Non-contact optical stiffness measurements in the organ of Corti in mice.

Cochlear macromechanics and micromechanics both rely on the mechanical properties of the cells and membranes in the organ of Corti (OC). These components' stiffness have been investigated primarily using contact-based tests, which require the organ or cells to be removed from the cochlea. The approach is not only challenging but may alter the cells' stiffness as they are moved into a non-native environment. Recently, optical Brillouin microscopy has emerged as a promising tool for quantifying the mechanical property of biological specimens. This contact-free modality encourages that the stiffness of the OC cells and other components can be measured in situ and even in vivo. In the present study, we validated the feasibility of in situ Brillouin measurement on the OC cells' stiffness using fixed mouse cochleae. The results demonstrate that Brillouin microscopy has sufficient penetration depth and mechanical sensitivity to probe the OC, allowing us to differentiate the stiffness between the bone, spiral ligament, and cells; the longitudinal modulus obtained from the experiment varies between different types of OC cells in a way expected from the cells' cytoskeletal composition. This pilot study paves the way for future application of Brillouin microscopy to quantify the stiffness of OC constituents in situ in living cochleae.