STRC Research

Cochlear outer hair cell horizontal top connectors mediate mature stereocilia bundle mechanics

4 min read

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)

ConditionOHC bundle stiffnessIHC bundle stiffness
With HTCs (Strc+/−/Tecta−/−)5.12 ± 0.46 pN/nm2.34 ± 0.64 pN/nm
Without HTCs (Strc−/−/Tecta−/−)2.05 ± 0.15 pN/nm2.74 ± 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.

Connections

Numbers that matter (2026-04-23 audit)

Direct AFM measurements confirmed (mouse, apical turn, P13–P15, Strc+/−/Tecta−/− vs Strc−/−/Tecta−/−):

ConditionOHC bundle stiffness
With HTCs5.12 ± 0.46 pN/nm
Without HTCs2.05 ± 0.15 pN/nm
HTC contribution3.07 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.

Full text retrieved 2026-04-24 via PMC (PMC6382404). Confirmed: the paper does NOT report a link count per bundle, a per-link stiffness value, or any TEM/SEM connector enumeration. The “~19 links” and “~0.16 pN/nm/link” values in stereocilia-mechanics.md are external derivations (3.07 pN/nm ÷ 19 assumed links from a 20-stereocilium chain assumption). These numbers do NOT appear anywhere in the Cartagena-Rivera 2019 paper. SI of the Science.org PDF returned 403 — not retrieved. The aggregate stiffness data (5.12 vs 2.05 pN/nm) is fully confirmed from PMC full text.

Author correction (2026-04-24): Paper note previously listed wrong authors (“Dulon D, Papal S…” from Paris/Pasteur Institut Pasteur). Correct authors: Cartagena-Rivera AX (NIH), Le Gal S, Richards K, Verpy E, Chadwick RS. Different research group. The correct wikilink for this paper is [[dulon-2019-htc-bundle-mechanics]] (filename retained for backward compatibility).

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