Hydrostatic pressure shapes and canalizes semicircular canal morphology to ensure vestibular function

Background

The organs in our inner ear, called semicircular canals, are crucial for our sense of balance (1). These canals are shaped like tiny, curved tubes and detect head movements by sensing the flow of fluid inside them. As small changes can alter how these organs function properly, their shape must be very precise. In zebrafish, semicircular canals form inside a fluid-filled sac called the otic vesicle, where the opposing pillars meet and fuse (2). As the vesicle fills with fluid, pressure builds up inside, stretching and shaping the tissue. Researchers have long wondered how this pressure might help create the exact shapes needed for balance, and what happens if this process is impaired.

In this preprint, the authors investigated how vesicle expansion influences semicircular canal morphology by combining approaches like perturbations, physical modelling, and behavioral assays in zebrafish.

Figure 1 Representative time course images showing hydrostatic pressure-driven otic vesicle expansion during semicircular canal morphogenesis. Preprint Figure 1A, 1B made available under a CC-BY-NC-ND 4.0 International license.

Key findings

Fluid pressure directly shapes semicircular canal structures in zebrafish

As the otic vesicle fills with fluid, the pressure inside increases. This pressure eventually molds the tissue pillars that will become the semicircular canal. When the preprint authors, by laser ablation, quickly deflated the fluid-filled sac in the developing otic vesicle, the pressure was released leading the tissue pillars to become shorter and wider. In contrast, inflating the fluid-filled sac by using a drug caused the pillars to become longer and narrower. This shows that the shape of these organs is directly determined by the amount of fluid pressure inside the canal. To explain this, the authors used real measurements and material properties to build a physical model that could predict pillar shape changes in response to increased or decreased pressure.

Figure 2 Representative images of otic vesicle perturbation studies, using laser ablation (A–D) and drug treatment (E–J), and the resulting quantification of lumen volume and pillar aspect ratio changes under these conditions. Preprint Figure 2A-I made available under a CC-BY-NC-ND 4.0 International license.

Pressure ensures consistent organ formation

When the tissue pillars first form, their shape and curvature vary quite a bit from embryo to embryo, since the buds don’t always meet at exactly the same angle. But as pressure builds, these differences disappear, the pillars straighten out and became uniform. By tracking individual pillars over time, the authors noticed that pillar straightening sped up when they added more pressure and stalled when they released it. The authors termed this process “canalization,” which helps ensure the canals develop consistently regardless of how variable their starting shape was. This is essential for reliable balance.

Figure 3 Representative confocal images showing variable pillar tissue morphologies over time, with buds forming variable curvatures, meeting and fusing at variable angles (A-D). Perturbation studies to investigate the effects of adding and releasing pressure in the otic vesicles (F-M). Preprint Figure 3 made available under a CC-BY-NC-ND 4.0 International license.

Wnt signalling maintains the pressure needed for proper development

The researchers then asked what keeps the pressure up in the otic vesicle. They found that Wnt signalling helps keep the barrier of the fluid-filled vesicle tight, so fluid and ions don’t leak out. This allows pressure to build up inside, which is needed for the correct shaping of the organ. Notch signalling acts downstream of Wnt, also supporting this barrier function. If either pathway is disrupted, the barrier weakens, pressure drops, and the canals form abnormally.

Proper canal shape is essential for proper movement

The authors also reported that fish with an abnormal canal shape due to fluid leakage and low otic vesicle pressure had trouble swimming and maintaining balance. In addition, their movements were less active and their escape responses, such as swimming away from a threat, were less straight, often looping back on themselves instead of shooting away in one direction. These observations confirmed that proper canal shape is directly linked to the balancing ability.

What I like about this preprint

Organogenesis involves both molecular and mechanical regulation throughout, with forces constantly being generated as tissues dynamically change shape. I liked how this elegant study confirms that such force generation (hydrostatic pressure-driven) does not just determine tissue geometry such as that of the epithelial pillar, but also acts as the mechanism for pillar tissue canalization. This process eventually reduces shape variability to ensure consistent organ formation. Altogether, the study offers a compelling physical explanation for the fidelity of tissue development.

Questions for the authors

• The present study proposes a Wnt-dependent regulation of epithelial barrier function, which in turn governs luminal hydrostatic pressure, which in turn governs epithelial morphology. Do you think a feedback mechanism may exist, in which pillar shape itself can influence the state of hydrostatic pressure inside?
• In humans, some people are born with malformed semicircular canals and they may or may not exhibit balance problems (2), suggesting that the body can sometimes compensate. Do you think these cases still involve some hidden disruption to pressure regulation, or is it possible that canal shape and pressure regulation can become uncoupled later in life?

References

• Zhao, X., Tan, R.-S., Tang, H.-C., Teo, S.-K., Su, Y., Wan, M., Leng, S., Zhang, J.-1484 , Allen, J., Kassab, G.S., et al. (2018). Left Ventricular Wall Stress Is Sensitive Marker1485 of Hypertrophic Cardiomyopathy With Preserved Ejection Fraction. Front. Physiol. 9, 250.1486 https://doi.org/10.3389/fphys.2018.00250
• Munjal, A., Hannezo, E., Tsai, T.Y.-C., Mitchison, T.J., and Megason, S.G. (2021). Extracellular hyaluronate pressure shaped by cellular tethers drives tissue morphogenesis. Cell 184, 6313-6325.e18. https://doi.org/10.1016/j.cell.2021.11.025.1523
• Yun JM, Kim SH and Bae SH (2024) Vestibular dysfunction in lateral semicircular canal dysplasia. Front. Neurol. 15:1341812. doi: 10.3389/fneur.2024.1341812.

Tags: biomechanics, epithelial barrier integrity, hydrostatic pressure, inner ear development, semicircular canals, tissue morphogenesis, vestibular function, wnt signaling, zebrafish

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