Visualizing Collagen Fibrils in the Cochlea's Tectorial and Basilar Membranes Using a Fluorescently Labeled Collagen-Binding Protein Fragment

PURPOSE

A probe that binds to unfixed collagen fibrils was used to image the shapes and fibrous properties of the TM and BM. The probe (CNA35) is derived from the bacterial adhesion protein CNA. We present confocal images of hydrated gerbil TM, BM, and other cochlear structures stained with fluorescently labeled CNA35. A primary purpose of this article is to describe the use of the CNA35 collagen probe in the cochlea.

METHODS

Recombinant poly-histidine-tagged CNA35 was expressed in Escherichia coli, purified by cobalt-affinity chromatography, fluorescence labeled, and further purified by gel filtration chromatography. Cochleae from freshly harvested gerbil bullae were irrigated with and then incubated in CNA35 for periods ranging from 2 h - overnight. The cochleae were fixed, decalcified, and dissected. Isolated cochlear turns were imaged by confocal microscopy.

RESULTS

The CNA35 probe stained the BM and TM, and volumetric imaging revealed the shape of these structures and the collagen fibrils within them. The limbal zone of the TM stained intensely. In samples from the cochlear base, intense staining was detected on the side of the TM that faces hair cells. In the BM pectinate zone, staining was intense at the upper and lower boundaries. The BM arcuate zone was characterized by a prominent longitudinal collagenous structure. The spiral ligament, limbus and lamina stained for collagen, and within the spiral limbus the habenula perforata were outlined with intense staining.

CONCLUSION

The CNA35 probe provides a unique and useful view of collagenous structures in the cochlea.

Calcium signaling in interdental cells during the critical developmental period of the mouse cochlea

The tectorial membrane (TM), a complex acellular structure that covers part of the organ of Corti and excites outer hair cells, is required for normal hearing. It consists of collagen fibrils and various glycoproteins, which are synthesized in embryonic and postnatal development by different cochlear cell types including the interdental cells (IDCs). At its modiolar side, the TM is fixed to the apical surfaces of IDCs, which form the covering epithelium of the spiral limbus. We performed confocal membrane imaging and Ca²⁺ imaging in IDCs of the developing mouse cochlea from birth to postnatal day 18 (P18). Using the fluorescent membrane markers FM 4-64 and CellMask™ Deep Red on explanted whole-mount cochlear epithelium, we identified the morphology of IDCs at different z-levels of the spiral limbus. Ca²⁺ imaging of Fluo-8 AM-loaded cochlear epithelia revealed spontaneous intracellular Ca²⁺ transients in IDCs at P0/1, P4/5, and P18. Their relative frequency was lowest on P0/1, increased by a factor of 12.5 on P4/5 and decreased to twice the initial value on P18. At all three ages, stimulation of IDCs with the trinucleotides ATP and UTP at 1 and 10 μM elicited Ca²⁺ transients of varying amplitude and shape. Before the onset of hearing, IDCs responded with robust Ca²⁺ oscillations. At P18, after the onset of hearing, ATP stimulation either caused Ca²⁺ oscillations or an initial Ca²⁺ peak followed by a plateau while the UTP response was unchanged from that at pre-hearing stage. Parameters of spontaneous and nucleotide-evoked Ca²⁺ transients such as amplitude, decay time and duration were markedly reduced during cochlear development, whereas the kinetics of the Ca²⁺ rise did not show relevant changes. Whether low-frequency spontaneous Ca²⁺ transients are necessary for the formation and maintenance of the tectorial membrane e.g. by regulating gene transcription needs to be elucidated in further studies.

Cochlear outer hair cell horizontal top connectors mediate mature stereocilia bundle mechanics

Outer hair cell (OHC) stereocilia bundle deflection opens mechanoelectrical transduction channels at the tips of the stereocilia from the middle and short rows, while bundle cohesion is maintained owing to the presence of horizontal top connectors. Here, we used a quantitative noncontact atomic force microscopy method to investigate stereocilia bundle stiffness and damping, when stimulated at acoustic frequencies and nanometer distances from the bundle. Stereocilia bundle mechanics were determined in stereocilin-deficient mice lacking top connectors and with detached tectorial membrane (Strc -/-/Tecta -/- double knockout) and heterozygous littermate controls (Strc +/-/Tecta -/-). A substantial decrease in bundle stiffness and damping by ~60 and ~74% on postnatal days P13 to P15 was observed when top connectors were absent. Additionally, we followed bundle mechanics during OHC top connectors development between P9 and P15 and quantified the observed increase in OHC bundle stiffness and damping in Strc +/-/Tecta -/- mice while no significant change was detected in Strc -/-/Tecta -/- animals.

A tectorin-based matrix and planar cell polarity genes are required for normal collagen-fibril orientation in the developing tectorial membrane

The tectorial membrane is an extracellular structure of the cochlea. It develops on the surface of the auditory epithelium and contains collagen fibrils embedded in a tectorin-based matrix. The collagen fibrils are oriented radially with an apically directed slant - a feature considered crucial for hearing. To determine how this pattern is generated, collagen-fibril formation was examined in mice lacking a tectorin-based matrix, epithelial cilia or the planar cell polarity genes Vangl2 and Ptk7 In wild-type mice, collagen-fibril bundles appear within a tectorin-based matrix at E15.5 and, as fibril number rapidly increases, become co-aligned and correctly oriented. Epithelial width measurements and data from Kif3acKO mice suggest, respectively, that radial stretch and cilia play little, if any, role in determining normal collagen-fibril orientation; however, evidence from tectorin-knockout mice indicates that confinement is important. PRICKLE2 distribution reveals the planar cell polarity axis in the underlying epithelium is organised along the length of the cochlea and, in mice in which this polarity is disrupted, the apically directed collagen offset is no longer observed. These results highlight the importance of the tectorin-based matrix and epithelial signals for precise collagen organisation in the tectorial membrane.

Tectorins crosslink type II collagen fibrils and connect the tectorial membrane to the spiral limbus

All inner ear organs possess extracellular matrix appendices over the sensory epithelia that are crucial for their proper function. The tectorial membrane (TM) is a gelatinous acellular membrane located above the hearing sensory epithelium and is composed mostly of type II collagen, and α and β tectorins. TM molecules self-assemble in the endolymph fluid environment, interacting medially with the spiral limbus and distally with the outer hair cell stereocilia. Here, we used immunogold labeling in freeze-substituted mouse cochleae to assess the fine localization of both tectorins in distinct TM regions. We observed that the TM adheres to the spiral limbus through a dense thin matrix enriched in α- and β-tectorin, both likely bound to the membranes of interdental cells. Freeze-etching images revealed that type II collagen fibrils were crosslinked by short thin filaments (4±1.5nm, width), resembling another collagen type protein, or chains of globular elements (15±3.2nm, diameter). Gold-particles for both tectorins also localized adjacent to the type II collagen fibrils, suggesting that these globules might be composed essentially of α- and β-tectorins. Finally, the presence of gold-particles at the TM lower side suggests that the outer hair cell stereocilia membrane has a molecular partner to tectorins, probably stereocilin, allowing the physical connection between the TM and the organ of Corti.

Porosity controls spread of excitation in tectorial membrane traveling waves

Cochlear frequency selectivity plays a key role in our ability to understand speech, and is widely believed to be associated with cochlear amplification. However, genetic studies targeting the tectorial membrane (TM) have demonstrated both sharper and broader tuning with no obvious changes in hair bundle or somatic motility mechanisms. For example, cochlear tuning of Tectb(-/-) mice is significantly sharper than that of Tecta(Y1870C/+) mice, even though TM stiffnesses are similarly reduced relative to wild-type TMs. Here we show that differences in TM viscosity can account for these differences in tuning. In the basal cochlear turn, nanoscale pores of Tecta(Y1870C/+) TMs are significantly larger than those of Tectb(-/-) TMs. The larger pore size reduces shear viscosity (by ∼70%), thereby reducing traveling wave speed and increasing spread of excitation. These results demonstrate the previously unrecognized importance of TM porosity in cochlear and neural tuning.

Autonomous functions of murine thyroid hormone receptor TRα and TRβ in cochlear hair cells

Thyroid hormone acts on gene transcription by binding to its nuclear receptors TRα1 and TRβ. Whereas global deletion of TRβ causes deafness, global TRα-deficient mice have normal hearing thresholds. Since the individual roles of the two receptors in cochlear hair cells are still unclear, we generated mice with a hair cell-specific mutation of TRα1 or deletion of TRβ using the Cre-loxP system. Hair cell-specific TRβ mutant mice showed normal hearing thresholds but delayed BK channel expression in inner hair cells, slightly stronger outer hair cell function, and slightly reduced amplitudes of auditory brainstem responses. In contrast, hair cell-specific TRα mutant mice showed normal timing of BK channel expression, slightly reduced outer hair cell function, and slightly enhanced amplitudes of auditory brainstem responses. Our data demonstrate that TRβ-related deafness originates outside of hair cells and that TRα and TRβ play opposing, non-redundant roles in hair cells. A role for thyroid hormone receptors in controlling key regulators that shape signal transduction during development is discussed. Thyroid hormone may act through different thyroid hormone receptor activities to permanently alter the sensitivity of auditory neurotransmission.

Structural and mechanical analysis of tectorial membrane Tecta mutants

The tectorial membrane (TM) is an extracellular matrix of the cochlea whose prominent role in hearing has been demonstrated through mutation studies. The C1509G mutation of the Tecta gene, which encodes for the α-tectorin protein, leads to hearing loss. The heterozygote TM only attaches to the first row of outer hair cells (OHCs), and the homozygote TM does not attach to any OHCs. Here we measured the morphology and mechanical properties of wild-type, heterozygous, and homozygous Tecta TMs. Morphological analyses conducted with second- and third-harmonic imaging, scanning electron microscopy, and immunolabeling revealed marked changes in the collagen architecture and stereocilin-labeling patterns of the mutant TMs. The mechanical properties of the mutant TM were measured by force spectroscopy. Whereas the axial Young's modulus of the low-frequency (apical) region of Tecta mutant TM samples was similar to that of wild-type TMs, it significantly decreased in the basal region to a value approaching that found at the apex. Modeling simulations suggest that a reduced TM Young's modulus is likely to reduce OHC stereociliary deflection. These findings argue that the heterozygote C1509G mutation results in a lack of attachment of the TM to the OHCs, which in turn reduces both the overall number of OHCs that are involved in mechanotransduction and the degree of mechanotransduction exhibited by the OHCs that remain attached to the TM.

Zona pellucida domain-containing protein β-tectorin is crucial for zebrafish proper inner ear development

Background

The zona pellucida (ZP) domain is part of many extracellular proteins with diverse functions from structural components to receptors. The mammalian β-tectorin is a protein of 336 amino acid residues containing a single ZP domain and a putative signal peptide at the N-terminus of the protein. It is 1 component of a gel-like structure called the tectorial membrane which is involved in transforming sound waves into neuronal signals and is important for normal auditory function. β-Tectorin is specifically expressed in the mammalian and avian inner ear.

Methodology/Principal Findings

We identified and cloned the gene encoding zebrafish β-tectorin. Through whole-mount in situ hybridization, we demonstrated that β-tectorin messenger RNA was expressed in the otic placode and specialized sensory patch of the inner ear during zebrafish embryonic stages. Morpholino knockdown of zebrafish β-tectorin affected the position and number of otoliths in the ears of morphants. Finally, swimming behaviors of β-tectorin morphants were abnormal since the development of the inner ear was compromised.

Conclusions/Significance

Our results reveal that zebrafish β-tectorin is specifically expressed in the zebrafish inner ear, and is important for regulating the development of the zebrafish inner ear. Lack of zebrafish β-tectorin caused severe defects in inner ear formation of otoliths and function.

Tectorial membrane material properties in Tecta(Y)(1870C/+) heterozygous mice

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.

Sharpened cochlear tuning in a mouse with a genetically modified tectorial membrane

Frequency tuning in the cochlea is determined by the passive mechanical properties of the basilar membrane and active feedback from the outer hair cells, sensory-effector cells that detect and amplify sound-induced basilar membrane motions. The sensory hair bundles of the outer hair cells are imbedded in the tectorial membrane, a sheet of extracellular matrix that overlies the cochlea's sensory epithelium. The tectorial membrane contains radially organized collagen fibrils that are imbedded in an unusual striated-sheet matrix formed by two glycoproteins, alpha-tectorin (Tecta) and beta-tectorin (Tectb). In Tectb(-/-) mice the structure of the striated-sheet matrix is disrupted. Although these mice have a low-frequency hearing loss, basilar-membrane and neural tuning are both significantly enhanced in the high-frequency regions of the cochlea, with little loss in sensitivity. These findings can be attributed to a reduction in the acting mass of the tectorial membrane and reveal a new function for this structure in controlling interactions along the cochlea.

Soluble megalin is accumulated in the lumen of the rat endolymphatic sac

The endolymphatic sac (ES) is believed to play an important role in maintaining homeostasis in the inner ear by the absorption and endocytosis of endolymph. Megalin is a 600-kDa multiligand endocytic receptor expressed in certain types of absorptive epithelia including kidney proximal tubules. We analyzed the immunoreactivity for megalin in rat ES by immunofluorescence, immunogold electron microscopy, and immunoblotting. With immunostaining, the luminal substances of the ES were strongly stained for megalin. Megalin was also localized in luminal macrophage-like cells and both types of epithelial cell (mitochondria-rich cells and ribosome-rich cells). In these cells, the megalin was localized in the lumen of endosomes, but was not membrane associated. This localization pattern indicates that the megalin in these cells is not a membrane receptor, but merely one of the constituents that are endocytosed from the lumen of the ES. Immunoblotting indicated that the megalin in the ES is a 210-kDa molecule lacking a cytoplasmic domain. This suggests that the megalin in the ES may be a soluble form, different from the 600-kDa membrane-bound receptor expressed in kidneys. Taken together, it is likely that the megalin in the ES lumen is a soluble component and may be endocytosed by the ES epithelial cells. Furthermore, we found that the tectorial membrane, an acellular structure in the cochlea, gave a strong megalin immunoreaction. Since the cochlea is connected to the ES, the megalin may be transported alone or with the components of the tectorial membrane from the cochlea to the ES lumen through longitudinal flow.

Mice doubly deficient in the midkine and pleiotrophin genes exhibit deficits in the expression of beta-tectorin gene and in auditory response

alpha-Tectorin and beta-tectorin are major noncollagenous proteins of the tectorial membrane, which plays a crucial role in the reception of sonic signals in the cochlea. Midkine and pleiotrophin are closely related proteins that serve as growth factors and cytokines. In mice doubly deficient in the midkine gene and pleiotrophin gene, expression of beta-tectorin mRNA was nearly abolished in the cochlea on day 1 and 7 after birth. Expression of alpha-tectorin mRNA was unaffected in the double knockout mice, and expression of beta-tectorin mRNA was not altered in mice deficient in only the midkine or pleiotrophin gene. In newborn wild-type mice, both midkine and pleiotrophin were expressed in the greater epithelial ridge of the cochlea, in which beta-tectorin mRNA was strongly expressed. These results indicate that either midkine or pleiotrophin is required for significant expression of beta-tectorin. In agreement with the view that beta-tectorin is essential for normal auditory function, mice doubly deficient in both midkine and pleiotrophin genes exhibited very severe auditory deficits. We observed that mice deficient in either midkine or pleiotrophin gene were also impaired in their auditory response, but the level of the deficit was generally low or moderate. The present finding illustrates the importance of growth factor expression in the cochlea for auditory function.

Displacement of the reticular lamina with and without the tectorial membrane in the guinea pig cochlea

OBJECTIVE

In an attempt to establish the mechanical relationships between the reticular lamina and tectorial membrane, we studied the morphological changes of the reticular lamina on a micrometer scale when an in vitro preparation of guinea pig cochlea with and without tectorial membrane was exposed to a potassium-rich medium.

MATERIAL AND METHODS

Using video-enhanced differential interference contrast microscopy, the radial displacement of the inner hair cells (IHCs) and outer hair cells (OHCs) in the reticular lamina was measured in real time after exposure to the potassium-rich medium for 3 min.

RESULTS

The amplitude of the displacement of the OHCs in preparations with an intact tectorial membrane was half of that observed in those in which the tectorial membrane had been removed. A similar displacement response was also observed for the IHCs, although it was smaller than that for the OHCs. There was no significant difference in the amplitude of the displacement among the three rows of OHCs.

CONCLUSIONS

These results suggest that the structure linking the OHCs to the pillar cells is very elastic and that the movement of the OHCs in situ is weakly mechanically coupled to the IHCs. The tectorial membrane provides increased compliance in the motion of the IHCs and OHCs.

Distribution of glycoconjugates during cochlea development in mice: light microscopic lectin study

During development, different epithelial cells in the mouse cochlea express different cell surface glycoconjugates, which may reflect membrane specialization. Some of the lectins tested in this study (SBA, succ-WGA, and PSA) labeled the sensory cells of the cochlea around birth. Other lectins (WGA, Con A, RCA-II, and PHA-E) labeled surfaces of the sensory cells, particularly the stereocilia, from early stages of development (gestation day (GD) 16) through 21 days after birth. These may be adhesion molecules needed to attach the newly forming tectorial membrane (TM) to the stereocilia. Lectin staining of the developing TM revealed that the substructures of the TM are biochemically distinct. Lectin staining also showed the temporal sequence of the expression of cytoplasmic glycoconjugates of the cochlear epithelium during development. Biochemical changes during development are probably the result of different cells being involved in the production of glycoconjugates, and may have functional significance, specifically with regard to the expression of adhesion and/or signaling molecules.

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.

A targeted deletion in alpha-tectorin reveals that the tectorial membrane is required for the gain and timing of cochlear feedback

alpha-tectorin is an extracellular matrix molecule of the inner ear. Mice homozygous for a targeted deletion in a-tectorin have tectorial membranes that are detached from the cochlear epithelium and lack all noncollagenous matrix, but the architecture of the organ of Corti is otherwise normal. The basilar membranes of wild-type and alpha-tectorin mutant mice are tuned, but the alpha-tectorin mutants are 35 dB less sensitive. Basilar membrane responses of wild-type mice exhibit a second resonance, indicating that the tectorial membrane provides an inertial mass against which outer hair cells can exert forces. Cochlear microphonics recorded in alpha-tectorin mutants differ in both phase and symmetry relative to those of wild-type mice. Thus, the tectorial membrane ensures that outer hair cells can effectively respond to basilar membrane motion and that feedback is delivered with the appropriate gain and timing required for amplification.

Selective and transient expression of a native chondroitin sulfate epitope in Deiters' cells, pillar cells, and the developing tectorial membrane

The tectorial membrane (TM) is an acellular connective tissue overlying the sensory hair cells of the organ of Corti. Association of the tectorial membrane with the stereocilia of the sensory hair cells is necessary for proper auditory function. During development, the mature tectorial membrane is thought to arise by fusion of a "major" and "minor" tectorial membrane (Lim, Hear Res 1986;22:117-146). Several proteins and glycoconjugates have been detected in the developing TM; however, the specific molecules which mediate fusion of the two components of the TM have not been identified. In the present study, a novel monoclonal antibody (TC2) that recognizes a native epitope on glycosaminoglycans enriched in chondroitin-4-sulfate revealed a transient and restricted expression in the developing gerbil TM. The localization patterns suggest that Deiters' and pillar cells secrete a TC2-positive matrix prior to birth that later becomes incorporated into the marginal band and superior layer (cover net) of the TM. The developmental timecourse and patterns of TC2 reactivity suggest that this molecule may play a critical role in the fusion of the minor TM with the major TM.

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.

Ultrastructural organization of proteoglycans and fibrillar matrix of the tectorial membrane

The molecular and supramolecular structure of the tectorial membrane (TM) was studied by transmission electron microscopy (TEM). Collagen (type A) fibrils in the TM were found associated with proteoglycans (PGs) and type B fibrils. Most PGs were orthogonally oriented and attached D-periodically to collagen fibrils. Computer averaged projections of PG particles and linear aggregates of PGs in crystalline arrays, stained with Cuprolinic blue, showed an elongated, electron-dense structure 50-65 nm in length and 10 nm in width. Image analysis of type B fibrils showed that they are constructed of globular domains arranged with a periodicity of 12-14 nm. Each globular domain contains two thin 'arms', extended in opposite directions, which contact the 'arms' of adjacent fibrils. Numerous type B fibrils were found between collagen fibrils. They are attached to adjacent collagen fibrils by the 'arms' of their globular domains. An association of type B fibrils and PGs with collagen seems to result in the local ordered arrangement of the TM matrix. A hypothetical model of the TM matrix supramolecular structure is presented.

Antibodies to the sulphated, high molecular mass mouse tectorin stain hair bundles and the olfactory mucus layer

Polyclonal antibodies were raised in chickens to the glycosylated forms of the high (H), medium (M) and low (L) molecular mass (MM) mouse tectorins. In the mouse cochlea, all three antibodies stained the tectorial membrane. Antibodies raised to HMM tectorin also stained the hair bundles of both inner and outer hair cells. A number of other mouse tissues were screened with the anti-tectorin antibodies to look for similar or antigenically related molecules. Staining was not observed in any other tissue type with the antibodies directed against the MMM and LMM tectorins. In the nose, the anti-HMM tectorin antibodies stained Bowman's glands and the mucus layer overlying the olfactory epithelium. The surface of the adjacent respiratory epithelium was not stained by these antibodies. HMM tectorin can be specifically radiolabelled by injecting neonatal mice with 35SO4 and undergoes a shift in electrophoretic mobility following treatment with keratanase, an endo-beta-galactosidase from Pseudomonas. However, when centrifuged on shallow CsCl gradients HMM tectorin has a buoyant density similar to that of glycoproteins and does not behave as a typical cartilage type proteoglycan. HMM tectorin does not react with mab 5D4, a monoclonal antibody that recognises keratan sulphate glycosaminoglycan from corneal and skeletal muscle proteoglycan. Unlike antibodies to HMM tectorin, mab 5D4 selectively stains the upper surface of the tectorial membrane, Hensen's stripe and the mucus layer overlying the respiratory epithelium. These studies indicate that the MMM and LMM tectorins may be unique to the cochlea, and that HMM may be a "light' keratan sulphate proteoglycan that is antigenically related to either the mucins or a more specific component of the olfactory mucus layer.

Distribution of beta-tectorin mRNA in the early posthatch and developing avian inner ear

Expression of beta-tectorin mRNA in the inner ear of the embryonic and early posthatch (PH) chick was studied by in situ hybridisation. In the PH chick, beta-tectorin mRNA is expressed in the basilar papilla, in the clear and the cuboidal cells that lie either side of the papilla, in the striolar regions of the maculae, and in two small groups of cells lying adjacent to the midline in the cristae of the anterior and posterior ampullae. Expression of beta-tectorin is not observed in the lateral ampulla. In the sensory epithelia of the PH chick in which beta-tectorin mRNA is detected, expression is restricted to the supporting cell population. During development of the cochlear duct, beta-tectorin expression begins between embryonic (E) days 5 and 6. At E6, expression is observed throughout the length of the duct but is highest at the distal end. By E7, the pattern of expression is reversed and is highest at the proximal end of the cochlea, suggesting that a wave of high beta-tectorin expression passes disto-proximally along the papilla during E6 and E7. Expression of beta-tectorin mRNA is not detected in the homogene cells at any stage during the development of the cochlear duct, indicating that these cells do not synthesise one of the two major proteins of the avian tectorial membrane. The distribution of supporting cells expressing beta-tectorin mRNA in the different epithelia was compared with the distribution of sensory cells that have type B hair bundles, those with shaft links restricted to basal regions of their stereocilia, and sensory cells that have type A bundles, those with shaft links all over the entire surface of their stereocilia. Hair cells with type A hair bundles are never found in association with supporting cells expressing beta-tectorin. Although there is a correspondence in the basilar papilla and the maculae of the utriculus and lagena between the distribution of supporting cells expressing beta-tectorin mRNA and hair cells with type B bundles, this correlation does not generalise to the other sensory epithelia.

Further studies on the mechanics of the cochlear partition in the mustached bat. II. A second cochlear frequency map derived from acoustic distortion products

It has been proposed that acoustic 2f1-f2 distortions reflect the frequency characteristics of a secondary cochlear filter mechanism (Brown et al., 1992; Allen and Fahey, 1993). This concept was used to construct a second cochlear frequency map that may represent aspects of tectorial membrane (TM) tuning. Within the frequency range of 15-105 kHz, for a given f2 frequency, f1 was varied and the frequency ratio f2/f1 determined that produced maximum levels of the 2f1-f2 distortion (best ratio). The second cochlear frequency map was derived by plotting the distortion frequency that corresponded to the best ratio f2/f1 against the cochlear place of f2 which was obtained from the HRP-frequency map of Pteronotus (Kössl and Vater, 1985b). Minimum best ratios of 1.0005 and hence practically identical characteristic frequencies of the putative tuning of basilar membrane (HRP) and TM (2f1-f2) were found at about 45% distance from the base, a point at which 62 kHz are represented on the BM. This frequency is associated with strong cochlear resonance and large evoked and spontaneous otoacoustic emissions. Between 45% and 20% distance from the base, the basilar membrane (BM) tuning progressively increases to about 70 kHz whereas the calculated TM tuning remains constant at a frequency close to 62 kHz. The range of constant TM tuning coincides with the sparsely innervated cochlear region of Pteronotus where BM thickness is maximal and TM mass and limbal attachment are reduced (Vater and Kössl, 1996). We suggest that here the TM oscillates strongly at 62 kHz and may carry most of the energy of cochlear resonance which is transferred into movement of the organ of Corti at and apical to the 45% location where the BM is tuned to 62 kHz.

Filtering of distortion-product otoacoustic emissions in the inner ear of birds and lizards

When the output magnitude of more than one order of distortion-product otoacoustic emission (DPOAE) is measured, they reach their maximum at the same DPOAE frequency. This fact led several authors to the assumption that, subsequent to their generation in the cochlea, the DPOAE are band-pass filtered. It was suggested that the tectorial membrane is the structure responsible for this filtering. In this report, we show that the same kind of "DPOAE tuning' is shown by animals which have hearing organs with tectorial structures of very different morphology, or even with no tectorial membrane at all. We therefore conclude that it is unlikely that the filter is the tectorial membrane.

Low-voltage field-emission scanning electron microscopy of non-coated guinea-pig hair cell stereocilia

The stereociliar structures of the guinea-pig cochlear organ of Corti were studied at low-voltage (1-5 kV) with field-emission scanning electron microscope (SEM) using various pre- and post-fixation methods, such as OTOTO (OsO4/thiocrbohydrazide/OsO4/thiocarbohydrazide/OsO4) and TAO (tannic acid/arginine/OsO4), and different dissection procedures of the cochlea. A perfusion and immersion pre-fixation with glutaraldehyde, in combination with removal of the bony wall and stria vascularis from the cochlea, followed by the TAO non-coating treatment gave the best result at 2 kV acceleration voltage. Due to these new technique, several interesting delicate structures of the stereocilia, in particular fine surface structures, were detected for the first time using SEM. These findings include the different types of cross-links and tip links, i.e., the fine surface morphology of the stereocilia and their attachments and imprints in the tectorial membrane (TM). One of the most interesting findings in this study is a network of long filamentous structures, which has been identified mainly at the top of the longest stereocilia and the undersurface of the TM and which may represent the glycocalyx. These findings and their possible implications in the process of mechanoelectrical transduction will be discussed.

The osmotic response of the isolated, unfixed mouse tectorial membrane to isosmotic solutions: effect of Na+, K+, and Ca2+ concentration

Changes in the size, shape, and structure of the isolated tectorial membrane (TM) of the mouse were measured in response to isosmotic changes in the ionic composition of the bathing solution. Substitution of artificial perilymph (AP) for artificial endolymph (AE) caused a small (approximately 1%) shrinkage of the TM's thickness. This substitution alters not only the predominate cation (from K+ to Na+) but also the Ca2+ concentration (from 20 mumol/l to 2 mmol/l). When the predominate cation was changed from K+ to Na+, while holding Ca2+ concentration constant, results depended on Ca2+ concentration: there was a small (approximately 1%) swelling for 20 mumol/l Ca2+, larger (approximately 14%) swelling for lower (< 7 mumol/l) concentrations of Ca2+, and little response for 2 mmol/l Ca2+ or for solutions containing the Ca2+ chelator EGTA. Addition of Ca2+ while holding the predominate cation constant caused shrinkage of the TM; both removal of Ca2+ and addition of the Ca2+ chelator EGTA caused swelling. Swelling responses were largely reversible if the magnitude of the swelling was small. Responses greater than a few percent were only partially reversible and caused long-lasting changes. Changes in ionic composition of the bath affected not only the thickness of the TM but also its other dimensions. Solution changes that increase TM thickness tend to cause radial shearing motions of the surfaces of the TM, which are accompanied by small decreases in width. Little change in length was observed. Although the responses were non-isotropic, increases in thickness were highly correlated with increases in volume. Swelling of the TM was also accompanied by a reduction in prominence of its radially oriented fibrillar structure. These results for the isolated TM of the mouse are qualitatively similar to those obtained previously for the isolated chick TM (Freeman et al., 1994) but different from those obtained for the in vitro mouse TM (Kronester-Frei, 1979a).

Developmental expression of proteoglycans in the tectorial and basilar membrane of the gerbil cochlea

The appearance and distribution of specific proteoglycans (PGs) was assessed during development and maturation of the tectorial (TM) and basilar membranes (BM) in the gerbil cochlea. At birth, monoclonal antibodies against keratan sulfate (KSPG) and chondroitin 4- or 6-sulfate (4S CSPG, 6S CSPG) reacted with the upper fibrous layer of the TM with staining for anti-KSPG predominating. Reactivity for 4S CSPG remained constant whereas that for 6S CSPG increased through day 20 when it exceeded that of 4S CSPG. The region of Köllikers organ near the developing tunnel of Corti stained positively with all three PG antibodies from birth through day 8. In contrast, cells in the developing inner spiral sulcus lacked immunoreactive KSPG but expressed CSPG. PGs were first detectable in the BM of the basal turn at day 8 and increased to near adult levels by 16 days after birth. Anti-KSPG again showed the strongest staining with labeling density for 4S and 6S CSPG being about equal at maturity. Staining with all three antibodies was localized along the margins of the BM. Reactivity of the TM and BM in the upper turns lagged behind that of the basal turns by 24-48 h. Our results show that the TM is relatively mature at birth, needing only minor changes in its PG content to reach adult levels. In contrast, the BM showed a marked increase in its content of PGs during a period corresponding to the onset and rapid development of auditory function.

Expression of the carbohydrate epitope 3-fucosyl-N-acetyl-lactosamine (CD 15) in the adult guinea pig inner ear

The expression of the CD 15 (3-fucosyl-N-acetyl-lactosamine) epitope was immunohistochemically studied on paraffin sections of adult guinea pig inner ears. Two regions of the inner ear expressed the epitope for CD 15: the tectorial membrane of the cochlea and the endolymphatic sac. The upper part of the main body of the tectorial membrane was deeply stained. In the rugosal and distal part of the endolymphatic sac several unevenly distributed cells showed strong intra- and extracellular localization of the CD15 epitope. The CD15 epitope is associated with a transduction structure (tectorial membrane) and with a "volume regulating" compartment (endolymphatic sac) and may be involved in the maintenance of the structural integrity of both.

Morphological changes in the tectorial and basilar membranes of aged rats

Using both light and transmission electron microscopy presbycusic degeneration of the cochlea was observed in particular in the tectorial and basilar membranes, in naturally aged rats. These animals showed a descending auditory pattern as determined by auditory brainstem response. Ultrastructurally, the number of collagen fibers in the tectorial membrane was reduced and straight type A fibers were increased relative to branched, coiled type B fibers. The basilar membrane in the basal turn was also thickened by an increased homogeneous ground substance. These findings indicate that the specificity of vibration of the tectorial and basal membranes is very different in aged and young rats.

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.

Pilocarpine-induced changes in the saccharide composition of the tectorial membrane and interdental cells of the organ of Corti: a study with gold-labeled lectins

The glycoconjugates in the cytoplasm of inner ear interdental cells and those constituting the limbal tectorial membrane were identified by a post-embedding cytochemical method using low-temperature embedding in Lowicryl K4M and labeling with biotinylated lectins, goat anti-biotin antibody, rabbit anti-goat antibody, and gold-labeled protein A in control animals, and after the systemic injection of pilocarpine. The lectins used were ConA, PHA-E, PSA, RCA, SBA, Succ-WGA, UEA, and WGA. In control animals, a semiquantitive analysis of gold particles showed that Succ-WGA produced the strongest labeling on the tectorial membrane, followed by SBA, ConA, WGA, RCA, PHA-E, and PSA. The lowest values were obtained with UEA. The cytoplasm of the interdental cells was also labeled with all the lectins, but the number of particles/microns2 was lower than on the tectorial membrane. The concentration of gold particles on the limbal tectorial membrane in pilocarpine-treated animals was higher than in control animals for some lectins (RCA, PSA, UEA) but lower for others (WGA, SBA, PHA-E, Succ-WGA). The changes in the labeling pattern of the cytoplasm of the interdental cells paralleled those in the tectorial membrane. These results demonstrate that the saccharide composition of the limbal tectorial membrane can be modified by systemic injection of pilocarpine. This action may take place through a change in either the secretion rate or the amount of some glycoconjugates by the interdental cells.

Incorporation of D3H glucosamine to the adult and developing cochlear tectorial membrane of normal and hypothyroid rats

The uptake of D-3H-glucosamine by the developing cochlea of normal and hypothyroid rats was examined using light microscopic radioautography. During postnatal development, normal and hypothyroid rat cochleas exhibited a layer of radiolabelling in the tectorial membrane (TM). This layer first appeared in the TM region which covers the spiral limbus and the Kölliker's organ (KO), then progressively reached the apical part of the TM covering the organ of Corti. Radiolabelling was significantly greater in hypothyroid than in normal cochleas. These findings suggests that the enormous size reached by the TM in the congenital hypothyroidism could be related to an increase of epithelial secretion, at least for carbohydrates. It also suggests that TM, in normal and hypothyroid cochleas, could be formed during development by the addition of successive layers. Older layers could be displaced upwards by the new ones. Cochleas of normal young adult rats, treated with D-3H-glucosamine, showed a very scarce and diffuse radiolabelling. Cochleas of hypothyroid young adult rats exhibited a thickened and distorted TM, which incorporated a significant amount of carbohydrates. These results suggest that TM secretion is highly reduced in young adult normal animals, while in young adult hypothyroid ones it is still active. During cochlear maturation, thyroxine seems to be necessary, not only for the synthesis of normal glycoproteins (as suggested by previous reports), but also for the control of glycoprotein secretion.

[Biochemical profile of proteoglycans and glucosaminoglycans of the mammalian tectorial membrane]

The tectorial membrane is an acellular connective tissue which plays an essential role in cochlear function. While a comparatively large amount of information is available on the collagen network of the tectorial membrane, studies on the biochemical nature of this highly hydrated matrix, which is composed of proteoglycans (PGs) and glycosaminoglycans (GAGs), have been quite limited. Previous reports on the biochemical analysis of the tectorial membrane have failed to detect uronic acid, which is present in large amounts in all mammalian GAGs except keratan sulfate. Applying a colorimetric assay based on the binding of GAGs to cationic dye Safranin-0 in combination with enzymatic techniques, we were able to measure GAGs in the murine tectorial membrane. Approximately 0.3% uronic acid-containing GAGs (mainly in the form of chondroitin/dermatan sulfate) and 0.17% keratan sulfate were detected in the tectorial membrane (both on a wet weight basis). In addition, various types of electrophoresis revealed one large PG with a molecular mass similar to that of the large type cartilage PGs and three small PGs, containing chondroitin sulfate and keratan sulfate side chains, respectively. Judging by coelution of standards, one of the small PGs seemed to correspond to fibromodulin, which has at least one keratan sulfate side chain, and binds to type I and type II collagen to regulate collagen organization in tissues. Our results suggest: (1) Donnan equilibrium is established in the tectorial membrane because sulfated GAGs are highly negatively charged and consequently bring about an influx of large amounts of water and cations into the matrix.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural localization and semiquantitative analysis of glycoconjugates in the tectorial membrane

The tectorial membrane of the gerbil cochlea was analyzed with lectin-gold cytochemical methods for demonstrating and characterizing glycoconjugates (GCs) in situ. Binding of lectins from Limax flavus (LFA), Lens culinaris (LCA), Datura stramonium (DSA), Ricinus communis (RCA I), Ulex europeus (UEA I) and Phaseolus vulgaris (PHA L) was assayed semiquantitatively on ultrathin sections. Binding occurred throughout the tectorial membrane with all lectins except UEA I but the labelling density with a given lectin differed among substructures. The cover net disclosed the highest level of GC with four lectins whereas the fibrous layer revealed the lowest level. DSA, LCA and PHA L demonstrated considerable similarity between the cover net and the marginal band in content of GC with N-linked oligosaccharide. The cover net differed from the marginal band, however, in containing more RCA I reactive GC with terminal lactosamine. Hensen's stripe, with which inner hair cell stereocilia are thought to interact, differed from other substructures in containing the highest level of PHA L-reactive traintennate N-linked chains and except for the basal layer the lowest concentration of GC with terminal lactosamine. Fucosylated GC detectable with UEA I-gold was present at low levels in all substructures except the cover net and marginal band. Distribution of GCs in the fibrous layer and less consistently in the cover net differed between limbal and middle zones. The differences observed here in the carbohydrate composition among substructures in the tectorial membrane support and extend previous cytochemical observations and imply a role for different classes of GCs in determining the biophysical and physiological properties of the tectorial membrane.

Incorporation of D-[3H]-glucosamine and L-[3H]-fucose into the developing rat cochlea

The uptake of two tritiated carbohydrates, D-[3H]-glucosamine and L-[3H]-fucose, to the developing rat cochlea was examined using light and electron microscopic radioautography. Both carbohydrates, administered to in vitro developing rat cochleas, shared a similar ultrastructural labeling pattern on the microvilli and apical cell region and on the tectorial membrane (TM) fibrils. On embryonic day 18, the radiolabeling appeared on the apical surface of the undifferentiated epithelium that will develop into both spiral limbus and Kölliker's organ (KO), while on postnatal day (PD) 1, it was only located on the apical surface of the KO. When D-[3H]-glucosamine was administered in vivo to newborn rats, the radiolabeling was observed in the TM covering the KO at PD 3. Lastly, D-[3H]-glucosamine administered in vivo to PD 7 rats, appeared at PD 9 in the TM region lying just above the organ of Corti. The present findings support the previously suggested leading role of the spiral limbus and KO in the secretion of the TM during cochlear development. The secretion of carbohydrates, and probably of other matrix components, starts on the spiral limbus and KO region and progressively extends to the organ of Corti.

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.

Mechanically induced length changes of isolated outer hair cells are metabolically dependent

Isolated outer hair cells from the organ of Corti of the guinea pig have been shown to change length in response to a mechanical stimulus in the form of a tone burst at a fixed frequency of 200 Hz (Canlon et al., 1988). In the present study, the threshold of movement for individual outer hair cells is related to the original length of the cell such that long cells are more sensitive than short cells for all cochlear locations studied. Length changes could be elicited when the stimulus was projected at any site along the longitudinal axis of the plasma membrane. Length changes were not elicited when the stereocilia were stimulated directly. These mechanically-induced length changes were found to be metabolically dependent. In the presence of either sodium cyanide or 2,4-dinitrophenol, the motile response of outer hair cells was completely blocked within 30 min. When the extracellular pH was altered in a graded fashion, the motile response decreased gradually. Furthermore, 3 microM poly-L-lysine or poly-D-lysine of different molecular weights were also effective in blocking the motile response, whereas the negatively charged polyaminoacid, poly-L-aspartate, was not effective. Fluorescently-labelled poly-lysine demonstrated that the plasma membrane, stereocilia, and nucleus were the most intensely stained structures of the outer hair cells. It is suggested that the passive influx of poly-lysine is responsible for the inhibition of the motile response. Finally, the finding that the bidirectional motile response of isolated outer hair cells induced by mechanical stimulation is dependent on the metabolic state of the cell distinguishes this type of motility from the electrically induced outer hair cell shape changes.

Immunohistochemical identification of proteoglycans in gelatinous membranes of cat and gerbil inner ear

Proteoglycans have been identified in gelatinous membranes of adult cat and gerbil inner ears using highly specific histochemical techniques. The tectorial and otoconial membranes and cupula of both species stained strongly with high iron diamine which is specific for sulfate esters and with monoclonal antibody against keratan sulfate proteoglycan (KSPG). The cat tectorial membrane also showed strong immunoreactivity with monoclonal antibody against chondroitin sulfate proteoglycan (CSPG) but the gerbil tectorial membrane reacted only weakly with this antibody. Otoconial membranes and the cupula of both species showed little if any immunostaining with antibodies against CSPG. Supporting cells in the vestibular neurosensory epithelium and planum semilunatum cells in the ampullae of the cat stained strongly with anti-KSPG, demonstrating the origin of KSPG in the cat. These cell types failed to stain in the gerbil, however, suggesting a different mechanism of secretion or a slower rate of turnover of membraneous KSPG in the gerbil. Interdental cells of both species failed to react with either antibody, leaving the origin of tectorial membrane proteoglycans in question. The approach used here provides a highly sensitive and reliable means of assessing the contribution of specific proteoglycans to inner ear structure and function.

Localization of anionic sulfate groups in the tectorial membrane

Colloidal iron hydroxide (CIH) staining demonstrates the existence of anionic sulfate groups of glycoconjugates associated with several constituents of the tectorial membrane (TM). In the adult animal, labelling in the main body of the TM appears as long, electron-dense patches surrounding type A fibrils which show alternating stained and unstained zones. On the other hand, labelling of the fibrils of the matrix of the TM appears as single, CIH particles with no special arrangement. Some of the structurally distinct regions of the TM are also labelled (limbal zone, Hensen's stripe and inner portions of the cover net), while others are not (marginal band and outer portions of the cover net). Staining of type A fibrils in the major TM is already present in newborn animals; while, both the outermost region of the TM closest to the cells of the organ of Kölliker and the minor TM are not labelled. The implications of these distributions of sulfated glycoconjugates for the electrochemical properties of the TM are discussed.

Effects of perinatal hypothyroidism in the carbohydrate composition of cochlear tectorial membrane

The presence of carbohydrates in the cochlear tectorial membrane (TM) of normal and hypothyroid rats was analyzed using fluorescent lectin probes. SBA and WGA lectins exhibited a similar reactivity in both normal and hypothyroid TMs. DBA, RCA1, UEA1 and Con A lectins were also reactive, although they showed a different distribution pattern between normal and hypothyroid TMs. Lastly, one of the lectins, PNA, was only labeled in hypothyroid TMs. These findings suggest that carbohydrate chains containing residues of N-acetyl-D-galactosamine (GalNAc) and N-acetyl-D-glucosamine, are similarly distributed in normal and hypothyroid TMs. Other carbohydrate residues as GalNAc alpha 1,3 GalNAc, D-galactose (Gal), L-fucose and D-mannose, are present, but are abnormally distributed in hypothyroid TMs. The Gal beta 1,3GalNAc residues, recognized by PNA, could be present only in the hypothyroid TMs. Alterations in glycosylation of the glycoproteins in the hypothyroid TM could be responsible for the abnormal distribution pattern of carbohydrate residues here described, and for the distorted shape of the hypothyroid TM.

Gelsolin immunoreactivity and development of the tectorial membrane in the cochlea of normal and hypothyroid rats

Gelsolin was localized by immunocytochemistry in the developing cochlea of the rat. In normal animals, the protein appeared at 18 th day in utero in cells of the Kölliker's organ, which are involved in the secretion of the tectorial membrane. The Kölliker's organ cells were not immunoreactive after the first postnatal week, which is when they cease their secretory activity. Gelsolin immunoreactivity was similar in thyroid-deficient rats until the second postnatal week but, at this age, Kölliker's organ did not transform and its gelsolin immunoreactivity persisted, together with its secretory activity. As a result, the tectorial membrane was greatly distorted and out of contact with the hair cells, which dramatically impaired the mechanical properties of the organ of Corti. The developing cochlea thus provides an example of the involvement of gelsolin in a secretory process that is of importance in the development of hearing.

Developmental aspects of glycoprotein secretion and migration in the endolymphatic space

Secretion and migration of sulphated glycoproteins during the early postnatal development of the rat inner ear was studied autoradiographically with the use of 35SO4. This isotope was found to be selectively incorporated into the embryonal sulcus cells and subsequently expelled into the tectorial membrane. This process decreased simultaneously with the replacement of the tall sulcus cells by cuboidal cells and the detachment of the tectorial membrane from these cells. A two stage development of the tectorial membrane is postulated: formation of filaments followed by incorporation of glycoproteins. Incorporation of isotope was also observed in the sensory epithelium of the vestibular part of the labyrinth, followed by migration into the cupulae and otolithic membranes. The accumulation of labelled material in the lumen of the endolymphatic sac 6 hours after isotope administration, suggests the existence of endolymph flow towards the endolymphatic sac in the early postnatal period.