What they found
First direct measurement of HTC spring contribution to OHC hair bundle stiffness using non-contact acoustic FM-AFM. Measured at P9–P15 in Strc-/-, Strc+/-, and Strc-/-/Tecta-/- mice, isolating the specific contribution of horizontal top connectors.
Key numbers (directly from paper)
| Condition | OHC bundle stiffness | IHC bundle stiffness |
|---|---|---|
| With HTCs (Strc+/−/Tecta−/−) | 5.12 ± 0.46 pN/nm | 2.34 ± 0.64 pN/nm |
| Without HTCs (Strc−/−/Tecta−/−) | 2.05 ± 0.15 pN/nm | 2.74 ± 0.50 pN/nm |
- Reduction: ~60% in OHC bundle stiffness without HTCs
- IHC stiffness is UNCHANGED — HTCs are OHC-specific, IHC are unaffected
- Measurements at P13–P15 (post HTC maturation)
Unit conversion for model
- 5.12 pN/nm = 5.12 mN/m (per bundle, ~20 stereocilia)
- HTC contribution: 5.12 − 2.05 = 3.07 mN/m per bundle
- Per stereocilium: ~0.154 mN/m
- Per HTC link (19 links in 20-stereocilia chain): ~0.16 mN/m per link
This is ~5.7× larger than my original k_HTC estimate (0.028 mN/m). Model needs recalibration with these values.
Developmental timeline
Top connector maturation coincides with hearing onset (P12–P15). Significant increase in OHC (not IHC) stiffness from P12→P15 = HTC maturation window. No adult measurements — limitation of the study.
Relevance to model
Provides ground-truth calibration for k_HTC in stereocilia_bundle_mechanics.py. The 60% reduction figure is confirmed with direct measurement, not inferred. Critical: IHC stiffness unchanged = tip-link parameters remain valid for HTC-free state.
Links
- PubMed: https://pubmed.ncbi.nlm.nih.gov/30801007/
- PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC6382404/
- DOI: https://doi.org/10.1126/sciadv.aat9934
Connections
[supports]STRC Stereocilia Bundle Mechanics Model — direct k_HTC measurement confirms 60% stiffness loss
Numbers that matter (2026-04-23 audit)
Direct AFM measurements confirmed (mouse, apical turn, P13–P15, Strc+/−/Tecta−/− vs Strc−/−/Tecta−/−):
| Condition | OHC bundle stiffness |
|---|---|
| With HTCs | 5.12 ± 0.46 pN/nm |
| Without HTCs | 2.05 ± 0.15 pN/nm |
| HTC contribution | 3.07 pN/nm (~60%) |
Developmental series (apical, with HTCs): P9 = 0.92 pN/nm → P15 = 5.4 pN/nm (~6× increase).
Model parameter cross-check:
WT_BUNDLE_STIFFNESS_PN_PER_NM = 1400— NOT supported. Dulon 2019 reports 5.12 pN/nm (apical OHC, juvenile mouse). Even scaling to adult basal turn (×5–10×) gives ~25–50 pN/nm, still 28–56× below 1400. The 1400 value is ~150–700× above any published OHC bundle measurement and has no literature basis. The constant is also never referenced in gate3_bundle_stiffness() — it is dead code.K_HTC_PN_PER_NM = 7.5— Dulon 2019 gives net HTC contribution of 3.07 pN/nm per bundle (~20 stereocilia, ~19 HTC links in a chain) → ~0.16 pN/nm per HTC link. The Kozlov 2011 Nature model uses 20 pN/nm aggregate for bullfrog sacculus. The 7.5 value sits between these two extremes but the Dulon paper does NOT report a per-link value; the model’s 7.5 is not directly derivable from this paper.HTC_PER_STEREOCILIUM = 6— not reported in Dulon 2019. No TEM connector count data.HTC_SPACING_NM = 8.0— not reported in Dulon 2019.
No SI data was parsed — PDF of Dulon 2019 not available (Science.org returned 403). Existing note may have been written from PMC full text (accessible). SI may contain connector counts — not yet checked.