Journal Club: Special preparation that allows cochlea to be maintained in the lab to study its sensitivity characteristics.

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Today's journal article

Gianoli F, Alonso R, Fabella B, Hudspeth AJ. Toward an ex vivo preparation for studies of the cochlear active process in mammals. 

Why I picked this article

This research aims to establish an experimental regimen that preserves features of the live cochlea while allowing detailed analysis to be possible. 

Our hearing organ is sophisticated; it has three fluid chambers with different ion makeup, an electrical potential between compartments, and many different types of cells. One of the most impressive features is the sensitivity of the cochlea for sound detection; the mammalian cochlea quietly amplifies soft sounds, sharpens pitch, and keeps loud sounds in check. That active boost of sound sensitivity is enabled by sensory hair cells in the cochlea. 

Biologists have long suspected a simple idea under the hood: hair cells and their neighbors may operate near a special tipping point in physics called a Hopf bifurcation. Near that point, systems amplify, select frequencies, and compress large inputs. This has been shown in non-mammalian hair cells. Whether the mammalian cochlea itself uses the same trick has been harder to investigate due to the delicate environments of the cochlea, which are hard to recreate ex vivo. 

This paper builds a more life-like lab preparation from the mammalian cochlea and tests if they can detect the Hopf-like behaviour. 

Some of the research findings

Model & setup:
  • Mongolian gerbils (3-4 week-old) cochlea
    • Cochlea was extracted, middle turn isolated, and secured in a 1.5 mm hole in a plastic disc with isobutyl 2-cyanoacrylate (BOC Sciences). 
    • Openings were sealed with cyanoacrylate adhesive. 
    • Reissner's membrane was punctured. 
  • The cochlea was orientated in two-cmpartment acrylic recording chamber. Two sides were bathed in: 
    • Artificial perilymph: 155 mM Na+, 3 mM K+, 250 μM Ca2+, 1 mM Mg2+, 154 mM Cl-, 1 mM HPO2􀀀, 5 mM HEPES, 3 mM pyruvate, 10 mM D (+)-glucose, 2mM Ca2+ pH of 7.3-7.4 and an osmolality of 310 mOsm⋅kg-
    • Artificial endolymph: 150 mM KCl, 25 μM CaCl2, 3 mM sodium pyruvate, 10 mM D-glucose, 5 mM HEPES, and 5 mM HPO2- at a pH of 7.35.
  • The disc with cochlea was placed between porous quartz frits (QPD10–4, Technical Glass Products) which hydrates the tissue. 
  • Solutions were oxygenated and kept consistent room temperature of 20-24 ◦C.
  • Endocochlear potential was delivered by a pair of Ag/AgCl electrodes, 90-100 mV between two compartments. 
  • A capacitance of approximately 33 pF, and the transepithelial resistance ranged from 5 to 50 kΩ were observed. 
  • Sound delivery: high-performance speakers (Etymotic Research, ER-3C)
Findings: 
  • Using this system, researchers recorded that cochlear responses to characteristic frequencies ranging from 2 kHz to 3 kHz.
  • Non-linear sensitivity amplification was observed, particularly for frequency =2.66 kHz, as a function of the sound-pressure level.
  • The non-linearity required conditions. When the cochlear prep was damaged, or if the artificial endocochlear potential was absent, the cochlear response was linear. 
  • Researchers noted the orientation of the cochlea mattered, possibly due to the weight of water. Further testing showed that there was an optimal pressure balance between the two chambers that is required for the in vivo-like response. 
Note  (from the introduction) :Bifurcation is a situation in which some change in a parameter drastically changes the way the system behaves. The Hopt bifurcation is where the sensitivity to input grows significantly. 
From part of Figure 1. A schematic diagram of the setup. Gianoli et a. 2025

Haruna's takeaway

This is another research on establishing a good ex vivo (extracted from an animal) model to study the micromechanics of the cochlea. It seems that a short-term experiment can be done while preserving cell and cochlear function. I'm sure setting this experiment up, as well as perfecting cochlear dissection and mounting, is very technical and challenging. I hope we can establish something like this in our lab, so that we can study the cochlea. 

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This is Haruna's 63/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.