Measurement of the mechanical properties of isolated tectorial membrane using atomic force microscopy

Abstract

The tectorial membrane (TM) is an extracellular matrix situated over the sensory cells of the cochlea. Its strategic location, together with the results of recent TM-specific mutation studies, suggests that it has an important role in the mechanism by which the cochlea transduces mechanical energy into neural excitation. A detailed characterization of TM mechanical properties is fundamental to understanding its role in cochlear mechanics. In this work, the mechanical properties of the TM are characterized in the radial and longitudinal directions using nano-and microindentation experiments conducted by using atomic force spectroscopy. We find that the stiffness in the main body region and in the spiral limbus attachment zone does not change significantly along the length of the cochlea. The main body of the TM is the softest region, whereas the spiral limbus attachment zone is stiffer, with the two areas having averaged Young's modulus values of 37 ؎ 3 and 135 ؎ 14 kPa, respectively. By contrast, we find that the stiffness of the TM in the region above the outer hair cells (OHCs) increases by one order of magnitude in the longitudinal direction, from 24 ؎ 4 kPa in the apical region to 210 ؎ 15 kPa at the basilar end of the TM. Scanning electron microscopy analysis shows differences in the collagen fiber arrangements in the OHC zone of the TM that correspond to the observed variations in mechanical properties. The longitudinal increase in TM stiffness is similar to that found for the OHC stereocilia, which supports the existence of mechanical coupling between these two structures.cochlea ͉ collagen fibers ͉ indentation ͉ hearing T he tectorial membrane (TM) is a heterogeneous and gelatinous extracellular matrix. It is located in the cochlea, which transduces mechanical audio stimuli into an electrical signal. Under physiological conditions, 97% of the weight of the TM is water, whereas the remainder is composed of proteins and protoglycans (1). Nearly half of these proteins are collagens, principally of type II, which form fibrils that are assembled along the radial direction (Fig. 1A); at the surface of the TM, which faces away from the hair-cells, they form a mesh-like structure called the covering net.Structurally, the TM spans the entire length of the cochlea and is Ϸ100 m wide and 50 m thick. It is strategically situated over the sensory cells of the cochlea: the outer (OHCs) and inner (IHCs) hair cells (Fig. 1B). The electromotility of OHCs is thought to play a central role in the cochlear mechanicalelectrical transduction process (2, 3), whereas IHCs synapse directly with the auditory nerve (4). Upon auditory stimulation, deflection of the stereocilium bundles, which exist both in OHCs and IHCs, directly converts the mechanical energy into an electrical signal. This conversion is the physiological function of these cells. An additional function of the OHCs is electromechanical transduction. Whereas the stereocilia of OHCs are physically embedded in the lower surface of the TM, the stereocilia of the IHC ar...

Basilar Membrane and Tectorial Membrane Stiffness in the CBA/CaJ Mouse

“…Whereas von Békésy (1953Békésy ( , 1960 and Shoelson et al (2004) found no systematic longitudinal stiffness changes in the TM, a systematic change in driving point stiffness measured at the TM has been reported by our group in the gerbil (Emadi et al 2002;Richter et al 2007) and by Gueta at al. (Gueta et al 2006 in the mouse.…”

“…(Gueta et al 2006 in the mouse. Gueta et al's (2006) published data on TM Young's modulus differed significantly from previously reported values (Gueta et al 2006(Gueta et al , 2008(Gueta et al , 2011. They measured Young's modulus using an atomic force microscope probe and demonstrated that the values decrease from base to apex by about TEUDT AND RICHTER: Basilar Membrane and Tectorial Membrane Stiffnessan order of magnitude (210 to 24 kPa) and vary along the radial axis by a factor of two.…”

“…Furthermore, stiffness measurements were made on isolated TM (Freeman et al 2003a, b;Gueta et al 2006Gueta et al , 2008Gueta et al , 2011Shoelson et al 2004), in vitro (Emadi et al 2002Richter et al 2007;von Békésy, 1960) and in vivo (Zwislocki and Cefaratti 1989;Zwislocki et al 1988). Some experimental sets were obtained from different sites along the cochlea, and a longitudinal gradient could be determined (Emadi et al 2002(Emadi et al , 2004Gueta et al 2006Gueta et al , 2011Richter et al 2007;von Békésy, 1960).…”

“…Some experimental sets were obtained from different sites along the cochlea, and a longitudinal gradient could be determined (Emadi et al 2002(Emadi et al , 2004Gueta et al 2006Gueta et al , 2011Richter et al 2007;von Békésy, 1960). Recently, efforts have been made to determine the impedance of isolated tectorial membranes (Gavara and Chadwick 2010;Jones et al 2013).…”

Enhancement of sensitivity gain and frequency tuning by coupling of active hair bundles

“…In the mammalian cochlea the Young's modulus of the tectorial membrane has been estimated and exhibits a gradient of tectorial stiffness (33)(34)(35)(36)(37). Values reported range from E ϭ 0.3 kPa at the apex to 3 kPa at the base in gerbil (37) and from E ϭ 24 kPa at the apex to 224 kPa at the base in mouse (36).…”