Supernumerary human hair cells-signs of regeneration or impaired development? A field emission scanning electron microscopy study.
BACKGROUND
Current attempts to regenerate cochlear sensorineural structures motivate further inspection of the human organ of hearing. Here, we analyzed the supernumerary inner hair cell (sIHC), a possible sign of regeneration and cell replacement.


METHODS
Human cochleae were studied using field emission scanning electron microscopy (FESEM; maximum resolution 2 nm) obtained from individuals aged 44, 48, and 58 years with normal sensorineural pure-tone average (PTA) thresholds (PTA <20 dB). The wasted tissue was harvested during trans-cochlear approaches and immediately fixed for ultrastructural analysis.


RESULTS
All specimens exhibited sIHCs at all turns except at the extreme lower basal turn. In one specimen, it was possible to image and count the inner hair cells (IHCs) along the cochlea representing the 0.2 kHz-8 kHz region according to the Greenwood place/frequency scale. In a region with 2,321 IHCs, there were 120 scattered one-cell losses or 'gaps' (5%). Forty-two sIHCs were present facing the modiolus. Thirty-eight percent of the sIHCs were located near a 'gap' in the IHC row (±6 IHCs).


CONCLUSIONS
The prevalence of ectopic inner hair cells was higher than expected. The morphology and placement could reflect a certain ongoing regeneration. Further molecular studies are needed to verify if the regenerative capacity of the human auditory periphery might have been underestimated.
a Department of Surgical Sciences, Head and Neck Surgery , Section of Otolaryngology, Uppsala University Hospital , Uppsala , Sweden.|b Department of Otolaryngology , Uppsala University Hospital , Uppsala , Sweden.
Helge
H
Rask-Andersen
a Department of Surgical Sciences, Head and Neck Surgery , Section of Otolaryngology, Uppsala University Hospital , Uppsala , Sweden.|b Department of Otolaryngology , Uppsala University Hospital , Uppsala , Sweden.
Hao
H
Li
c Department of Otolaryngology , Medical University of Innsbruck , Innsbruck , Austria.|d Medical Campus University of Oldenburg School of Medicine and Health Sciences, European Medical School , Oldenburg , Germany.|e Research Center of Neurosensory Science, University of Oldenburg , Oldenburg , Germany.
Hubert
H
Löwenheim
c Department of Otolaryngology , Medical University of Innsbruck , Innsbruck , Austria.|d Medical Campus University of Oldenburg School of Medicine and Health Sciences, European Medical School , Oldenburg , Germany.|e Research Center of Neurosensory Science, University of Oldenburg , Oldenburg , Germany.
Marcus
M
Müller
f Cluster of Excellence Hearing4all , University of Oldenburg , Oldenburg , Germany.
Kristian
K
Pfaller
g Department of Histology and Molecular Cell Biology , Institute of Anatomy and Histology, Medical University of Innsbruck , Innsbruck , Austria.
Annelies
A
Schrott-Fischer
g Department of Histology and Molecular Cell Biology , Institute of Anatomy and Histology, Medical University of Innsbruck , Innsbruck , Austria.
Rudolf
R
Glueckert
D000328:Adult; D000818:Animals; D002294:Carcinoma, Squamous Cell / Q000473:pathology; D003051:Cochlea / Q000473:pathology / Q000502:physiology* / Q000648:ultrastructure; D003884:Dermoid Cyst / Q000473:pathology; D004428:Ear Neoplasms / Q000473:pathology; D005260:Female; D006198:Hair Cells, Auditory / Q000502:physiology*; D006199:Hair Cells, Auditory, Inner / Q000502:physiology*; D006309:Hearing / Q000502:physiology*; D006801:Humans; D008579:Meningioma / Q000473:pathology; D008855:Microscopy, Electron, Scanning; D008875:Middle Aged; D012038:Regeneration
Human cochlea; SEM; inner hair cell; regeneration; supernumerary hair cells