Journal Club: Optical coherence tomography to visualize the low-frequency zone of the gerbil cochlea

on

Today's journal article

Page S, Ghaffari R, Freeman DM. Visualization of relative cochlear motions using high resolution optical coherence microscopy. 

Why I picked this article

This research uses a technique called the Optical Coherence Tomography (OCT) with high resolution to visualise the movement induced by sound within the cochlea, particularly in the area that is responsible for low-frequency hearing. 

The cochlea is a super-sensitive sound detection device in our inner ear. The sensitivity of the cochlea and the broad range of frequencies it can detect are enabled by the vibratory properties of the Organ of Corti sitting on the basilar membrane. The motion of the Organ of Corti differs between the high-frequency zone and the low-frequency zone of the cochlea. Studying how the organ of Corti moves in response to sound, or the micromechanics of the cochlea, is very challenging because of the size, speed and location of such movements taking place within the cochlea. 

OCT has emerged and is increasingly used to image live movement of the cochlear compartments in response to sound stimuli. However, many of the studies to date have focused on the high-frequency zone because it is easier to visualise this area. This research has developed a new way to image the low-frequency zone of the cochlea. 

Some of the research findings

Animal model: 

  • Mongolian gerbil (3-5 week-old, either sex) 
  • Cochleae were removed from the skull and fixed to a petridish, and bathed in the artificial perilymph.
  • Artificial perilymph (mimic of natural cochlear fluid): "7 mM sodium chloride (NaCl), 163.4 mM sodium gluconate (C6H11NaO7), 3 mM potassium chloride (KCl), 0.1 mM calcium chloride dihydrate (CaCl2⋅2H2O), 0.1 mM magnesium chloride (MgCl2), 2 mM sodium sulfate (Na2SO4), 0.5 mM sodium dihydrogen phosphate (NaH2PO4), 5 mM HEPES (C8H18N2O4S), 5 mM dextrose (C6H12O6), and 4 mM L-glutamine (H2NCOCH2CH2 CH(NH2) CO2H)"
  • Stapes, one of the ossicular chain attached to the oval window, was connected to a titanium probe mounted on a micromanipulator, so that sinusoidal mechanical movement can be applied to the stapes. 
  • A small hole was made on the bone to get better visualisation of the apical end of the cochlea. 

OCT setup:

  • Time-domain DOCM system 
  • 843 nm wavelength 
  • Two lenses were used:
    • low NA (= numerical aperture) 0.13 lens: ~6.5μm axial resolution, ~2.4μm lateral resolution
    • high NA 0.8 lens: ~2.4μm axial resolution, ~340 nm lateral resolution
Part of Fig 4. Cross-section of the sensory part of the cochlea, in the low-frequency hearing zone imaged using high-resolution OCT. Page et al. (2025).

Finding:
  • High-resolution image enabled visualisation of sensory cells in the Organ of Corti, the sensory domain within the cochlea, in the region corresponding to low frequency hearing around 275Hz. 
  • Axial displacement in nm could be resolved relative to the visualisation axis. It shows that the most displacement occurred in the Reissner's membrane near the outer hair cells, followed by the outer (lateral) region of the organ of Corti.
  • Quantification of the movement of the reticular lamina, the surface along the outer and inner hair that is important for sensory transduction, showed that the maximum average motion occurred around the area slightly more lateral from where outer hair cells are. The maximum magnitude was around ~80 nm. 

Haruna's takeaway

What's cool about this research is the push for "high resolution OCT" use, which gives much more detailed visualisation of the cellular component in the cochlea, and the establishment of a perfused cochlea in a petri dish model approach to visualise the low-frequency zone of the cochlea. The technique gives additional data points for those wanting to model and understand the cochlear micromechanics. 

Personally, very interested in how they set up the stapes to be controlled by the micromanipulator, as I'd love to be able to set something up like that for our lab. 

 ------- 

This is Haruna's 28/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.