Morphometric analysis of the inner ear of mammals can provide information for cochlear frequency mapping, a species-specific designation of locations in the cochlea at which different sound frequencies are encoded. Morphometric variation occurs in the hair cells of the organ of Corti along the cochlea, with the base encoding the highest frequency sounds and the apex encoding the lowest frequencies. Changes in cell shape and spacing can yield additional information about the biophysical basis of cochlear tuning mechanisms. Here, we investigate how morphometric analysis of hair cells in mammals can be used to predict the relationship between frequency and cochlear location. We used linear and geometric morphometrics to analyze scanning electron micrographs of the hair cells of the cochleae in Parnell’s mustached bat (Pteronotus parnellii) and Wistar rat (Rattus norvegicus) and determined a relationship between cochlear morphometrics and their frequency map. Sixteen of twenty-two of the morphometric parameters analyzed showed a significant change along the cochlea, including the distance between the rows of hair cells, outer hair cell width, and gap width between hair cells. A multiple linear regression model revealed that nine of these parameters are responsible for 86.9 % of the variation in these morphometric data. Determining the most biologically relevant measurements related to frequency detection can give us a greater understanding of the essential biomechanical characteristics for frequency selectivity during sound transduction in a diversity of animals.
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The authors thank Christina Harvey for help with R and geomorph, and Vikram Baliga for helping with the geomorph coding that he and C.H. developed. We also thank Derrick Horne (UBC Bioimaging Facility) and Dr. Chantal Cazevieille (COMET) for technical assistance and Dr. Marc Lenoir (Institute for Neurosciences of Montpellier) for dissecting and processing the rat cochleae.
Funding for the research described herein was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) (RGPIN 312039-13 and RGPAS 446012-13 to R.E.S.) and the Department of Zoology at the University of British Columbia. The research leading to these results has also received funding from the Marie Skłodowska-Curie Individual Post-doctoral Fellowship 751284-H2020-MSCA-IF-2016 to M.M.
All experiments described herein regarding the bats, were carried out in accordance with current laws for animal experimentation in Germany (Regierungspräsidium Darmstadt) and according to the Declaration of Helsinki. Experiments on the rats were carried out in accordance with the animal welfare guidelines of the Institut National de la Santé et de laRecherche Médicale and approved by the French Ministère de l’Agriculture et de la Forêt.
The authors declare that they have no conflict of interest.
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Girdlestone, C.D., Ng, J., Kössl, M. et al. Correlating Cochlear Morphometrics from Parnell’s Mustached Bat (Pteronotus parnellii) with Hearing. JARO 21, 425–444 (2020). https://doi.org/10.1007/s10162-020-00764-1
- inner ear
- organ of Corti
- geometric morphometrics
- linear morphometrics
- frequency map