Journal Club: Cell polarity and arrangement of cells during development is driven by stiffness difference between adjacent cells.

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Today's journal article

Prakash A, Weninger J, Singh N, Raman S, Rao M, Kruse K, Ladher RK. Junctional force patterning drives both positional order and planar polarity in the auditory epithelia. 

Why I picked this article

Hair cells are the sensors of sound. Supporting cells are the neighbouring cells to the hair cells, which support the environment surrounding hair cells. Hair cells and supporting cells are arranged in ordered rows and columns; the ordered arrangement of cells, as well as arranged "hairs" or stereocilia on hair cells, is critical for sensitive detection of sounds and sound transduction. 

The orientation within the cell that establishes how small features like stereocilia form in a particular location, side of the cell, facing the right orientation, length... is called "planar cell polarity". When planar cell polarity is disrupted by genetic manipulation, it results in the disruption of the ordered architecture of hair cells and loss of cochlear function. During development, a lot happens to establish the polarity of the cells. Stretching and growth of the structure can influence the cell; cells divide and start to differentiate into hair cells and supporting cells; cells migrate; and at some stage during those processes, planar cell polarities are established. 

This research asks how the inner ear auditory epithelium achieves ordered architecture made by a collection of cells, and planar cell polarity within cells, while it is still growing and shifting. Researchers used the chick auditory epithelium developing in the egg as the model. 

Some of the research findings

Animal model:
  • Developing auditory epithelium of chick (the basilar papilla) 
  • Fertilised eggs from Kallinga chickens were incubated, and inner ear from staged embryos were used for experiments. 
  • Electroporation was used to genetically manipulate some cells in ovo (in the egg).
  • Ex-ovo explants: inner ear from staged embryos were cultured in cultured media ex ovo. 
Mapping the development of the chick auditory epithelium:
  • At embryonic day 8, hair cells were not polarised. 
  • At embryonic day 10, the polarity was observed as visible by the location of special cilia called kinocilium. 
  • At embryonic day 14, the polarity was further refined.  
  • Hexatic order parameter = 1 for a perfect hexagonal lattice. The order increased with development from 0.2 (E8) to 0.65 (E14). 
  • Variation of polarity decreased from E8 to E14, meaning cells were a lot more aligned with development. 
  • Quantification of the cell area and number showed an increasing cell area for hair cells with development, and decreasing neighbour number. 
2D vertex modelling: 
  • 2D vertex modelling (simulation) was performed based on the observed changes in cells. 
    • the cell areas = Aα
    • normalised = cell perimeters (termed cell shape indices, pα)
    • junctional = lengths, li,j.
    • e.g. Early development (E10): Að0Þ α = Að0Þ H = Að0Þ S = 1.
  • Finding: simulation showed that junctional contractility being different between hair cells and supporting cells can help generate positional differences during development. 
  • The increase in hair cell surface area was also contributor to positioning of the cell. 
  • Code used for simulation and data analysis are available: 
    • Zenodo: https://doi.org/10.5281/zenodo.14917594.
    • https://doi.org/10.5281/zenodo.14917594.
Inhibition of 
  • Rho-Associated Kinase (ROCK) and Myosin-Light Chain Kinase (MLCK) are important proteins regulating cell motility, contraction and cell architecture. 
  • This research used pharmacological inhibitors of ROCK(Y-27632) or MLCK (ML-7) to inhibit these proteins in cultured chick basilar papilla. 
  • This led to a reduction of planar cell polarity. 

Overall, research suggests that the spatial control of NM2–RLC phosphorylation tunes junction stiffness, steering local rearrangements into global order. Tissue-scale patterns emerge from distributed, junction-level rules rather than a single organiser that tunes the orientation of cells. 

Part of Figure 1B. hair cells and surrounding cells acquire different arrangements and polarities with development. Prakash et al. 2025

Haruna's takeaway

This is a fascinating publication, starting from simple but comprehensive geometric profiling of developing hair cells and supporting cells, using them for a simulation and then trying to identify molecular (proteins) responsible. I couldn't fully appreciate and understand the whole publication yet, as there is so much data, so I need to revisit it again, especially for the modelling aspect. It's very interesting as the patterning of the cells and organs in development tends to be discussed from particular proteins like transcription factors or receptors serving as the guiding cue. But in reality, cells are constantly packed without any space, and the real constraint may be the density, number and tension between cells. I did not know about the huge surface area increase in hair cells with development in the basilar papilla. I would imagine such mechanical constraints also translate to the mammalian organ of corti elongation during early stage. 

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This is Haruna's 62/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.