Measuring material properties of tectorial membranes from normal and genetically modified mice

Abstract

With the discovery of hearing disorders caused by mutations in proteins expressed in the tectorial membrane (TM), the importance of the TM in cochlear mechanics has never been clearer. However, the exact role of the TM in cochlear mechanics remains a mystery. In this thesis, I have investigated material properties of two mouse models of genetic hearing disorders that affect proteins found in the TM, a-tectorin and type XI collagen. The Tecta mutants had a missense mutation in ac-tectorin, a protein found exclusively in the TM in the organ of Corti. The effect of the mutation was to decrease the fixed charge concentration, which was found to be the primary determinant of the bulk modulus. However, the shear modulus was not affected. Collla2 is one of the genes that encodes for type XI collagen. Mutation in this gene causes no significant change in fixed charge concentration and, therefore, bulk modulus. However, the radial shear impedance is lowered. These measurements suggest that TM shear impedance is dominated by radially oriented collagen fibers and plays a key role in driving outer hair cell (OHC) bundle deflection. At the same time, the TM bulk modulus is dominated by the presence of fixed charge and may play a key role in coupling energy from outer to inner hair cells.