Measurement of Anisotropic Mechanical Properties of the Tectorial Membrane

The solid component of the tectorial membrane (TM) is a porous matrix made up of the radial collagen fibers and the striated sheet matrix. The striated sheet matrix is believed to contribute to shear impedance in both the radial and longitudinal directions, but the molecular mechanisms involved have not been determined. A missense mutation in Tecta, a gene that encodes for the α-tectorin protein in the striated sheet matrix, causes a 60-dB threshold shift in mice with relatively little reduction in outer hair cell amplification. Here, we show that this threshold shift is coupled to changes in shear impedance, response to osmotic pressure, and concentration of fixed charge of the TM. In Tecta(Y)(1870C/+) mice, the tectorin content of the TM was reduced, as was the content of glycoconjugates reacting with the lectin wheat germ agglutinin. Charge measurements showed a decrease in fixed charge concentration from -6.4±1.4 mmol/L in wild-types to -2.1±0.7 mmol/L in Tecta(Y)(1870C/+) TMs. TMs from Tecta(Y)(1870C/+) mice showed little volume change in response to osmotic pressure compared to those of wild-type mice. The magnitude of both radial and longitudinal TM shear impedance was reduced by 10±1.6 dB in Tecta(Y)(1870C/+) mice. However, the phase of shear impedance was unchanged. These changes are consistent with an increase in the porosity of the TM and a corresponding decrease of the solid fraction. Mechanisms by which these changes can affect the coupling between outer and inner hair cells are discussed.

Section: The Contribution Of Striated-sheet Matrix To Tm Shear Impedancementioning

confidence: 99%

Tectorial Membrane Material Properties in Tecta1870/+ Heterozygous Mice

Masaki

Ghaffari

et al. 2010

“…Among hair cell organs, the presence of collagen in the overlying gelatinous matrix is unique to the mammalian TM (14,34). A computational model of TM material properties suggests that the radial stiffness of the TM is dominated by these collagen fibrils (13). Moreover, radial variations in TM shear modulus are correlated with the local density of collagen fibrils (35).…”

Section: Tm Mechanical Anisotropy Is Due To Collagenmentioning

confidence: 99%

“…It has been shown that the shear impedance of the TM is roughly twice as large in the radial direction as in other directions (9-13). A model suggests that this anisotropy is due to the presence of radially oriented collagen fibrils (13), a feature that is unique to the mammalian TM among hair cell tectorial structures (14). However, the extent to which these fibers contribute to TM mechanical properties has not been determined experimentally.…”

Section: Introductionmentioning

confidence: 99%

Col11a2 Deletion Reveals the Molecular Basis for Tectorial Membrane Mechanical Anisotropy

Masaki

Ghaffari

et al. 2009

The tectorial membrane (TM) has a significantly larger stiffness in the radial direction than other directions, a prominent mechanical anisotropy that is believed to be critical for the proper functioning of the cochlea. To determine the molecular basis of this anisotropy, we measured material properties of TMs from mice with a targeted deletion of Col11a2, which encodes for collagen XI. In light micrographs, the density of TM radial collagen fibers was lower in Col11a2 -/- mice than wild-types. Tone-evoked distortion product otoacoustic emission and auditory brainstem response measurements in Col11a2 -/- mice were reduced by 30-50 dB independent of frequency as compared with wild-types, showing that the sensitivity loss is cochlear in origin. Stress-strain measurements made using osmotic pressure revealed no significant dependence of TM bulk compressibility on the presence of collagen XI. Charge measurements made by placing the TM as an electrical conduit between two baths revealed no change in the density of charge affixed to the TM matrix in Col11a2 -/- mice. Measurements of mechanical shear impedance revealed a 5.5 +/- 0.8 dB decrease in radial shear impedance and a 3.3 +/- 0.3 dB decrease in longitudinal shear impedance resulting from the Col11a2 deletion. The ratio of radial to longitudinal shear impedance fell from 1.8 +/- 0.7 for TMs from wild-type mice to 1.0 +/- 0.1 for those from Col11a2 -/- mice. These results show that the organization of collagen into radial fibrils is responsible for the mechanical anisotropy of the TM. This anisotropy can be attributed to increased mechanical coupling provided by the collagen fibrils. Mechanisms by which changes in TM material properties may contribute to the threshold elevation in Col11a2 -/- mice are discussed.

“…While the inner ears of all vertebrates have gelatinous structures overlying the hair cell bundles, the mammalian TM has a number of unique specializations that are believed to affect its mechanical properties. The TM is anisotropic, with an increased stiffness in the radial direction that has been attributed to the presence of collagen fibers (1)(2)(3). The TM also contains a number of other proteins that contribute to its mechanical properties.…”

Section: Introductionmentioning

confidence: 99%

Frequency-Dependent Shear Impedance of the Tectorial Membrane

Hemmert²,

Freeman

et al. 2008

Microscale mechanical probes were designed and bulk-fabricated for applying shearing forces to biological tissues. These probes were used to measure shear impedance of the tectorial membrane (TM) in two dimensions. Forces were applied in the radial and longitudinal directions at frequencies ranging from 0.01-9 kHz and amplitudes from 0.02-4 microN. The force applied was determined by measuring the deflection of the probes' cantilever arms. TM impedance in the radial direction had a magnitude of 63 +/- 28 mN x s/m at 10 Hz and fell with frequency by 16 +/- 0.4 dB/decade, with a constant phase of -72 +/- 6 degrees . In the longitudinal direction, impedance was 36 +/- 9 mN x s/m at 10 Hz and fell by 19 +/- 0.4 dB/decade, with a constant phase of -78 +/- 4 degrees . Impedance was nearly constant as a function of force except at the highest forces, for which it fell slightly. These results show that the viscoelastic properties of the TM extend over a significant range of audio frequencies, consistent with a poroelastic interpretation of TM mechanics. The shear modulus G' determined from these measurements was 17-50 kPa, which is larger than in species with a lower auditory frequency range. This value suggests that hair bundles cannot globally shear the TM, but most likely cause bulk TM motion.