tissue. These arrangements allow obtaining not only 3D tomographic
images but also various vibration parameters such as spatial amplitude, phase, and frequency, with high temporal and
transverse resolutions over a wide field. The axial resolution and the accuracy of vibration amplitude measurement were
estimated to be 2.5 μm and 3 nm, respectively. This wide-field tomographic sensing method can be applied for
measuring microdynamics of a variety of biological samples, thus contributing to the progress in life sciences research.
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