The tectorial membrane: one slice of a complex cochlear sandwich

Abstract

Purpose of review: The review is both timely and relevant, as recent findings have shown the tectorial membrane plays a more dynamic role in hearing than hitherto suspected, and that many forms of deafness can result from mutations in tectorial membrane proteins.

Recent findings: Main themes covered are the molecular composition, the structural organization and properties of the tectorial membrane, the role of the tectorial membrane as a second resonator and a structure within which there is significant longitudinal coupling, and how mutations in tectorial membrane proteins cause deafness in mice and men.

Conclusion: Findings from experimental models imply that the tectorial membrane plays multiple, critical roles in hearing. These include coupling elements along the length of the cochlea, supporting a travelling wave and ensuring the gain and timing of cochlear feedback are optimal. The clinical findings suggest stable, moderate-to-severe forms of hereditary hearing loss may be diagnostic of a mutation in TECTA, a gene encoding one of the major, noncollagenous proteins of the tectorial membrane.

Figure 1

a) Diagram showing how the hair cells of the organ of Corti lie sandwiched between two extracellular matrices, the basilar (BM) and tectorial (TM) membranes. Three zones can distinguished in the TM, the limbal zone (LZ) by which the TM is attached to the spiral limbus, the medial zone (MZ) that stretches over both the internal sulcus (IS) and organ of Corti (OC), and the lateral marginal band (MB). HS = Hensen’s stripe, CN = covernet. The longitudinal, radial and transverse axes of the cochlear partition are indicated to the right. b) Domain structures of the three non-collagenous glycoproteins of the TM. Red = vWF type D repeat, purple = vWF type B repeat, green = ZP domain, C = cysteine knot, TSP = threonine/serine/proline rich region, ENT = nidogen/entactin G1 domain, black = hydrophobic N-terminal signal peptide (right) or hydrophobic C-termini (left), yellow = region lacking any significant homology to other proteins. c) A Toluidin blue stained section from the mouse cochlear duct (centre) and transmission electron micrographs (surrounding) showing the ultrastructure of the different regions and peripheral features of the TM.

Figure 1

a) Diagram showing how the hair cells of the organ of Corti lie sandwiched between two extracellular matrices, the basilar (BM) and tectorial (TM) membranes. Three zones can distinguished in the TM, the limbal zone (LZ) by which the TM is attached to the spiral limbus, the medial zone (MZ) that stretches over both the internal sulcus (IS) and organ of Corti (OC), and the lateral marginal band (MB). HS = Hensen’s stripe, CN = covernet. The longitudinal, radial and transverse axes of the cochlear partition are indicated to the right. b) Domain structures of the three non-collagenous glycoproteins of the TM. Red = vWF type D repeat, purple = vWF type B repeat, green = ZP domain, C = cysteine knot, TSP = threonine/serine/proline rich region, ENT = nidogen/entactin G1 domain, black = hydrophobic N-terminal signal peptide (right) or hydrophobic C-termini (left), yellow = region lacking any significant homology to other proteins. c) A Toluidin blue stained section from the mouse cochlear duct (centre) and transmission electron micrographs (surrounding) showing the ultrastructure of the different regions and peripheral features of the TM.

Figure 1

a) Diagram showing how the hair cells of the organ of Corti lie sandwiched between two extracellular matrices, the basilar (BM) and tectorial (TM) membranes. Three zones can distinguished in the TM, the limbal zone (LZ) by which the TM is attached to the spiral limbus, the medial zone (MZ) that stretches over both the internal sulcus (IS) and organ of Corti (OC), and the lateral marginal band (MB). HS = Hensen’s stripe, CN = covernet. The longitudinal, radial and transverse axes of the cochlear partition are indicated to the right. b) Domain structures of the three non-collagenous glycoproteins of the TM. Red = vWF type D repeat, purple = vWF type B repeat, green = ZP domain, C = cysteine knot, TSP = threonine/serine/proline rich region, ENT = nidogen/entactin G1 domain, black = hydrophobic N-terminal signal peptide (right) or hydrophobic C-termini (left), yellow = region lacking any significant homology to other proteins. c) A Toluidin blue stained section from the mouse cochlear duct (centre) and transmission electron micrographs (surrounding) showing the ultrastructure of the different regions and peripheral features of the TM.

Figure 2

Basic mechanical model of a cochlear segment incoporating the TM resonance. Masses of the tectorial and basilar membrane in each segment are M_TM and M_BM, respectively. K_BM denotes stiffness of the basilar membrane. K_TM is stiffness of the TM’s limbal attachment. Movements of the masses are damped by corresponding dash-pots R_TM and R_BM. Vibrations of the basilar membrane, driven by pressure difference, P, between the scalae, are transmitted to the TM via stiffness K_OHC of the outer-hair-cell hair bundles.

Figure 3

Diagram illustrating hypothetical interactions between the pressure driven basilar membrane travelling wave (blue) and the TM travelling wave (red) propagating along the TM due to elastic coupling. f_BM and f_TM indicate places of the basilar and tectorial membrane resonances for a give frequency of stimulation. The radial TM travelling wave may determine the extent (shaded area of equal wavelength) to which feedback from the outer hair cells can amplify motion of the basilar membrane (BM).