OHC Bundle Geometry — Apical Mouse Cochlea, Cartagena-Rivera 2019
Geometric parameters of the developing mouse OHC stereocilia bundle in the apical ¼ turn, required as inputs to the FM-AFM stiffness/damping equations (see Recipe — Noncontact FM-AFM Hair Bundle Mechanics). All numbers from SEM (effective area ) and confocal cryosections (sensory epithelium angle ).
Effective area at the top of the bundle ()
| Postnatal age | (with HTC) | (no HTC) | Notes |
|---|---|---|---|
| P14 | 5.3 ± 0.2 µm² | 6.4 ± 0.4 µm² | Mutant bundles are ~21% larger — splayed without HTC cohesion |
Source: [Cartagena-Rivera 2019, “Estimation of hair bundle geometrical parameters…” section + fig. S2]. Method: ImageJ contour selection on SEM images, apical ¼ turn, P10/P12/P14 sampled.
Grouping convention used by the authors: P9–P10 use P10 area; P11–P12 use P12 area; P13–P15 use P14 area. Reasoning: stereocilin appears around P11 and HTCs are mature by P13 onward.
Sensory epithelium angle (Tecta⁻/⁻ single-knockout)
| Postnatal age | (degrees) |
|---|---|
| P10 | 4° ± 0.5° |
| P12 | 6.7° ± 0.9° |
| P14 | 15.2° ± 0.7° |
Source: [Cartagena-Rivera 2019, “Estimation of hair bundle geometrical parameters…” section + fig. S3]. Method: maximum-intensity Z-projection confocal, intersecting lines fit to basilar membrane and reticular lamina in the apical turn.
is the angle between the basilar membrane and the reticular lamina; assumed equal in and littermates because stereocilin is not involved in epithelial morphology, but measured on Tecta⁻/⁻ background because the tectorial membrane could shift the angle.
Sensitivity of bundle stiffness to geometric inputs
| Input | Fractional uncertainty | Propagated uncertainty in |
|---|---|---|
| – | ||
| – |
Source: [Cartagena-Rivera 2019, section S3 (sensitivity analysis)]. These dominate the systematic error budget of the FM-AFM stiffness measurement.
Reticular lamina apical surface stiffness (PFT-AFM, P17)
The apical reticular-lamina normal stiffness is ~5–10× larger than the measured hair-bundle normal stiffness ([Cartagena-Rivera 2019, “FM-AFM measures the bundle stiffness normal to the hair bundle” section + fig. S1]). Confirms that FM-AFM over a bundle measures bundle stiffness, not surface stiffness.
Why these matter
- Eq. 1 uses as the effective coupling area between the oscillating bead and the bundle. A factor-of-three change in (4° → 15° between P10 and P14) shifts the geometric prefactor by ~3.8× — geometry alone explains a major part of apparent stiffness change with age, separate from molecular maturation.
- Any STRC-research model that uses Cartagena-Rivera 2019 stiffness numbers without these geometric corrections is implicitly assuming P14 geometry. Earlier postnatal data needs the matched , pair.
- Splayed-bundle area in confirms the structural role of HTCs as a cohesion element, independent of mechanical (stiffness/damping) loss.
Connections
[source]dulon-2019-htc-bundle-mechanics[part-of]stereocilia-mechanics[see-also]Recipe — Noncontact FM-AFM Hair Bundle Mechanics[see-also]OHC Bundle Damping Cartagena 2019[applies]h09-hydrogel[about]Stereocilin