The tympanic covering layer contributes to basilar membrane elasticity potentially influencing human frequency resolution and speech perception

EMILIN family extracellular glycoproteins in the basilar membrane (BM) have so far only been identified in the mouse. It may influence the stiffness and extracellular filamentous architecture essential for cochlear frequency tuning (Russell et al., 2020, Science Advances, 6, eaba2634). In our study, we explored whether elastic components, such as EMILIN-2 (elastin microfiber interface-located protein 2) and elastin, are also present in the human BM. In addition, we analyzed the fine morphology of the BM and tympanic covering layer (TCL) at different frequency locations using light, transmission electron, and scanning electron microscopy (LM, TEM, and SEM) in well-preserved human cochlear specimens. Tonotopic estimations were made from model extrapolations using synchrotron radiation phase-contrast imaging (SR-PCI) and 3D reconstruction of matched cochleae with delineated octave bands constructed using Greenwood's formula (Greenwood, 1961, Journal of the Acoustical Society of America, 33, 1344). Our immunohistochemistry and gene RNA sequencing (RNA-seq) data support that EMILIN-2 and elastin are expressed in the human BM. Expression pattern of both proteins that influence BM architecture vary along the frequency range. TEM analysis suggests that TCL cells are actively involved in the deposition of the amorphous substance and fibrillary network, conceivably serving to assemble extracellular and elastic constituents along the BM. Thus, our findings suggest that TCL cells may play an important functional role in human cochlear tuning, particularly at low frequencies potentially linked to our remarkable speech and music perception.