An ultrastructural study of the guinea pig tectorial membrane 'type A' protofibril

Sound-evoked deflections of outer hair cell stereocilia arise from tectorial membrane anisotropy

The exceptional performance of mammalian hearing is due to the cochlea's amplification of sound-induced mechanical stimuli. During acoustic stimulation, the vertical motion of the outer hair cells relative to the tectorial membrane (TM) is converted into the lateral motion of their stereocilia. The actual mode of this conversion, which represents a fundamental step in hearing, remains enigmatic, as it is unclear why the stereocilia are deflected when pressed against the TM, rather than penetrating it. In this study we show that deflection of the stereocilia is a direct outcome of the anisotropic material properties of the TM. Using force spectroscopy, we find that the vertical stiffness of the TM is significantly larger than its lateral stiffness. As a result, the TM is more resistant to the vertical motion of stereocilia than to their lateral motion, and so they are deflected laterally when pushed against the TM. Our findings are confirmed by finite element simulations of the mechanical interaction between the TM and stereocilia, which show that the vertical outer hair cells motion is converted into lateral stereocilia motion when the experimentally determined stiffness values are incorporated into the model. Our results thus show that the material properties of the TM play a central and previously unknown role in mammalian hearing.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Dynamic material properties of the tectorial membrane: a summary

Dynamic material properties of the tectorial membrane (TM) have been measured at audio frequencies in TMs excised from the apical portions of mouse cochleae. We review, integrate, and interpret recent findings. The mechanical point impedance of the TM in the radial, longitudinal, and transverse directions is viscoelastic and has a frequency dependence of the form 1/(K(j2pif)(alpha)) for 10<or=f<or=4000 Hz, where f is frequency, K is a constant, j=-1, and alpha approximately 0.66. Comparison with other connective tissues shows that the TM is a relatively lossy viscoelastic material. The median magnitudes of the point impedance at 10 Hz in the radial, longitudinal, and transverse directions are 4.6 x 10(-3) N.s/m, 1.8 x 10(-3) N.s/m, and 2.7 x 10(-3) N.s/m. Consistent with osmotic responses (Freeman et al., 2003), the TM point impedance is anisotropic - the TM is stiffer in the radial than in the longitudinal and transverse directions. The mechanical space constant of the TM is approximately 20 microm. Comparisons reveal that in the apical region of the mouse cochlea, the TM dynamic stiffness at 10 Hz is 10 times larger than the static stiffness of the aggregate hair cells in a mechanical space constant and roughly comparable to the stiffness of the basilar membrane. We conclude that the TM provides a mechanical load on the basilar membrane and that the lability of the TM to changes in endolymph composition may well be reflected in changes in basilar membrane motion.

Static material properties of the tectorial membrane: a summary

The tectorial membrane (TM) is a polyelectrolyte gel. Hence, its chemical, electrical, mechanical, and osmotic properties are inextricably linked. We review, integrate, and interpret recent findings on these properties in isolated TM preparations. The dimensions of the TM in alligator lizard, chick, and mouse are sensitive to bath ion concentrations of constituents normally present in the cochlear fluids - an increase in calcium concentration shrinks the TM, and an increase in sodium concentration swells the TM in a manner that depends competitively on the calcium concentration. The sodium-induced swelling is specific; it does not occur with other alkali metal cations. We interpret these findings as due to competitive binding of sodium and calcium to TM macromolecules which causes a change in their conformation that leads to a change in mechanical properties. In mouse TM, decreasing the bath pH below 6 or increasing it above 7 results in swelling of the TM. Electric potential measurements are consistent with the notion that the swelling is caused by a pH-driven increase in positive fixed charge at low pH and an increase in the magnitude of the negative fixed charge at high pH which is consistent with the known protonation pattern of TM macromolecules. Increasing the osmotic pressure of the bathing solution with polyethylene glycol shrinks the TM and decreasing the ionic strength of the bathing solution swells the TM. Both results are qualitatively consistent with predictions of a polyelectrolyte gel model of the TM.

The otoconia of the guinea pig utricle: internal structure, surface exposure, and interactions with the filament matrix

A unique feature of the vertebrate gravity receptor organs, the saccule and utricle, is the mass of biomineral structures, the otoconia, overlying a gelatinous matrix also called "otoconial membrane" on the surface of the sensory epithelium. In mammals, otoconia are deposits of calcium carbonate in the form of composite calcite crystals. We used quick-freezing, deep etching to examine the otoconial mass of the guinea pig utricle. The deep-etching step exposed large expanses of intact and fractured otoconia, showing the fine structure and relationship between their internal crystal structure, their surface components, and the filament matrix in which they are embedded. Each otoconium has a compact central core meshwork of filaments and a composite outer shell of ordered crystallites and macromolecular aggregates. A distinct network of 20-nm beaded filaments covers the surface of the otoconia. The otoconia are interconnected and secured to the gelatinous matrix by surface adhesion and by confinement within a loose interotoconial filament matrix. The gelatinous matrix is a dense network made of yet another type of filament, 22 nm in diameter, which are cross-linked by shorter filaments, characteristically 11 nm in diameter. Our freeze-etching data provide a structural framework for considering the molecular nature of the components of the otoconial complex, their mechanical properties, and the degree of biological versus chemical control of otoconia biosynthesis.

Type A fibril of the mouse tectorial membrane shows D-periodicity: an atomic force microscopic examination

This study demonstrated that type A fibrils of the mouse tectorial membrane showed a morphology characteristic of collagen, as demonstrated using an atomic force microscope. In the topographical imaging mode, the surface of the type A fibrils showed a periodic pattern, consisting of alternating grooves and ridges. The periodicity of the type A fibrils was 69.1 +/- 0.6 nm, which is in accordance with characteristic collagen D-periodicity. The difference in height between grooves and ridges was 1.6 +/- 0.3 nm. In the variable deflection imaging mode, the type A fibrils showed a clear banding pattern, which consisted of alternating light and dark zones, with D-periodicity. In addition, the type A fibrils exhibited one minor dark band in the light zone and one minor light band in the dark zone.

Tectorial membrane-organ of Corti relationship during cochlear development

The development of stereociliary attachment to the tectorial membrane was investigated in the mouse cochlea using transmission and scanning electron microscopy. At the 18th gestational day, only the major tectorial membrane can be identified covering the greater epithelial ridge and the inner hair cells in all turns. At the 19th gestational day, the minor tectorial membrane was first seen in the basal turn, over the outer hair cells. During early stages of development, the stereocilia of hair cells were surrounded by a loose fibrillar material underneath the tectorial membrane. After the 10th postnatal day, the outer hair cells' stereocilia were attached to Kimura's (or Hardesty's) membrane, while inner hair cells' stereociliary bundles were attached to the undersurface of the tectorial membrane near the Hensen's stripe. Between the 10th and the 14th postnatal days, the space between the inner hair cells and the first row of outer hair cells widened by virtue of the growth of the heads of pillar cells, and the inner hair cells' stereocilia were displaced towards the Hensen's stripe. After the 14th postnatal day, the inner hair cells' stereociliary bundles detached from the tectorial membrane, while the outer hair cells' stereocilia remained attached to it. The tip-link system, which connects the tips of the stereocilia to the next tallest stereocilia, is present at birth in the outer hair cells. The marginal pillar, that anchored the tectorial membrane to the underlying organ of Corti during development, first appeared on the 6th postnatal day and disappeared on the 14th-15th postnatal day. The present data together with other reports support the idea that although some structures, such as hair cells' stereocilia and innervation, are already formed early during development, the cochlear microarchitecture is not fully developed morphologically and ready to function normally until the end of the second postnatal week in the mouse.

Crystalline arrays of proteoglycan and collagen in the tectorial membrane

Ultrastucture of the tectorial membrane in the chinchilla cochlea was studied by transmission electron microscopy using different fixatives and staining procedures. It was shown that the tectorial membrane is a highly structured matrix composed of collagen type A fibrils, noncollagenous type B fibrils and proteoglycan. The localization of type B fibrils surrounding bundles of parallel type A fibrils was observed. Staining of the tectorial membranes with the cationic dye Cuprolinic blue in a "critical electrolyte concentration" method revealed proteoglycan, D-periodically associated with collagen type A fibrils and orthogonal to them. The appearance and size of the proteoglycan, and its binding to collagen, were similar to small proteoglycans observed in cartilage and other tissues. In many regions of the tectorial membrane the collagen-bound proteoglycan forms crystalline-like arrays. The images of these arrays processed by Fourier analysis show long linear aggregates of proteoglycan arranged parallel each other.

Immunohistochemical localization of keratan sulfate and chondroitin 4- and 6-sulfate proteoglycans in subregions of the tectorial and basilar membranes

Proteoglycans containing keratan sulfate (KSPG) and 4- and 6-sulfated epitopes of chondroitin sulfate (CSPG) were identified in distinct domains of the tectorial and basilar membranes by ultrastructural immunogold labeling with monoclonal antibodies. In the tectorial membrane (TM), the highest concentration of gold particles was present in the upper fibrous layers of the limbal, middle and marginal zones with all three antibodies. Reactivity with anti-KSPG exceeded that with anti-4S and anti-6S CSPG, especially in the marginal zone. The cover net showed no reactivity for any antibody. Labeling density of gold particles with all three antibodies increased markedly from base to apex. In the basilar membrane (BM), all three PGs were most highly concentrated in regions of amorphous ground substance bordering the upper and lower filamentous bands. As in the TM, reactivity for anti-KSPG in the BM exceeded that for either CSPG antibody and staining with all three antibodies was stronger and more widespread in the apical as compared to the basal turns. These results provide the first ultrastructural demonstration of KSPG and CSPG in distinct subregions of the TM and BM. The preferential distribution and marked increase in PGs from base to apex in both TM and BM supports a role for these macromolecules in regulating structural and mechanical properties of these highly specialized extracellular membranes.

A radial gradient of fibril density in the gerbil tectorial membrane

The tectorial membrane plays a key role in the transduction of mechanical to neural energy in the inner ear. To better understand the transduction process the composition of the tectorial membrane needs to be elucidated. This study was done to determine if Type A collagen fibrils are distributed homogeneously in the tectorial membrane or if there are longitudinal or radial gradients of fibril concentrations. Our results suggest that while there is no longitudinal gradient, there is a radial gradient of fibril concentration. The concentration of fibrils in the limbal (inner) zone of the tectorial membrane exceeds that in the marginal (outer) zone in all cochlear locations examined. This gradient is most marked in the basal, high frequency coding region of the cochlea. While fibril gradients in the tectorial membrane have not been the focus of previous investigations, several findings by other authors support the proposition that the marginal zone of the tectorial membrane is more compliant than the limbal zone. This radial gradient of tectorial membrane stiffness is likely to contribute to the characteristics of movement of the cochlear partition.

Glycoconjugates in the cochlea as revealed by the silver methenamine method

The glycoconjugates in the cochlea of the guinea pig were studied by staining samples by the silver methenamine method as well as after periodic acid-Schiff (PAS) staining. Results obtained by the two methods were similar but not identical. The silver methenamine method was much better in terms of resolution. However, this method of staining seemed less specific than the PAS reaction. When the silver methenamine method was used, the tectorial membrane and outer hair cells were specifically stained. Two types of fibrils were observed in the tectorial membrane. Thick fibrils were located in the fibrous layer. Thin fibrils were situated in the marginal band, the cover net, Hensen's stripe and the fibrous layer. The thick and thin fibrils appeared to correspond to type A and type B protofibrils, respectively. The outer hair cells were found to contain strongly stained particles which, presumably, consisted of glycogen. The basement membrane of the capillaries in the stria vascularis also gave a positive reaction, while that of other capillaries was essentially unstained. This finding suggests structural differences between these capillaries.

Collagen of accessory structures of organ of Corti

It was previously demonstrated that about 40% of the protein of the tectorial membrane of the guinea pig consists of collagen type II, with lesser amounts of type IX and XI. In this paper we extend these studies on the tectorial membrane to the basilar membrane and to other accessory structures, the spiral ligament and spiral limbus. Earlier immunohistochemical data indicated that no collagen type II is present in the basilar membrane of the newborn guinea pig, but that it is present in the area of the basilar membrane in the embryo. However, by means of stringent extraction procedures we have determined biochemically that collagen type II and lesser amounts of type XI are present in the basilar membrane of the adult guinea pig, at similar levels (on the basis of total protein) to the tectorial membrane. Levels of collagen type II are much lower in the spiral ligament and spiral limbus. The presented studies demonstrate that classical techniques of collagen chemistry can be applied at the microscale on minute tissue elements. The significance of the presence of collagen in the tectorial membrane and basilar membrane is discussed in the light of known mechanical properties of these structures.

Histochemical analysis of glycoconjugates in gelatinous membranes of the gerbil's inner ear

The gelatinous membranes of the gerbil inner ear were analyzed histochemically for glycoconjugates with a battery of twenty horseradish peroxidase-conjugated lectins. Glycoconjugates with mannose (Man) and/or glucose (Glc), galactose (Gal), fucose (Fuc), N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc) and N-acetylneuraminic acid (NeuAc) were detected in the tectorial and otolithic membranes and cupula. Differences in lectin reactivity were observed between tectorial and vestibular membranes and also among zones and between the medial and lateral regions of the middle zone of the tectorial membrane. The distribution of staining differed markedly for several lectins that bind specifically to GalNAc or to GlcNAc but vary in affinity for oligosaccharides containing these sugars in different sequences or linkages. The findings suggest presence of the terminal disaccharides GalNAc alpha 1,3Gal in tectorial membrane and Gal beta 1,3GalNAc in vestibular membranes. Lectin binding profiles provided evidence that the limbal zone's fibrous and attachment layers contain mainly O-glycosidically linked oligosaccharides whereas the middle zone's medial fibrous layer contains both O- and N-linked chains. The remaining regions of the tectorial membrane contain mainly N-linked oligosaccharides with bisected biantennary type chains predominating. Additionally, the marginal band and the middle zone's basal layer contain abundant N-linked oligosaccharides with a triantennary structure.