The ears are not just for hearing
When we think about our ears, the sense of hearing comes to mind first. Yet, that is just a part of what the cells in our ears can do. Let’s take a closer look at the vestibular hair cells. They are located in the inner ear, on the surface of a tiny but complex network of the vestibular system, comprising the otolith organs called utricle and saccule, and three semicircular ducts. This system is so tiny that it could comfortably sit on the end of a pencil, but it cares for our body’s balance control. It allows us to orient our body to gravity, walk, run, and dance (1). Even though the number of vestibular hair cells is significantly smaller (around 16,000) compared to auditory hair cells (about 3.5 million), they play an equally important role in our daily lives (2).
Decoding Movements: The Brain and Vestibular Hair Cells
Vestibular hair cells are mechanoreceptors designed to respond specifically to mechanical energy — movement (3). They are called hair cells due to the hair-like structures on top of them called stereocilia (4). When we move our head, the stereocilia of the vestibular hair cells move too, sending signals by opening up ion channels at the tips of the stereocilia. When these channels open, ions rush into the hair cell. This sudden influx changes the electrical charge within the cell, creating an electrical signal transmitted to the vestibular nerve. Our brain interprets these electrical signals, transforming them into information that it can use to understand our body’s position and motion in space. Doing so gives us a sense of balance and spatial orientation (5, 6). Cells in the semicircular ducts monitor rotational movements, and the ones in otolith organs track linear movements and gravity (7).
Tiny hard-working cells
The vestibular system’s sensitivity is impressive. The hair cells can detect movements smaller than the diameter of an atom. And they are on duty 24/7, even when we sleep. While their primary function is maintaining balance, vestibular hair cells also help to coordinate our eye movements thanks to the vestibulo-ocular reflex. They allow our eyes to focus on an object even when our head moves. This is why we can read a book while shaking our heads or riding in a vehicle (8).
Delicate balance
Unlike some cells in our body that constantly divide and replace themselves, the amount of vestibular hair cells we were born with stays the same throughout the life. These cells degenerate with age and cannot regenerate significantly once damaged (9). Significant vestibular hair cell loss can cause nausea, imbalance, dizziness, and incapacitation (10). The majority of people have vestibular impairments. According to estimates, they afflict 35% of Americans over 40, and their prevalence rises sharply with age (11). But there is hope. In rodent models, after damage, specific type II hair cells have been proven to regenerate and become re-innervated, providing potential for regaining vestibular function (12–14). The regeneration process is studied excessively by numerous laboratories around the world.
Recognizing and appreciating the labs working in this space:
- Navaratnam Lab, Yale School of Medicine, USA, https://medicine.yale.edu/lab/navaratnam/
- Alan Cheng Lab, Stanford University School of Medicine, USA, https://med.stanford.edu/achenglab.html
- Vestibular Hair Cell Lab, CU Anschutz, University of Colorado, USA, https://medschool.cuanschutz.edu/otolaryngology (Twitter: @CUOTO, https://www.facebook.com/cuotolaryngology, insta: cu_otolaryngology, Linkedin: https://www.linkedin.com/company/cuotolaryngology/)
- State Key Laboratory of Medical Neurobiology, Department of Otorhinolaryngology, Eye and ENT Hospital, MOE Frontiers Center for Brain Science, ENT Institute, Fudan University, Shanghai, China, https://eye.hms.harvard.edu/shanghaieent
- UCL Ear Institute, University College London, London, United Kingdom, https://www.ucl.ac.uk/ear/ucl-ear-institute ( Twitter: @UCLEarInstitute)
- Boston University Hearing Research Center, Boston, USA, https://www.bumc.bu.edu/cotanchelab/
References
- Day BL, Fitzpatrick RC. The vestibular system. Current biology. 2005;15(15):R583-R6.
- Zhang LW, Cang XH, Chen Y, Guan MX. In vitro culture of mammalian inner ear hair cells. Journal of Zhejiang University Science B. 2019;20(2):170–9.
- Gillespie PG, Müller U. Mechanotransduction by hair cells: models, molecules, and mechanisms. Cell. 2009;139(1):33–44.
- Khan S, Chang R. Anatomy of the vestibular system: a review. NeuroRehabilitation. 2013;32(3):437–43.
- Colclasure JC, Holt JR. Transduction and adaptation in sensory hair cells of the mammalian vestibular system. Gravit Space Biol Bull. 2003;16(2):61–70.
- Eatock RA, Songer JE. Vestibular hair cells and afferents: two channels for head motion signals. Annual review of neuroscience. 2011;34:501–34.
- Haggerty SE, King WM. The Interaction of Pre-programmed Eye Movements With the Vestibulo-Ocular Reflex. Frontiers in Systems Neuroscience. 2018;12.
- Huang Y, Mao H, Chen Y. Regeneration of Hair Cells in the Human Vestibular System. Frontiers in Molecular Neuroscience. 2022;15.
- Brandt T, Dieterich M. The dizzy patient: don’t forget disorders of the central vestibular system. Nature Reviews Neurology. 2017;13(6):352–62.
- Burns JC, Stone JS. Development and regeneration of vestibular hair cells in mammals. Seminars in Cell & Developmental Biology. 2017;65:96–105.
- Kawamoto K, Izumikawa M, Beyer LA, Atkin GM, Raphael Y. Spontaneous hair cell regeneration in the mouse utricle following gentamicin ototoxicity. Hearing research. 2009;247(1):17–26.
- Lin V, Golub JS, Nguyen TB, Hume CR, Oesterle EC, Stone JS. Inhibition of Notch activity promotes nonmitotic regeneration of hair cells in the adult mouse utricles. The Journal of neuroscience: the official journal of the Society for Neuroscience. 2011;31(43):15329–39.
- Slowik AD, Bermingham-McDonogh O. Hair cell generation by notch inhibition in the adult mammalian cristae. Journal of the Association for Research in Otolaryngology : JARO. 2013;14(6):813–28.
About the author:
DR. AGA SZMITKOWSKA
Content Editor The League of Extraordinary Celltypes, Sci-Illustrate Stories
Aga did her Ph.D. in Biochemistry at the CEITEC/Masaryk University in Brno, Czech Republic, where she was a part of the Laboratory of Genomics and Proteomics of Plant Systems. She is a passionate public speaker and science communicator. After graduation, she became a freelance content coordinator and strategist in a start-up environment focused on lifestyle and longevity.
About the artist:
NELLY AGHEKYAN
Contributing Artist The League of Extraordinary Celltypes, Sci-Illustrate Stories
Nelli Aghekyan, did Bachelor’s and Master’s degrees in Architecture in Armenia, after studying architecture and interior design for 6 years, she concentrated on her drawing skills and continued her path in the illustration world. She works mainly on children’s book illustrations, some of her books are now being published. Currently living in Italy, she works as a full-time freelance artist, collaborating with different companies and clients.
About the animator:
DR. EMANUELE PETRETTO
Animator The League of Extraordinary Celltypes, Sci-Illustrate Stories
Dr. Petretto received his Ph.D. in Biochemistry at the University of Fribourg, Switzerland, focusing on the behaviour of matter at nanoscopic scales and the stability of colloidal systems. Using molecular dynamics simulations, he explored the delicate interaction among particles, interfaces, and solvents.
Currently, he is fully pursuing another delicate interaction: the intricate interplay between art and science. Through data visualisation, motion design, and games, he wants to show the wonders of the complexity surrounding us.
About the series:
The League of Extraordinary Celltypes
The team at Sci-Illustrate and Endosymbiont bring to you an exciting series where we dive deep into the wondrous cell types that make our bodies tick ❤.
